Feeds:
Posts
Comments

Posts Tagged ‘pentose phosphate shunt’


Metabolic insight into cancer cell survival

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

Revised 4/20/2016

AACR 2016: Novel Epigenetic Drug Therapeutics Revealed

http://www.genengnews.com/gen-news-highlights/aacr-2016-epigenetic-drug-therapeutics/81252634/

As the 2016 American Association for Cancer Research meeting begins to downshift toward a close, the presentation sessions certainly did not suffer from a lack of enthusiasm from attendees or high-quality research from presenters. Of particular note was a major symposium that discussed next-generation epigenetic therapeutics.

In the past several years, there have been a variety of epigenetic targets exploited by newly developed drug compounds, many of which have already progressed into clinical trials. Often these compounds will target specific classes of epigenetic regulators like acetylases and histone demethylases, for instance, the small-molecule inhibitors of protein interacting bromodomains—implicated in a diverse range of cancers—and methyltransferase inhibitors, such as lysyl demethylases (KDM).

However, for all of their recently achieved success, researchers are continually searching for increasingly rapid methods to validate epigenetic drug targets. Session Chairperson Udo Oppermann, Ph.D., principal investigator at the University of Oxford, stressed that open access research and continued investigator cooperation were key factors for driving rapid development of novel therapeutics in the field. Anecdotally, Dr. Oppermann noted that if biologists were a bit more like the international cooperative teams of physicists that discovered the Higgs boson or gravitational waves, many biological endeavors would advance rather quickly.

After providing the audience with a brief introduction to the symposium’s topic, Dr. Oppermann described his current research on histone demethylase inhibitors in multiple myeloma and the connection to metabolic pathways. He surmised that tricarboxylic acid (TCA) cycle-derived metabolites can link cellular metabolism to cancer—impacting epigenetic landscapes. Specifically, the TCA intermediates are inhibitors of KDMs, ultimately controlling epigenetic and metabolic regulation.

Furthermore, Dr. Oppermann’s group was able to show that treatment of myeloma cell lines with the potent and specific histone demethylase inhibitor GSK-J4 was able to reverse the Myc-driven metabolic dependency, forcing a selected amino acid depletion. This deficiency led to the integrated stress response and the activation of proapoptotic genes. This work helps to solidify further the potent nature of GSK-J4 in cancer while simultaneously uncovering the metabolic mechanisms that cancer cells employ to keep their overproliferative phenotypes progressing forward.

Next, Tomasz Cierpicki, Ph.D., assistant professor at the University of Michigan, described his work on targeting leukemic stem cells with small-molecule inhibitors of the protein regulator of cytokinesis 1 (PRC1). Dr. Cierpicki took the audience through his research design, which was to target BMI1, an oncogene that determines the proliferative capacity and self-renewal potential of normal and leukemic stem cells. BMI1 has been implicated in a variety of tumors and is essential for the Polycomb Repressive Complex 1 (PRC1). Moreover, BMI1 interacts with the RING1B protein to form an active E3 ubiquitin ligase that targets histone H2A, modifying epigenetic regulation mechanisms.

Dr. Cierpicki’s laboratory looked at inhibitors of the RING1B–BMI1 E3 ligase complex as potential therapeutic agents targeting cancer stem cells. Using an array of techniques from fragment screening to medicinal chemistry, the researchers were able to identify potent compounds that bound to RING1B–BMI1 and inhibit its E3 ubiquitin ligase activity with low micromolar affinities. When testing in vitro, the inhibitors revealed robust downregulation of H2A ubiquitination. Dr. Cierpicki and his colleagues found that the RING1–BMI1 inhibitor blocked the self-renewal capacity of the stem cells and induced cellular differentiation—validating RING ligases as a novel epigenetic drug target.

Finishing up the session was William Sellers, M.D., vice president and global head of oncology for the Novartis Institutes for BioMedical Research. Dr. Sellers’ research is focused on what genes are necessary for epigenetic regulation of cancer and how they are linked to essential metabolic processes. He and his colleagues accomplished their studies through the use of large-scale shRNA screening across a diverse set of 390 cancer cell lines.

Utilizing deep small hairpin RNA (shRNA) screening libraries, at 20 shRNAs per genome, provided the investigators with highly robust gene-level data, which resulted in the emergence of several distinct classes of cancer-dependent genes. For example, Dr. Sellers’ group found that several known oncogenes fell into the genetic dependence class, whereas other genes were sorted into lineage, paralog, and collateral synthetic lethality dependent classes.

An interesting example from Dr. Sellers’ work was the link his laboratory discovered between polyamine metabolism and salvage and the protein arginine methyltransferase 5 (PRMT5). In particular, the loss of methylthioadenosine phosphorylase (MTAP)—which has been observed in many solid tumors and hematologic malignancies—resulted in the accumulation of S-methyl-5′-thioadenosine (MTA), which specifically inhibited the epigenetic mechanisms of PRMT5. The culmination Dr. Sellers’ analysis led to the finding that PRMT5 is a novel target for therapeutic development in MTAP deleted cancers.

These three presentations represented some of the excellent, cutting-edge research that is not only looking to develop novel drug therapeutics but also trying to uncover the underlying molecular mechanisms of epigenetic regulation and cancer.

 

 

 

A Metabolic Twist that Drives Cancer Survival

http://www.technologynetworks.com/Metabolomics/news.aspx?ID=190262

A novel metabolic pathway that helps cancer cells thrive in conditions that are lethal to normal cells has been identified.

 

“It’s long been thought that if we could target tumor-specific metabolic pathways, it could lead to effective ways to treat cancer,” said senior author Dr. Ralph DeBerardinis, Associate Professor of CRI and Pediatrics, Director of CRI’s Genetic and Metabolic Disease Program, and Chief of the Division of Pediatric Genetics and Metabolism at UT Southwestern. “This study finds that two very different metabolic processes are linked in a way that is specifically required for cells to adapt to the stress associated with cancer progression.”

The study, available online today in the journal Nature, reveals that cancer cells use an alternate version of two well-known metabolic pathways called the pentose phosphate pathway (PPP) and the Krebs cycle to defend against toxins. The toxins are reactive oxygen species (ROS) that kill cells via oxidative stress.

This work builds on earlier studies by Dr. DeBerardinis’ laboratory that found the Krebs cycle, a series of chemical reactions that cells use to generate energy, could reverse itself under certain conditions to nourish cancer cells.

Dr. DeBerardinis said most normal cells and tumor cells grow by attaching to nutrient-rich tissue called a matrix. “They are dependent on matrix attachment to receive growth-promoting signals and to regulate their metabolism in a way that supports cell growth, proliferation, and survival,” he said.

Detachment from the matrix results in a sudden increase in ROS that is lethal to normal cells, he added. Cancer cells seem to have a workaround.

The destruction of healthy cells when detached from the matrix was reported in a landmark 2009 Nature study by Harvard Medical School cell biologist Dr. Joan Brugge. Intriguingly, that same study found that inserting an oncogene – a gene with the potential to cause cancer – into a normal cell caused it to behave like a cancer cell and survive detachment, said Dr. DeBerardinis, who also is affiliated with the Eugene McDermott Center for Human Growth & Development, holds the Joel B. Steinberg, M.D. Chair in Pediatrics, and is a Sowell Family Scholar in Medical Research at UT Southwestern.

“Another Nature study, this one from CRI Director Dr. Sean Morrison’s laboratory in November 2015, found that the rare skin cancer cells that were able to detach from the primary tumor and successfully metastasize to other parts of the body had the ability to keep ROS levels from getting dangerously high,” Dr. DeBerardinis said. Dr. Morrison, also a CPRIT Scholar in Cancer Research and a Howard Hughes Medical Institute Investigator, holds the Mary McDermott Cook Chair in Pediatrics Genetics at UT Southwestern.

Working under the premise that the two findings were pieces of the same puzzle, a crucial part of the picture seemed to be missing, he said.

It was known for decades that the PPP was a major source of NADPH, which provides a source of reducing equivalents (that is, electrons) to scavenge ROS; however, the PPP produces NADPH in a part of the cell called the cytosol, whereas the reactive oxygen species are generated primarily in another subcellular structure called the mitochondria.

“If you think of ROS as fire, then NADPH is like the water used by cancer cells to douse the flames,” Dr. DeBerardinis said. But how could NADPH from the PPP help deal with the stress of ROS produced in a completely different part of the cell? “What we did was to discover how this happens,” Dr. DeBerardinis said.

The current study in Nature demonstrates that cancer cells use a “piggybacking” system to carry reducing equivalents from the PPP into the mitochondria. This movement involves an unusual reaction in the cytosol that transfers reducing equivalents from NADPH to a molecule called citrate, similar to a reversed reaction of the Krebs cycle, he said. The citrate then enters the mitochondria and stimulates another pathway that results in the release of reducing equivalents to produce NADPH right at the location of ROS creation, allowing the cancer cells to survive and grow without the benefit of matrix attachment.

“We knew that both the PPP and Krebs cycle provide metabolic benefits to cancer cells.  But we had no idea that they were linked in this unusual fashion,” he said. “Strikingly, normal cells were unable to transport NADPH by this mechanism, and died as a result of the high ROS levels.”

Dr. DeBerardinis stressed that the findings were based on cultured cell models, and more research will be necessary to test the role of the pathway in living organisms. “We are particularly excited to test whether this pathway is required for metastasis, because cancer cells need to survive in a matrix-detached state in the circulation in order to metastasize,” he said.

CRI scientists find novel metabolic twist that drives cancer survival
http://www.utsouthwestern.edu/newsroom/news-releases/year-2016/april/cancer-metabolism.html

https://pharmaceuticalintelligence.com/2016/04/09/programmed-cell-death-and-cancer-therapy/5 days ago Cancer Cell Survival Driven by Novel Metabolic Pathway … This new study describes an alternate version of two wellknown metabolic pathways, the pentose phosphate pathway (PPP) and the Krebs cycle,…

http://www.nature.com/nature/journal/v481/n7381/abs/nature10642.html Jan 19, 2012 Nature | Letter …. DeBerardinis, R. J. et al. Beyond aerobic …. Andrew R. Mullen,; Pei-Hsuan Chen,; Tzuling Cheng &; Ralph J. DeBerardinis ..   

Haematopoietic stem cells require a highly regulated protein – Nature
http://www.nature.com/nature/journal/v509/n7498/abs/nature13035.html May 1, 2014 Nature | Article. Print; Share/ ….. synthesis. Nature Methods 6, 275–277 (2009) …. Robert A. J. Signer,; Jeffrey A. Magee &; Sean J. Morrison …       
http://www.nature.com/nature/journal/v527/n7577/abs/nature15726.html Nov 12, 2015 Nature | Article ….. Multistep nature of metastatic inefficiency: dormancy of solitary cells after successful extravasation and ….. Sean J. Morrison …     
Deep imaging of bone marrow shows non-dividing stem Nature
http://www.nature.com/nature/journal/v526/n7571/abs/nature15250.html   Oct 1, 2015 Nature | Letter. Print; Share/ …… Morrison, S. J. & Scadden, D. T. The bone marrow niche for ….. Kiranmai S. Kocherlakota &; Sean J. Morrison …
https://pharmaceuticalintelligence.com/category/cancer-biology-innovations-in-cancer-therapy/genomic-expression/Glutamine and cancer: cell biology, physiology, and clinical opportunities …. Metabolism of glutamine-derived α-ketoglutarate in the TCA cycle serves … known as hexosamines, that are used to glycosylate growth factor receptors and …… of two wellknown metabolic pathways, the pentose phosphate pathway (PPP )
The Mitochondrial Warburg Effect: A Cancer Enigma – IBC Journal
http://www.ibc7.org/article/file_down.php?pid=48&mode=article This feature of cancer cells is known as the Warburg effect, named … new paradigm of collaboration and a well-designed systemic approach will supply … Krebs cycle. … The pentose phosphate pathway uses glucose to produce ribose, which is used … glucose is taken up into cells, it is used in two main metabolic pathways

 

New paper offers intriguing insights into tumor metabolism     William G. Gilroy    August 19, 2009

Posted In: Research

A paper appearing in this week’s edition of the journal Nature by a team of researchers that includes University of Notre Dame biologist Zachary T. Schafer has important new implications for understanding the metabolism of tumors.

Schafer, an assistant professor of biological sciences and Coleman Junior Chair of Cancer Biology, points out that in the early stages of tumor formation some cells become detached from their normal cellular matrix. These “homeless” cells tend to develop certain defects that stop them from becoming cancerous. In a process known as apoptosis, these precancerous cells essentially kill themselves, allowing them to be destroyed by immune system cells.

The prevailing wisdom among researchers has been that apoptosis was the only way that cells could die.

In studies conducted prior to the research described in the Nature paper, it was found that even when apoptosis was inhibited in detached, precancerous cells, they still eventually died. Intrigued by these results, a team of researchers led by Joan S. Brugge, Louise Foote Pfieffer Professor of Cell Biology at Harvard Medical School, and Schafer decided to take a closer look.

They report in this week’s Nature paper that they found that even when apoptosis was inhibited in detached cells endowed with a cancer-causing gene, they still were incapable of absorbing glucose, their primary energy source. Additionally, the cells displayed signs of oxidative stress, which is a harmful accumulation of oxygen-derived molecules called reactive oxygen species (ROS). The research also revealed decreased ATP production, a key factor in energy transport in the cells.

Schafer notes that this combination of loss of glucose transport, decreased ATP production and heightened oxidative stress reveal a manner of cell death that hadn’t been previously demonstrated to play a role in this context.

In the next phase of the study, Schafer engineered the cells to express a high level of HER2, a gene known to be hyperactive in many breast cancer tumors. He also treated the cells with antioxidants to relieve oxidative stress.

Both approaches helped the cells survive. The HER2-treated cells regained glucose transport, avoided oxidative stress and recovered ATP levels.
Most surprisingly, the antioxidants restored metabolic activity in the cells by allowing fatty acids to be effectively used instead of glucose as an energy source, providing them with a chance to survive.

“Our results raise the possibility that antioxidant activity might allow early stage tumor cells to survive where they would otherwise die from these metabolic defects,” Schafer said.

He also cautions that while the antioxidant findings were surprising, their research was done solely in cell cultures and more research needs to be done before there are clear implications for individuals and their diets.

The paper does, however, offer important new clues about the metabolism of tumor cells and important information that may lead to drugs that can developed to target them.

 

Antioxidant and Oncogene Rescue of Metabolic Defects Caused by
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2931797/Aug 19, 2009
Nature. Author manuscript; available in PMC 2010 Sep 2. Published in … Y. Irie,1 Sizhen Gao,1 Pere Puigserver,1,2 and Joan S. Brugge1,*.

http://www.nature.com/cdd/journal/v10/n8/full/4401251a.html
Proteasome inhibitors have been shown to be effective in cancer treatment, an ability … a specific inhibitor of 26S proteasome, also reduced cell viability ( 80% with 10 mu …
be a consequence of the increased generation of ROS caused by MG132. …. vectors endowed with the wild type forms of RB or p53 genes (Figure 1f).

 

The Metastasis-Promoting Roles of Tumor-Associated Immune Cells

Tumor metastasis is driven not only by the accumulation of intrinsic alterations in malignant cells, but also by the interactions of cancer cells with various stromal cell components of the tumor microenvironment. In particular, inflammation and infiltration of the tumor tissue by host immune cells, such as tumor-associated macrophages, myeloid-derived suppressor cells, and regulatory T cells have been shown to support tumor growth in addition to invasion and metastasis. Each step of tumor development, from initiation through metastatic spread, is promoted by communication between tumor and immune cells via the secretion of cytokines, growth factors and proteases that remodel the tumor microenvironment. Invasion and metastasis requires neovascularization, breakdown of the basement membrane, and remodeling of the extracellular matrix for tumor cell invasion and extravasation into the blood and lymphatic vessels. The subsequent dissemination of tumor cells to distant organ sites necessitates a treacherous journey through the vasculature, which is fostered by close association with platelets and macrophages. Additionally, the establishment of the pre-metastatic niche and specific metastasis organ tropism is fostered by neutrophils and bone marrow-derived hematopoietic immune progenitor cells and other inflammatory cytokines derived from tumor and immune cells, which alter the local environment of the tissue to promote adhesion of circulating tumor cells. This review focuses on the interactions between tumor cells and immune cells recruited to the tumor microenvironment, and examines the factors allowing these cells to promote each stage of metastasis.

 

Once established, tumors are quite adept at preventing anti-tumor immune responses, and several defense mechanisms to circumvent immune detection have been described including antigen loss, down-regulation of major histocompatibility molecules (MHC), deregulation or loss of components of the endogenous antigen presentation pathway, and tumor-induced immune suppression mediated through cytokine secretion or direct interactions between tumor ligands and immune cell receptors [2]. These mechanisms contribute to the process of immunoediting in which tumor cell subpopulations susceptible to immune recognition are lysed and eliminated, while resistant tumor cells proliferate and increase their frequency in the developing neoplasia [3]. However, tumors not only effectively escape immune recognition, they also actively subvert the normal anti-tumor activity of immune cells to promote further tumor growth and metastasis.

During early stages of cancer development, infiltrating immune cell populations are primarily tumor suppressive, but depending on the presence of accessory stromal cells, the local cytokine milieu, and tumor-specific interactions, these immune cells can undergo phenotypic changes to enhance tumor cell dissemination and metastasis. For instance, CD4+ T cells, macrophages, and neutrophils have all been shown to possess opposing properties depending on the inflammatory state of the tumor environment, the tissue context, and other cellular stimuli intrinsic to the altered tumor cells [4, 5]. These features are dependent upon the inherent plasticity of immune cells in response to stimulatory or suppressive cytokines [6]. Notably, the switch from a Th1 tumor-suppressive phenotype such as CD4+ “helper” T cells, which aid cytotoxic CD8+ T cells in tumor rejection, to a Th2 tumor-promoting “regulatory” phenotype, which blocks CD8+ T-cell activity, is a characteristic outcome in the inflammatory, immune-suppressive tumor microenvironment [5, 7]. Likewise, M1 macrophages and N1 neutrophils are known to have pronounced anti-tumor activity; however, these immune cells are often subverted to a tumor-promoting M2 and N2 phenotype, respectively, in response to immune-suppressive cytokines secreted by tumor tissue [8].

 

The crosstalk that occurs between tumor and immune cells within the tumor microenvironment, the circulation, or at distant metastatic sites has been clearly shown to foster metastatic dissemination. Immune cells as well as the suppressive factors that they secrete represent potential targets for therapeutic intervention. Regardless of their source, cytokines, chemokines, proteases, and growth factors are some of the main factors contributing to immunosuppression and immune-mediated tumor progression. These molecules can be produced by immune, stromal, or malignant cells and can act in paracrine and autocrine fashion to promote each stage of tumor cell invasion and metastasis by enhancing inflammation, angiogenesis, tumor proliferation, and recruitment of additional immunosuppressive and tumor-promoting immune cells. These secreted factors provide the malignant cells with an abundant source of growth and survival signals that perpetuate a supportive microenvironment for tumor metastasis and represent some of the most attractive targets for directed anti-tumor therapy. Immune pathways provide numerous soluble targets for cancer treatment, and indeed, many drugs to target immune-suppressive molecules are moving forward in clinical trials. For instance, the anti-RANKL (Denosumab) antibody has been shown to effectively inhibit bone metastasis in prostate cancer patients [201], while a variety of neutralizing antibodies to IL-1β and IL-1 receptor have been shown to have efficacy in treating metastasis in pre-clinical animal models [202]. Several agents that target IL-1 or other immune-suppressive cytokines are already approved for the treatment of some inflammatory diseases and are prime candidates for human trails [202]. Additionally, other proteins involved in tumor progression that are induced directly or indirectly by immune cell populations, such as EMT-associated transcription factors, adhesion molecules, and tumor receptors and ligands which mediate immune suppression, could also be targeted with small molecules or blocking antibodies. Antibodies against two surface molecules expressed by suppressive lymphoid cells, anti-CTLA-4 (ipiliumimab) [203, 204] and anti-PD-1 have been recently gaining increasing support from clinical trials for their effective treatment for many forms of cancer including advanced melanoma and prostate cancer [205, 206]. Specifically, anti-CTLA-4 has been shown to be particularly efficacious in metastatic melanoma, while anti-PD-1 has only just begun a comprehensive evaluation in clinical trials [204, 207]. Likewise, non-steroidal anti-inflammatory drugs (NSAIDS) to prevent or treat chronic inflammation and lymphangiogenesis [208210], and anti-coagulants to prevent platelet aggregation on circulating tumor cells [211] are just two examples of a multitude of therapeutic agents that could be utilized to prevent immune-mediated tumor progression at unique stages of metastasis. Of course, new methods or biomarkers for the detection of patients at risk of tumor progression or metastasis are also desperately needed to tailor personalized therapy for patients to obtain the best possible clinical outcome.

 

  1. https://pharmaceuticalintelligence.com/category/cancer-and-therapeutics/Mar 26, 2016 This turns your immune systems ability to attack and kill cancer cells back on” …. the rare skin cancer cells that were able to detach from theprimary tumor and successfully metastasize to other parts of the body had the ability to keep ROS levels from getting dangerously high,” Dr. DeBerardinis remarked.

  2. https://pharmaceuticalintelligence.com/tag/histone-deacetylase-inhibitors-hdac/The HDAC-inhibiting agent romidepsin significantly increased T-cell tumor … skin cancer cells that were able to detach from the primary tumor and successfully … of the body had the ability to keep ROS levels from getting dangerously high,” Dr. …. Sensitivity for EGFR or KRAS was higher in patients with multiplemetastatic …

  3. https://pharmaceuticalintelligence.com/category/cancer-biology-innovations-in-cancer-therapy/genomic-expression/In a study involving 320 patients, the researchers were able to infer cell death in …. Glutamine and cancer: cell biology, physiology, and clinical opportunities … On the other hand, GLS2 expression is enhanced in some neuroblastomas, …… of the body had the ability to keep ROS levels from getting dangerously high,” Dr.

 

Read Full Post »


Summary, Metabolic Pathways

Author: Larry H. Bernstein, MD, FCAP 

 

This portion of a series of chapters on metabolism, proteomics and metabolomics dealt mainly with carbohydrate metabolism. Amino acids and lipids are presented more fully in the chapters that follow. There are features on the

  • functioning of enzymes and proteins,
  • on sequential changes in a chain reaction, and
  • on conformational changes that we shall also cover.

These are critical to developing a more complete understanding of life processes.

I needed to lay out the scope of metabolic reactions and pathways, and their complementary changes. These may not appear to be adaptive, if the circumstances and the duration is not clear. The metabolic pathways map in total
is in interaction with environmental conditions – light, heat, external nutrients and minerals, and toxins – all of which give direction and strength to these reactions. A developing goal is to discover how views introduced by molecular biology and genomics don’t clarify functional cellular dynamics that are not related to the classical view.  The work is vast.

Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by photosynthesis storing the absorbed energy internally, often in the form of starch or lipids. Plant components are consumed by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy and from lipids about 9 kcal. Energy obtained from metabolism (e.g. oxidation of glucose) is usually stored temporarily within cells in the form of ATP. Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts.

Carbohydrates are used for short-term fuel, and even though they are simpler to metabolize than fats, they don’t produce as equivalent energy yield measured by ATP.  In animals, the concentration of glucose in the blood is linked to the pancreatic endocrine hormone, insulin. . In most organisms, excess carbohydrates are regularly catabolized to form acetyl-CoA, which is a feed stock for the fatty acid synthesis pathway; fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates.

Glucose is metabolized obtaining ATP and pyruvate by way of first splitting a six-carbon into two three carbon chains, which are converted to lactic acid from pyruvate in the lactic dehydrogenase reaction. The reverse conversion is by a separate unidirectional reaction back to pyruvate after moving through pyruvate dehydrogenase complex.

Pyruvate dehydrogenase complex (PDC) is a complex of three enzymes that convert pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular respiration, and this complex links the glycolysis metabolic pathway to the citric acid cycle. This multi-enzyme complex is related structurally and functionally to the oxoglutarate dehydrogenase and branched-chain oxo-acid dehydrogenase multi-enzyme complexes. In eukaryotic cells the reaction occurs inside the mitochondria, after transport of the substrate, pyruvate, from the cytosol. The transport of pyruvate into the mitochondria is via a transport protein and is active, consuming energy. On entry to the mitochondria pyruvate decarboxylation occurs, producing acetyl CoA. This irreversible reaction traps the acetyl CoA within the mitochondria. Pyruvate dehydrogenase deficiency from mutations in any of the enzymes or cofactors results in lactic acidosis.

PDH-rxns The acetyl group is transferred to coenzyme A

PDH-rxns The acetyl group is transferred to coenzyme A

http://guweb2.gonzaga.edu/faculty/cronk/biochem/images/PDH-rxns.gif

Typically, a breakdown of one molecule of glucose by aerobic respiration (i.e. involving both glycolysis and Kreb’s cycle) is about 33-35 ATP. This is categorized as:

Glycogenolysis – the breakdown of glycogen into glucose, which provides a glucose supply for glucose-dependent tissues.

Glycogenolysis in liver provides circulating glucose short term.

Glycogenolysis in muscle is obligatory for muscle contraction.

Pyruvate from glycolysis enters the Krebs cycle, also known as the citric acid cycle, in aerobic organisms.

Anaerobic breakdown by glycolysis – yielding 8-10 ATP

Aerobic respiration by Kreb’s cycle – yielding 25 ATP

The pentose phosphate pathway (shunt) converts hexoses into pentoses and regenerates NADPH. NADPH is an essential antioxidant in cells which prevents oxidative damage and acts as precursor for production of many biomolecules.

Glycogenesis – the conversion of excess glucose into glycogen as a cellular storage mechanism; achieving low osmotic pressure.

Gluconeogenesis – de novo synthesis of glucose molecules from simple organic compounds. An example in humans is the conversion of a few amino acids in cellular protein to glucose.

Metabolic use of glucose is highly important as an energy source for muscle cells and in the brain, and red blood cells.

The hormone insulin is the primary glucose regulatory signal in animals. It mainly promotes glucose uptake by the cells, and it causes the liver to store excess glucose as glycogen. Its absence

  1. turns off glucose uptake,
  2. reverses electrolyte adjustments,
  3. begins glycogen breakdown and glucose release into the circulation by some cells,
  4. begins lipid release from lipid storage cells, etc.

The level of circulatory glucose (known informally as “blood sugar”) is the most important signal to the insulin-producing cells.

  • insulin is made by beta cells in the pancreas,
  • fat is stored n adipose tissue cells, and
  • glycogen is both stored and released as needed by liver cells.
  • no glucose is released to the blood from internal glycogen stores from muscle cells.

The hormone glucagon, on the other hand, opposes that of insulin, forcing the conversion of glycogen in liver cells to glucose, and then release into the blood. Growth hormone, cortisol, and certain catecholamines (such as epinepherine) have glucoregulatory actions similar to glucagon.  These hormones are referred to as stress hormones because they are released under the influence of catabolic proinflammatory (stress) cytokines – interleukin-1 (IL1) and tumor necrosis factor α (TNFα).

Net Yield of GlycolysisThe preparatory phase consumes 2 ATP

The pay-off phase produces 4 ATP.

The gross yield of glycolysis is therefore

4 ATP – 2 ATP = 2 ATP

The pay-off phase also produces 2 molecules of NADH + H+ which can be further converted to a total of 5 molecules of ATP* by the electron transport chain (ETC) during oxidative phosphorylation.

Thus the net yield during glycolysis is 7 molecules of ATP*
This is calculated assuming one NADH molecule gives 2.5 molecules of ATP during oxidative phosphorylation.

Cellular respiration involves 3 stages for the breakdown of glucose – glycolysis, Kreb’s cycle and the electron transport system. Kreb’s cycle produces about 60-70% of ATP for release of energy in the body. It directly or indirectly connects with all the other individual pathways in the body.

The Kreb’s Cycle occurs in two stages:

  1. Conversion of Pyruvate to Acetyl CoA
  2. Acetyl CoA Enters the Kreb’s Cycle

Each pyruvate in the presence of pyruvate dehydrogenase (PDH) complex in the mitochondria gets converted to acetyl CoA which in turn enters the Kreb’s cycle. This reaction is called as oxidative  decarboxylation as the carboxyl group is removed from the pyruvate molecule in the form of CO2 thus yielding 2-carbon acetyl group which along with the coenzyme A forms acetyl CoA.

The PDH requires the sequential action of five co-factors or co-enzymes for the combined action of dehydrogenation and decarboxylation to take place. These five are TPP (thiamine phosphate), FAD (flavin adenine dinucleotide), NAD (nicotinamide adenine dinucleotide), coenzyme A (denoted as CoA-SH at times to depict role of -SH group) and lipoamide.

Acetyl CoA condenses with oxaloacetate (4C) to form a citrate (6C) by transferring its acetyl group in the presence of enzyme citrate synthase. The CoA liberated in this reaction is ready to participate in the oxidative decarboxylation of another molecule of pyruvate by PDH complex.

Isocitrate undergoes oxidative decarboxylation by the enzyme isocitrate dehydrogenase to form oxalosuccinate (intermediate- not shown) which in turn forms α-ketoglutarate (also known as oxoglutarate) which is a five carbon compound. CO2 and NADH are released in this step. α-ketoglutarate (5C) undergoes oxidative decarboxylation once again to form succinyl CoA (4C) catalysed by the enzyme α-ketoglutarate dehydrogenase complex.

Succinyl CoA is then converted to succinate by succinate thiokinase or succinyl coA synthetase in a reversible manner. This reaction involves an intermediate step in which the enzyme gets phosphorylated and then the phosphoryl group which has a high group transfer potential is transferred to GDP to form GTP.

Succinate then gets oxidised reversibly to fumarate by succinate dehydrogenase. The enzyme contains iron-sulfur clusters and covalently bound FAD which when undergoes electron exchange in the mitochondria causes the production of FADH2.

Fumarate is then by the enzyme fumarase converted to malate by hydration(addition of H2O) in a reversible manner.

Malate is then reversibly converted to oxaloacetate by malate dehydrogenase which is NAD linked and thus produces NADH.

The oxaloacetate produced is now ready to be utilized in the next cycle by the citrate synthase reaction and thus the equilibrium of the cycle shifts to the right.

The NADH formed in the cytosol can yield variable amounts of ATP depending on the shuttle system utilized to transport them into the mitochondrial matrix. This NADH, formed in the cytosol, is impermeable to the mitochondrial inner-membrane where oxidative phosphorylation takes place. Thus to carry this NADH to the mitochondrial matrix there are special shuttle systems in the body. The most active shuttle is the malate-aspartate shuttle via which 2.5 molecules of ATP are generated for 1 NADH molecule. This shuttle is mainly used by the heart, liver and kidneys. The brain and skeletal muscles use the other shuttle known as glycerol 3-phosphate shuttle which synthesizes 1.5 molecules of ATP for 1 NADH.

Glucose-6-phosphate Dehydrogenase is the committed step of the Pentose Phosphate Pathway. This enzyme is regulated by availability of the substrate NADP+. As NADPH is utilized in reductive synthetic pathways, the increasing concentration of NADP+ stimulates the Pentose Phosphate Pathway, to replenish NADPH. The importance of this pathway can easily be underestimated.  The main source for energy in respiration was considered to be tied to the high energy phosphate bond in phosphorylation and utilizes NADPH, converting it to NADP+. The pentose phosphate shunt is essential for the generation of nucleic acids, in regeneration of red cells and lens – requiring NADPH.

NAD+ serves as electron acceptor in catabolic pathways in which metabolites are oxidized. The resultant NADH is reoxidized by the respiratory chain, producing ATP.

The pyridine nucleotide transhydrogenase reaction concerns the energy-dependent reduction of TPN by DPNH. In 1959, Klingenberg and Slenczka made the important observation that incubation of isolated liver mitochondria with DPN-specific substrates or succinate in the absence of phosphate acceptor resulted in a rapid and almost complete reduction of  the intramitochondrial TPN. These and related findings led Klingenberg and co-workers (1-3) to postulate the occurrence of a ATP-controlled transhydrogenase reaction catalyzing the reduction of TPN by DPNH.  (The role of transhydrogenase in the energy-linked reduction of TPN.  Fritz Hommes, Ronald W. Estabrook, The Wenner-Gren Institute, University of Stockholm, Stockholm, Sweden. Biochemical and Biophysical Research Communications 11, (1), 2 Apr 1963, Pp 1–6. http://dx.doi.org:/10.1016/0006-291X(63)90017-2/).

Further studies observed the coupling of TPN-specific dehydrogenases with the transhydrogenase and observing the reduction of large amounts of diphosphopyridine nucleotide (DPN) in the presence of catalytic amounts of triphosphopyridine nucleotide (TPN). The studies showed the direct interaction between TPNHz and DPN, in the presence of transhydrogenase to yield products having the properties of TPN and DPNHZ. The reaction involves a transfer of electrons (or hydrogen) rather than a phosphate. (Pyridine Nucleotide Transhydrogenase  II. Direct Evidence for and Mechanism of the Transhydrogenase Reaction* by  Nathan 0. Kaplan, Sidney P. Colowick, And Elizabeth F. Neufeld. (From The Mccollum-Pratt Institute, The Johns Hopkins University, Baltimore, Maryland) J. Biol. Chem. 1952, 195:107-119.) http://www.JBC.org/Content/195/1/107.Citation
Notation: TPN, NADP; DPN, NAD+; reduced pyridine nucleotides: TPNH (NADPH2), DPNH (NADH).

Note: In this discussion there is a detailed presentation of the activity of lactic acid conversion in the mitochondria by way of PDH. In a later section there is mention of the bidirectional reaction of lactate dehydrogenase.  However, the forward reaction is dominant (pyruvate to lactate) and is described. This is not related to the kinetics of the LD reaction with respect to the defining characteristic – Km.

Biochemical Education Jan 1977; 5(1):15. Kinetics of Lactate Dehydrogenase: A Textbook Problem.
K.L. MANCHESTER. Department of Biochemistry, University of Witwatersrand, Johannesburg South Africa.

One presupposes that determined Km values are meaningful under intracellular conditions. In relation to teaching it is a simple experiment for students to determine for themselves the Km towards pyruvate of LDH in a post-mitochondrial supernatant of rat heart and thigh muscle. The difference in Km may be a factor of 3 or 4-fold.It is pertinent then to ask what is the range of suhstrate concentrations over which a difference in Km may be expected to lead to significant differences in activity and how these concentrations compare with pyruvate concentrations in the cell. The evidence of Vesell and co-workers that inhibition by pyruvate is more readily seen at low than at high enzyme concentration is important in emphasizing that under intracellular conditions enzyme concentrations may be relatively large in relation to the substrate available. This will be particularly so in relation to [NADH] which in the cytoplasm is likely to be in the ~M range.

A final point concerns the kinetic parameters for LDH quoted by Bergmeyer for lactate estimations a pH of 9 is recommended and the Km towards lactate at that pH is likely to be appreciably different from the quoted values at pH 7 — Though still at pH 9 showing a substantially lower value for lactate with the heart preparationhttp://onlinelibrary.wiley.com/doi/10.1016/0307-4412%2877%2990013-9/pdf

Several investigators have established that epidermis converts most of the glucose it uses to lactic acid even in the presence of oxygen. This is in contrast to most tissues where lactic acid production is used for energy production only when oxygen is not available. This large amount of lactic acid being continually produced within the epidermal cell must be excreted by the cell and then carried away by the blood stream to other tissues where the lactate can be utilized. The LDH reaction with pyruvate and NADH is reversible although at physiological pH the equilibrium position for the reaction lies very far to the right, i.e., in favor of lactate production. The speed of this reaction depends not only on the amount of enzyme present but also on the concentrations of the substances involved on both sides of the equation. The net direction in which the reaction will proceed depends solely on the relative concentrations of the substances on each side of the equation.
In vivo there is net conversion of pyruvate (formed from glucose) to lactate. Measurements of the speed of lactate production by sheets of epidermis floating on a medium containing glucose indicate a rate of lactate production of approximately 0.7 rn/sm/
mm/mg of fresh epidermis.Slice incubation experiments are presumably much closer to the actual in vivo conditions than
the homogenate experiments. The discrepancy between the
two indicates that in vivo conditions are far from optimal for the conversion of pyruvate to lactate. Only 1/100th of the maximal activity of the enzyme present is being achieved. The concentrations of the various substances involved are not
optimal in vivo since pyruvate and NADH concentrations are
lower than lactate and NAD concentrations and this might explain the in vivo inhibition of LDH activity. (Lactate Production And Lactate Dehydrogenase In The Human Epidermis*. KM. Halprin, A Ohkawara. J Invest Dermat 1966; 47(3): 222-6.)
http://www.nature.com/jid/journal/v47/n3/pdf/jid1966133a.pdf

Read Full Post »


Carbohydrate Metabolism

Author and Curator: Larry H. Bernstein, MD, FCAP

This is the portion of the discussion in a series of articles that began with signaling and signaling pathways. There are features on the functioning of enzymes and proteins, on sequential changes in a chain reaction, and on conformational changes that we shall return to.  These are critical to developing a more complete understanding of life processes.  I have indicated that many of the protein-protein interactions or protein-membrane interactions and associated regulatory features have been referred to previously, but the focus of the discussion or points made were different.  Even though I considered placing this after the discussion of proteins and how they play out their essential role, I needed to lay out the scope of metabolic reactions and pathways, and their complementary changes. These may not appear to be adaptive, if the circumstances and the duration is not clear. The metabolic pathways map in total is in interaction with environmental conditions – light, heat, external nutrients and minerals, and toxins – all of which give direction and strength to these reactions. I shall again take from Wikipedia, as needed, and also follow mechanisms and examples from the literature, which give insight into the developments in cell metabolism. A developing goal is to discover how views introduced by molecular biology and genomics don’t clarify functional cellular dynamics that are not related to the classical view.  The work is vast.

  1. Signaling and signaling pathways
  2. Signaling transduction tutorial.
  3. Carbohydrate metabolism
  4. Lipid metabolism
  5. Protein synthesis and degradation
  6. Subcellular structure
  7. Impairments in pathological states: endocrine disorders; stress hypermetabolism; cancer.

Carbohydrate metabolism

Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms.

The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by photosynthesis storing the absorbed energy internally, often in the form of starch or lipids. Plant components are consumed by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy and from lipids about 9 kcal. Energy obtained from metabolism (e.g. oxidation of glucose) is usually stored temporarily within cells in the form of ATP.[1] Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts.

Complex carbohydrates contain three or more sugar units linked in a chain, with most containing hundreds to thousands of sugar units. They are digested by enzymes to release the simple sugars.
I shall not go into the digestion, breakdown and absorption of these sugar molecules. Carbohydrates are used for short-term fuel, and the most important is glucose.  Even though they are simpler to metabolize than fats or those amino acids (components of proteins) that can be used for fuel, they do not produce as effect an energy yield measured by ATP.  In animals, The concentration of glucose in the blood is linked to the pancreatic endocrine hormone, insulin.

Carbohydrates are typically stored as long polymers of glucose molecules with glycosidic bonds for structural support (e.g. chitin, cellulose) or for energy storage (e.g. glycogen, starch). However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. In most organisms, excess carbohydrates are regularly catabolised to form acetyl-CoA, which is a feed stock for the fatty acid synthesis pathway; fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates. However, animals, including humans, lack the necessary enzymatic machinery and so do not synthesize glucose from lipids, though glycerol can be converted to glucose.[6]

Metabolic pathways in eukaryotes

  • Carbon fixation, or photosynthesis, in which CO2 is reduced to carbohydrate.  [omitted]
  • Glycolysis – the metabolism of glucose molecules to obtain ATP and pyruvate[7] by way of first splitting a six-carbon into two three csrbon chains, which are converted to lactic acid from pyruvate in the lactic dehydrogenase reaction. The reverse conversion is by a separate unidirectional reaction back to pyruvate after moving through pyruvate dehydrogenase complex.[8]
  • Krebs, tricarboxylic acic, or citric acid cycle
    • Typically, a breakdown of one molecule of glucose by aerobic respiration (i.e. involving both glycolysis and Kreb’s cycle) is about 33-35 ATP.[1] This is categorized as:
  • Glycogenolysis – the breakdown of glycogen into glucose, which provides a glucose supply for glucose-dependent tissues.
    • Glycogenolysis in liver provides circulating glucose short term.
    • Glycogenolysis in muscle is obligatory for muscle contraction.
    •     Anaerobic breakdown by glycolysis – yielding 8-10 ATP
    •     Aerobic respiration by kreb’s cycle – yielding 25 ATP
  • The pentose phosphate pathway (shunt) converts hexoses into pentoses and regenerates NADPH.[9] NADPH is an essential antioxidant in cells which prevents oxidative damage and acts as precursor for production of many biomolecules.
  • Glycogenesis – the conversion of excess glucose into glycogen as a cellular storage mechanism; achieving low osmotic pressure.
  • Gluconeogenesisde novo synthesis of glucose molecules from simple organic compounds. An example in humans is the conversion of a few amino acids in cellular protein to glucose.
    Metabolic use of glucose is highly important as an energy source for muscle cells and in the brain, and red blood cells.

Glucoregulation

The hormone insulin is the primary glucose regulatory signal in animals. It mainly promotes glucose uptake by the cells,  and causes liver to store excess glucose as glycogen. Its absence turns off glucose uptake, reverses electrolyte adjustments, begins glycogen breakdown and glucose release into the circulation by some cells, begins lipid release from lipid storage cells, etc. The level of circulatory glucose (known informally as “blood sugar”) is the most important signal to the insulin-producing cells. Because the level of circulatory glucose is largely determined by the intake of dietary carbohydrates, diet controls major aspects of metabolism via insulin. In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells.

The hormone glucagon, on the other hand, opposes that of insulin, forcing the conversion of glycogen in liver cells to glucose, and then release into the blood. Muscle cells, however, lack the ability to export glucose into the blood. The release of glucagon is precipitated by low levels of blood glucose. Other hormones, notably growth hormone, cortisol, and certain catecholamines (such as epinepherine) have glucoregulatory actions similar to glucagon.  These hormones are referred to as stress hormones because they are released under the influence of catabolic proinflammatory (stress) cytokines – interleukin-1 (IL1) and tumor necrosis factor α (TNFα).

metabolic pathways

metabolic pathways

Glycemic control in DM

Glycemic control in DM

  1. Catabolic proinflammatory cytokines. Argilés JM1López-Soriano FJ. Curr Opin Clin Nutr Metab Care.1998 May;1(3):245-51.
  2. Tumor necrosis factor as a mediator of shock, cachexia and inflammation. Cerami A. Blood Purif. 1993; 11(2):108-17.
  3. Mediators of cytokine-induced insulin resistance in obesity and other inflammatory settings. Marette A. Curr Opin Clin Nutr Metab Care. 2002 Jul; 5(4):377-83.
  4. Inflammation: the link between insulin resistance, obesity and diabetes. Dandona P, Aljada A, Bandyopadhyay A. Trends Immunol. 2004 Jan; 25(1):4-7
  5. Proinflammatory cytokines and skeletal muscle. Späte U1, Schulze PC. Curr Opin Clin Nutr Metab Care. 2004 May;7(3):265-9.
  6. Insulin-like growth factor-1 and muscle wasting in chronic heart failure. Schulze PC, Späte U. Int J Biochem Cell Biol. 2005 Oct; 37(10):2023-35.
  7. IGF-I stimulates muscle growth by suppressing protein breakdown and expression of atrophy-related ubiquitin ligases, atrogin-1 and MuRF1. Sacheck JM, Ohtsuka A, McLary SC, Goldberg AL. Am J Physiol Endocrinol Metab. 2004 Oct; 287(4):E591-601. Epub 2004 Apr 20.

Glycolysis – Animation and Notes

By Sweety Mehta – Sept 20, 2011  in: Animations, Biochemistry Animations, Biochemistry Notes

http://pharmaxchange.info/press/2011/09/glycolysis-animation-and-notes/

Cellular respiration involves breaking the bonds of glucose to produce energy in the form of ATP (adenosine triphosphate). The total energy produced during glucose metabolism is described at Energetics of Cellular Respiration. Glycolysis is the most critical phase in glucose metabolism during cellular respiration. The term “glycolysis” literally means breakdown of glucose and sugars. Biochemically, it involves the breakdown of glucose to pyruvate (or pyruvic acid) via a series of enzymes. Glycolysis does not require molecular oxygen and is hence considered anaerobic. Therefore, it is a common pathway for all living organisms.

Glycolysis is followed by

Kreb’s cycle in the stages of cellular respiration.

Glycolysis is said to occur in two phases:

  1. The Preparatory Phase: From glucose till formation of Glyceraldehyde 3-Phosphate (GADP)
  2. The Pay-off Phase: From Glyceraldehyde-3-Phosphate (GADP) to the final product pyruvate

.

.

glycolysis

The animation below gives an outline of the entire pathway of glucose metabolism by glycolysis

Note – The animation is best played in full screen. To go forward in the animation, press the Play button. To skip the whole section press the forward button. To go back press the rewind button.

The Preparatory Phase

In this stage of the cycle, ATP or energy is actually consumed and is hence also known as the investment phase of glycolysis.

Step 1, involves the conversion of glucose to glucose-6-phosphate (G6P) with the help of the enzyme hexokinase and the consumption of 1 molecule of ATP. This reaction helps keep the concentration of glucose low in the cell, allowing for more absorption of glucose into it. Additionally, G6P is not transported out of the cell as there are no G6P transporters on the cell.

Step 2 involves the rearrangement of glucose-6-phosphate to fructose-6-phosphate (F6P) with the help of the enzyme phosphohexose isomerase in a reversible manner. Fructose can directly enter the glycolysis pathway at this point. This isomerization to a keto-sugar such as fructose is essential for carbanion stabilization required for the next step.

Step 3 involves the phosphorylation of fructose-6-phosphate to fructose-1,6-biphosphate (F1,6BP) by the use of 1 molecule of ATP and the enzyme phosphofructokinase-1 (PPK1). This phosphorylation step destabilizes the molecule and helps drive the next reaction which ensures breakdown of the molecule to a 3-carbon unit.

Step 4 involves the breakdown of fructose-1,6-biphosphate (6 carbons) to two molecules of 3-carbon units i.e. glyceralde 3-phosphate (GADP) and Dihydroxyacetone phosphate (DHAP). The GADP can be interconverted to DHAP by enzyme triose phosphate isomerase.

The Pay-Off Phase

In this stage of the cycle, ATP or energy is produced either in the form of ATP alone or in the form of NADH + H+ which can be later converted to ATP via the electron transport chain (ETS). In this since energy is restored it is known as the pay-off phase of glycolysis. All steps in this phase occur with 2 molecules of the substrates each as indicated in the brackets by the name of the molecules.

Step 1, involves the dehydrogenation of glyceraldehyde-3-phosphate (GADP) to 1,3-biphophoglycerate (1,3BPG) by the use of 2 molecules of inorganic phosphate (Pi) with the production of 2 molecules of NADH + H+ in the presence of the enzyme glyceraldehyde 3-phosphate dehydrogenase.

Step 2, in this step dephosphorylation of 1,3-biphosphoglycerate (1,3BPG) to 3-phospoglycerate (3PG) produces 2 molecules of ATP by the enzyme phosphoglycerate kinase.

Step 3, involves the isomerisation of 3-phosphoglycerate (3PG) to 2-phosphoglycerate (2PG) by the enzyme phosphoglycerate mutase in a reversible manner.

Step 4 involves the enolization of 2-phosphoglycerate (2PG) to phosphoenolpyruvate (PEP) with the loss of one molecule of water in the presence of enzyme enolase.

Step 5 is the final step of the glycolysis pathway and it involves the dephosphorylation of the phosphoenolpyruvate (PEP) to pyruvate by enzyme pyruvate kinase to produce 2 more molecules of ATP.

Net Yield of Glycolysis

  1. The preparatory phase consumes 2 ATP
  2. The pay-off phase produces 4 ATP.
  3. The gross yield of glycolysis is therefore
    4 ATP – 2 ATP = 2 ATP
  4. The pay-off phase also produces 2 molecules of NADH + H+ which can be further converted to a total of 5 molecules of ATP* by the electron transport chain (ETC) during oxidative phosphorylation.
  5. Thus the net yield during glycolysis is 7 molecules of ATP.

* This is calculated assuming one NADH molecule gives 2.5 molecules of ATP during oxidative phosphorylation.

References

  1. David L. Nelson and Michael M. Cox, Lehninger Principles of Biochemistry, 4th Ed.
  2. Jeremy M. Berg, John L. Tymockzo and Luber Stryer, Biochemistry, 7th Ed.

Tags: cellular respiration, electron transport chain, etc, glucose, glycolysis, metabolism, pay-off phase

Kreb’s Cycle or Citric Acid Cycle or Tricarboxylic Acid Cycle (with Animation)

By Sweety Mehta  – Sept 21, 2013  in: Animations, Biochemistry Animations, Biochemistry Notes
http://pharmaxchange.info/press/2013/09/krebs-cycle-citric-acid-cycle-tricarboxylic-acid-cycle-animation/

Introduction

Cellular respiration involves 3 stages for the breakdown of glucose – glycolysis, Kreb’s cycle and the electron transport system. The total energy produced during glucose metabolism is described at Energetics of Cellular Respiration. We have seen the glycolysis pathway with animation previously. The Kreb’s cycle is named after Adolf Krebs who studied the utilization of oxygen in a pigeon. It is also commonly known as the citric acid cycle or the tricarboxylic acid cycle. Kreb’s cycle is a very important step in the metabolic pathway as it produces about 60-70% of ATP for release of energy in the body. It directly or indirectly connects with all the other individual pathways in the body too. It takes place in the mitochondria as all the enzymes and co-enzymes required are present there.

The Kreb’s Cycle occurs in two stages:

1. Conversion of Pyruvate to Acetyl CoA

Glycolysis of 1 molecule of glucose produces 2 molecules of pyruvate. Each pyruvate in the presence of pyruvate dehydrogenase (PDH) complex in the mitochondria gets converted to acetyl CoA which in turn enters the Kreb’s cycle. This reaction is called as oxidative  decarboxylation as the carboxyl group is removed from the pyruvate molecule in the form of CO2 thus yielding 2-carbon acetyl group which along with the coenzyme A forms acetyl CoA.

The pyruvate dehydrogenase complex (PDH) comprises of three enzymes – pyruvate dehydrogenase, dihydrolipoyl transacetylase and dihydrolipoyl dehydrogenase each one playing an important role in the reaction as shown below. The PDH requires the sequential action of five co-factors or co-enzymes for the combined action of dehydrogenation and decarboxylation to take place. These five are TPP (thiamine phosphate), FAD (flavin adenine dinucleotide), NAD (nicotinamide adenine dinucleotide), coenzyme A (denoted as CoA-SH at times to depict role of -SH group) and lipoamide.

Conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex

pyruvate_dehydrogenase_complex_new2

Conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex

Pyruvate reacts with the TPP (Thiamine Phosphate) bound part of pyruvate dehydrogenase and undergoes decarboxylation to give hydroxyethyl-TPP.

This hydroxyethyl-TPP in turn gets oxidised to acetyl lipoamide by the same enzyme pyruvate dehydrogenase by the transfer of two electrons. These electrons then reduce the disulfide bond of the enzyme dihydrolipoyl transacetylase with the transfer of the acetyl group as highlighted in purple.

Dihydrolipoyl transacetylase catalyses the transesterification forming acetyl CoA by transfer of acetyl group to coenzyme A.

When acetyl CoA is being formed, at the same time reduced lipoamide is getting converted to oxidised lipoamide due to enzyme dihydrolipoyl dehydrogenase by the transfer of 2 hydrogen atoms to FAD.

Dihydrolipoyl dehydrogenase transfers the reduced equivalents (2 hydrogen atoms) to FAD thus forming FADH2. FADH2 in turn transfers a hydride ion to NAD+ to form NADH+H+.

2. Acetyl CoA Enters the Kreb’s Cycle

The acetyl CoA produced from the pyruvate dehydrogenase complex enters the Kreb’s cycle.

The animation below describes the Kreb’s cycle in detail followed by the discussion. A static image of the cycle can be found next to the discussion for reference. Press the play button to progress in the animation.

Discussion

Krebs1 cycle

The Kreb’s Cycle or Citric Acid Cycle or Tricarboxylic Acid Cycle in a static image version of the animation.

Acetyl CoA condenses with oxaloacetate (4C) to form a citrate (6C) by transferring its acetyl group in the presence of enzyme citrate synthase. The CoA liberated in this reaction is ready to participate in the oxidative decarboxylation of another molecule of pyruvate by PDH complex.

  • Citrate is then isomerised to Isocitrate by the enzyme aconitase through the formation of the intermediate cis-aconitate. This is a reversible reaction as aconitase has an iron-sulfur center which can promote reversible addition of H2O to the double bond of enzyme-bound cis-aconitate in 2 different ways, one forming citrate and the other isocitrate.
  • Isocitrate undergoes oxidative decarboxylation by the enzyme isocitrate dehydrogenase to form oxalosuccinate (intermediate- not shown) which in turn forms α-ketoglutarate (also known as oxoglutarate) which is a five carbon compound. CO2 and NADH are released in this step.
  • α-ketoglutarate (5C) undergoes oxidative decarboxylation once again to form succinyl CoA (4C) catalysed by the enzyme α-ketoglutarate dehydrogenase complex. α-ketoglutarate dehydrogenase complex is similar to PDH complex and is made up of 3 enzymes and is dependent on five co-enzymes TPP, FAD, NAD, bound lipoate and conenzyme A. In this step once again NADH and CO2 are liberated. So in all 2 molecules of NADH and 2 molecules of CO2 is produced till now.
  • Succinyl CoA is then converted to succinate by succinate thiokinase or succinyl coA synthetase in a reversible manner. This reaction involves an intermediate step in which the enzyme gets phosphorylated and then the phosphoryl group which has a high group transfer potential is transferred to GDP to form GTP. This GTP is converted to ATP by the enzyme nucleoside diphosphate kinase by donating its phosphoryl group to ADP. This reaction which involves the formation of GTP is a substrate level phosphorylation as it happens by using the energy formed by the oxidative decarboxylation of α-ketoglutarate.
  • Succinate then gets oxidised reversibly to fumarate by succinate dehydrogenase. The enzyme contains iron-sulfur clusters and covalently bound FAD which when undergoes electron exchange in the mitochondria causes the production of FADH2.
  • Fumarate is then by the enzyme fumarase converted to malate by hydration(addition of H2O) in a reversible manner.
  • Malate is then reversibly converted to oxaloacetate by malate dehydrogenase which is NAD linked and thus produces NADH.
  • The oxaloacetate produced is now ready to be utilized in the next cycle by the citrate synthase reaction and thus the equilibrium of the cycle shifts to the right.
Schrodingers_cat

Schrodingers_cat

Energetics of the Kreb’s Cycle

Keeping in mind that 1 molecule of glucose would produce 2 molecules of pyruvate via glycolysis. Hence the net energy produced by the Kreb’s cycle for each molecule of pyruvate is doubled for each molecule of glucose. Thus net energy yield in Kreb’s cycle can be summarized as follows for each molecule of glucose:

Reaction                                                              Number of ATP or                                        Number of ATP
reduced coenzyme formed                        ultimately formed

2 Pyruvate → 2 acetyl CoA                                  2 NADH                                                             5

2 Isocitrate → 2 α- ketoglutarate                     2 NADH                                                              5

2 α- ketoglutarate → 2 succinyl CoA             2 NADH                                                               5

2 Succinyl CoA → 2 succinate                           2 ATP                                                                  2

2 Succinate → 2 fumarate                               2 FADH2                                                               3

2 Malate → 2 oxaloacetate                            2 NADH                                                                  5

TOTAL                                                                                                                                                 25 ATP

* Note- This is calculated as 2.5 ATP per NADH and 1.5 ATP per FADH2. This is because there are multiple electron transport shuttle pathways through which these can be broken to ATP.

Regulation of Kreb’s Cycle

The amount of ADP and ATP largely control the citric acid cycle along with the activity of three key enzymes within the cycle:

Availability of ADP: ADP is a key substrate which finally gets converted to ATP that is essential for the energetics of the cell. A drop in ADP levels would result in inhibition of the electron transport system leading to accumulation of NADH and FADH2. These in turn inhibit the enzymes below.

Citrate Synthase: inhibited by ATP, acetyl CoA, NADH, and succinyl CoA.
Isocitrate Dehydrogenase: activated by ADP, and inhibited by NADH and ATP.
α-ketoglutarate dehydrogenase: inhibited by NADH and succinyl CoA.

Recommended Texts

David L. Nelson and Michael M. Cox, Lehninger Principles of Biochemistry 6th Edition
Jeremy M. Berg, John L. Tymockzo and Luber Stryer, Biochemistry 7th Edition

Tags: acetyl coA, animation, cellular respiration, citric acid cycle, energy, kreb’s cycle, pyruvate, pyruvate dehydrogenase, TCA cycle, tricarboxylic acid cycle

Energetics of Cellular Respiration (Glucose Metabolism)

By Sweety Mehta   – Oct 9, 2013 in: Biochemistry Notes, Notes
http://pharmaxchange.info/press/2013/10/energetics-of-cellular-respiration-glucose-metabolism/

energetics-of-cellular-respiration

electron transport chain in the mitochondrion energetics-of-cellular-respiration

Important Note: The NADH formed in the cytosol can yield variable amounts of ATP depending on the shuttle system utilized to transport them into the mitochondrial matrix. This NADH, formed in the cytosol, is impermeable to the mitochondrial inner-membrane where oxidative phosphorylation takes place. Thus to carry this NADH to the mitochondrial matrix there are special shuttle systems in the body. The most active shuttle is the malate-aspartate shuttle via which 2.5 molecules of ATP are generated for 1 NADH molecule. This shuttle is mainly used by the heart, liver and kidneys. The brain and skeletal muscles use the other shuttle known as glycerol 3-phosphate shuttle which synthesizes 1.5 molecules of ATP for 1 NADH.

Note: The above calculations are done considering that one NADH molecules produces 2.5 ATP and one FADH2 molecule produces 1.5 ATP in the ETS cycle (See full reasoning above). This is because the Kreb’s cycle occurs within the mitochondria and therefore does not require any shuttle pathway for the transport of the NADH into the mitochondrial matrix. Hence there is optimal conversion of NADH to ATP.

Development of the acetylation problem: a personal account

FRITZ LI P M A N N  Nobel Prize  1953

After my  apprenticeship with Otto Meyerhof, a first interest on my own became the phenomenon we call the Pasteur effect, this peculiar depression of the wasteful fermentation in the respiring cell. By looking for a chemical explanation of this economy measure on the cellular level, I was prompted into a study of the mechanism of pyruvic acid oxidation, since it is at the pyruvic stage where respiration branches off from fermentation. For this study I chose as a  promising system a relatively simple looking pyruvic acid oxidation enzyme in a certain strain of Lactobacillus delbrueckii1.

The most important event during this whole period, I now feel, was the accidental observation that in the L. delbrueckii system, pyruvic acid oxidation was completely dependent on the presence of inorganic phosphate. This observation was made in the course of attempts to replace oxygen by methylene blue. To measure the methylene blue reduction manometrically,
I had to switch to a bicarbonate buffer instead of the otherwise routinely used In bicarbonate, to my surprise, as shown in Fig. 1, pyruvate oxidation was very slow, but the addition of a little phosphate caused a remarkable increase in rate. The next figure, Fig. 2, shows the phosphate effect more drastically, using a preparation from which all phosphate was removed by washing with acetate buffer. Then it appeared that the reaction was really fully dependent on phosphate.

In spite of such a phosphate dependence, the phosphate balance measured by the ordinary Fiske-Subbarow procedure did not at first indicate any phosphorylative step. Nevertheless, the suspicion remained that phosphate in  some manner was entering into the reaction and that a phosphorylated intermediary was formed. As a first approximation, a coupling of this pyruvate oxidation with adenylic acid phosphorylation was attempted. And,indeed, addition of adenylic acid to the pyruvic oxidation system brought out a net  disappearance of inorganic phosphate, accounted for as adenosine triphosphate (Table 11).

I  now concluded that the missing link in the reaction chain was acetyl phosphate. In partial confirmation it was shown that a crude preparation of acetyl phosphate, synthesized by the old method of Kämmerer and Carius 2 would transfer phosphate to adenylic acid (Table 2). However, it still took quite some time from then on to identify acetyl phosphate definitely as the initial product of the pyruvic oxidation in this system3,4

At the time when these observations were made, about a dozen years ago,there was, to say the least, a tendency to believe that phosphorylation was rather specifically coupled with the glycolytic reaction. Here, however, we had found a coupling of phosphorylation with a respiratory system. This observation immediately suggested a rather sweeping biochemical significance, of transformations of electron transfer potential, respiratory or fermentative, to phosphate bond energy and therefrom to a wide range of biosynthetic reactions7.

There was a further unusual feature in this pyruvate oxidation system in that the product emerging from the process not only carried an energy-rich phosphoryl radical such as already known, but the acetyl phosphate was even more impressive through its energy-rich acetyl. It rather naturally became a contender for the role of “active” acetate, for the widespread existence of which the isotope experience had already furnished extensive evidence. I became, therefore, quite attracted by the possibility that acetyl phosphate could serve two rather different purposes, either to transfer its phosphoryl group into the phosphate pool, or to supply its active acetyl for biosynthesisof carbon structures. Thus acetyl phosphate should be able to serve as acetyldonor as well as phosphoryl donor, transferring, as shown in Fig. 3, on either side of the oxygen center, such as indicated by Bentley’s early experiments on cleavage7a of acetyl phosphate in H218O.

Phosphate dependence of pyruvate oxidation

Phosphate dependence of pyruvate oxidation

These two novel aspects of the energy problem, namely

(1) the emergence of an energy-rich phosphate bond from a purely
respiratory reaction; and

(2) the presumed derivation of a metabolic building-block through this same
reaction, prompted me to propose not only

  • the generalization of the phosphate bond as a versatie energy distributing system, but also to
    aim from there towards
  • a general concept of transfer of activated groupings by carrier as the fundamental reaction in
    biosynthesis8,9.

Although in the related manner the appearance of acetyl phosphate as a
metabolic intermediary first

  •  focused attention to possible mechanisms for the metabolic elaboration of  group activation,

it soon turned out that the relationship between acetyl phosphate and
acetyl transfer was much more complicated than anticipated.

Acetyl phosphate as acetyl and phosphoryl donor.

Although acetylation was found with rabbit liver homogenate, the
reaction was rather weak. In search of a more active system,   Pigeon
liver homogenate was tried and found to harbour an exceedingly potent
acetylation system (Ref. 11, cf. also Ref. 12). This finding of a particularly
active acetylation reaction in cell-free pigeon liver preparations was most
fortunate and played a quite important part in the development of the
acetylation problem.

[portion of lecture]

The pentose phosphate pathway is the major source for the NADPH
required for anabolic processes.
Pentose Phosphate Pathway

http://chemwiki.ucdavis.edu/Biological_Chemistry/Metabolism/Pentose_
Phosphate_Pathway

  • There are three distinct phases each of which has a distinct outcome.
  • Depending on the needs of the organism the metabolites of that outcome
    can be fed into many other pathways.
  • Gluconeogenesis is directly connected to the pentose phosphate pathway.
  • As the need for glucose-6-phosphate (the beginning metabolite in the pentose
    phosphate pathway) increases so does the activity of gluconeogenesis.

 pentose-phosphate-pathway

http://images.tutorvista.com/content/respiration/pentose-phosphate-pathway.jpeg

Introduction

The main molecule in the body that makes anabolic processes possible is NADPH.  Because of the structure of this molecule it readily donates hydrogen ions to metabolites thus reducing them and making them available for energy harvest at a later time. The PPP is the main source of synthesis for NADPH.  The pentose phosphate pathway (PPP) is also responsible for the production of Ribose-5-phosphate which is an important part of nucleic acids. Finally the PPP can also be used to produce glyceraldehyde-3-phosphate which can then be fed into the TCA and ETC cycles allowing for the harvest of energy. Depending on the needs of the cell certain enzymes can be regulated and thus increasing or decreasing the production of desired metabolites. The enzymes reasonable for catalyzing the steps of the PPP are found most abundantly in the liver (the major site of gluconeogenesis) more specifically in the cytosol. The cytosol is where fatty acid synthesis takes place which is a NADPH dependent process.

 

Oxidation Phase

  • The beginning molecule for the PPP is glucose-6-P which is the second intermediate metabolite in glycolysis. Glucose-6-P is oxidized in the presence of glucose-6-P dehydrogenase and NADP+.  This step is irreversible and is highly regulated.  NADPH and fatty acyl-CoA are strong negative inhibitors to this enzyme.  The purpose of this is to decrease production of NADPH when concentrations are high or the synthesis of fatty acids is no longer necessary.
  • The metabolic product of this step is gluconolactone which is hydrolytrically unstable.  Gluconolactonase causes gluconolactone to undergo a ring opening hydrolysis.  The product of this reaction is the more stable sugar acid, 6-phospho-D-gluconate.
  • 6-phospho-D-gluconate is oxidized by NADP+ in the presence of 6-phosphogluconate dehydrogenase which yields ribulose-5-phosphate.
  • The oxidation phase of the PPP is solely responsible for the production of the NADPH to be used in anabolic processes.

Isomerization Phase

  •  Ribulose-5-phosphate can then be isomerized by phosphopentose isomerase to produce ribose-5-phosphate.  Ribose-5-phosphate is one of the main building blocks of nucleic acids and the PPP is the primary source of production of ribose-5-phosphate.
  • If production of ribose-5-phosphate exceeds the needs of required ribose-5-phosphate in the organism, then phosphopentose epimerase catalyzes a chiralty rearrangement about the center carbon creating xylulose-5-phosphate.
  • The products of these two reactions can then be rearranged to produce many different length carbon chains.  These different length carbon chains have a variety of metabolic fates.

Rearrangement Phase 

  •  There are two main classes of enzymes responsible for the rearrangement and synthesis of the different length carbon chain molecules.  These are transketolase and transaldolase.
  • Transketolase is responsible for the cleaving of a two carbon unit from xylulose-5-P and adding that two carbon unit to ribose-5-P thus resulting in glyceraldehyde-3-P and sedoheptulose-7-P.
  • Transketolase is also responsible for the cleaving of a two carbon unit from xylulose-5-P and adding that two carbon unit to erythrose-4-P resulting in glyceraldehyde-3-P and fructose-6-P.
  • Transaldolase is responsible for cleaving the three carbon unit from sedoheptulose-7-P and adding that three carbon unit to glyceraldehyde-3-P thus resulting in erythrose-4-P and fructose-6-P.
  • The end results of the rearrangement phase is a variety of different length sugars which can be fed into many other metabolic processes.  For example, fructose-6-P is a key intermediate of glycolysis as well as glyceraldehyde-3-P.

References

  1. Garrett, H., Reginald and Charles Grisham. Biochemistry. Boston: Twayne Publishers, 2008.
  2. Raven, Peter. Biology. Boston: Twayne Publishers, 2005.

Glycogen Metabolism

Glycogen is a readily mobilized storage form of glucose. It is a very large, branched polymer of glucose residues (Figure 21.1) that can be broken down to yield glucose molecules when energy is needed. Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds. Branches at about every tenth residue are created by α-1,6-glycosidic bonds. Recall that α-glycosidic linkages form open helical polymers, whereas β linkages produce nearly straight strands that form structural fibrils, as in cellulose (Section 11.2.3).

http://www.ncbi.nlm.nih.gov/books/NBK21190/bin/ch21f1.jpg

Figure 21.1

Glycogen Structure ch21f1

Glycogen Structure. In this structure of two outer branches of a glycogen molecule, the residues at the nonreducing ends are shown in red and residue that starts a branch is shown in green. The rest of the glycogen molecule is represented by R.

Glycogen is not as reduced as fatty acids are and consequently not as energy rich. Why do animals store any energy as glycogen? Why not convert all excess fuel into fatty acids? Glycogen is an important fuel reserve for several reasons. The controlled breakdown of glycogen and release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels. Glycogen’s role in maintaining blood-glucose levels is especially important because glucose is virtually the only fuel used by the brain, except during prolonged starvation. Moreover, the glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity.

Gluconeogenesis
ChemWiki: The Dynamic Chemistry E-textbook > Biological Chemistry > Metabolism > Gluconeogenesis

Gluconeogenesis is much like glycolysis only the process occurs in reverse. However, there are exceptions. In glycolysis there are three highly exergonic steps (steps 1,3,10). These are also regulatory steps which include the enzymes hexokinase, phosphofructokinase, and pyruvate kinase. Biological reactions can occur in both the forward and reverse direction. If the reaction occurs in the reverse direction the energy normally released in that reaction is now required. If gluconeogenesis were to simply occur in reverse the reaction would require too much energy to be profitable to that particular organism. In order to overcome this problem, nature has evolved three other enzymes to replace the glycolysis enzymes hexokinase, phosphofructokinase, and pyruvate kinase when going through the process of gluconeogenesis:

  1. The first step in gluconeogenesis is the conversion of pyruvate to phosphoenolpyruvic acid (PEP). In order to convert pyruvate to PEP there are several steps and several enzymes required. Pyruvate carboxylase, PEP carboxykinase and malate dehydrogenase are the three enzymes responsible for this conversion. Pyruvate carboxylase is found on the mitochondria and converts pyruvate into oxaloacetate. Because oxaloacetate cannot pass through the mitochondria membranes it must be first converted into malate by malate dehydrogenase. Malate can then cross the mitochondria membrane into the cytoplasm where it is then converted back into oxaloacetate with another malate dehydrogenase. Lastly, oxaloacetate is converted into PEP via PEP carboxykinase. The next several steps are exactly the same as glycolysis only the process is in reverse.
  2. The second step that differs from glycolysis is the conversion of fructose-1,6-bP to fructose-6-P with the use of the enzyme fructose-1,6-phosphatase. The conversion of fructose-6-P to glucose-6-P uses the same enzyme as glycolysis, phosphoglucoisomerase.
  3. The last step that differs from glycolysis is the conversion of glucose-6-P to glucose with the enzyme glucose-6-phosphatase. This enzyme is located in the endoplasmic reticulum.

Glycolysis

File:Glycolysis overview.svg

Regulation

Because it is important for organisms to conserve energy, they have derived ways to regulate those metabolic pathways that require and release the most energy. In glycolysis and gluconeogenesis seven of the ten steps occur at or near equilibrium. In gluconeogenesis the conversion of pyruvate to PEP, the conversion of fructose-1,6-bP, and the conversion of glucose-6-P to glucose all occur very spontaneously which is why these processes are highly regulated. It is important for the organism to conserve as much energy as possible. When there is an excess of energy available, gluconeogenesis is inhibited. When energy is required, gluconeogenesis is activated.

  1. The conversion of pyruvate to PEP is regulated by acetyl-CoA. More specifically pyruvate carboxylase is activated by acetyl-CoA. Because acetyl-CoA is an important metabolite in the TCA cycle which produces a lot of energy, when concentrations of acetyl-CoA are high organisms use pyruvate carboxylase to channel pyruvate away from the TCA cycle. If the organism does not need more energy, then it is best to divert those metabolites towards storage or other necessary processes.
  2. The conversion of fructose-1,6-bP to fructose-6-P with the use of fructose-1,6-phosphatase is negatively regulated and inhibited by the molecules AMP and fructose-2,6-bP. These are reciprocal regulators to glycolysis’ phosphofructokinase. Phosphofructosekinase is positively regulated by AMP and fructose-2,6-bP. Once again, when the energy levels produced are higher than needed, i.e. a large ATP to AMP ratio, the organism increases gluconeogenesis and decreases glycolysis. The opposite also applies when energy levels are lower than needed, i.e. a low ATP to AMP ratio, the organism increases glycolysis and decreases gluconeogenesis.
  3. The conversion of glucose-6-P to glucose with use of glucose-6-phosphatase is controlled by substrate level regulation. The metabolite responsible for this type of regulation is glucose-6-P. As levels of glucose-6-P increase, glucose-6-phosphatase increases activity and more glucose is produced. Thus glycolysis is unable to proceed.

 

Read Full Post »