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Glycolysis

At lower exercise intensities it can sustain muscle activity in diving animals , such as seals, whales and other aquatic vertebrates, for very much longer periods of time. But the speed at which ATP is produced in this manner is about times that of oxidative phosphorylation. The pH in the cytoplasm quickly drops when hydrogen ions accumulate in the muscle, eventually inhibiting the enzymes involved in glycolysis.

Step 1: Hexokinase

The burning sensation in muscles during hard exercise can be attributed to the release of hydrogen ions during the shift to glucose fermentation from glucose oxidation to carbon dioxide and water, when aerobic metabolism can no longer keep pace with the energy demands of the muscles. These hydrogen ions form a part of lactic acid. The body falls back on this less efficient but faster method of producing ATP under low oxygen conditions.

The liver in mammals gets rid of this excess lactate by transforming it back into pyruvate under aerobic conditions; see Cori cycle. Fermentation of pyruvate to lactate is sometimes also called "anaerobic glycolysis", however, glycolysis ends with the production of pyruvate regardless of the presence or absence of oxygen. In the above two examples of fermentation, NADH is oxidized by transferring two electrons to pyruvate. However, anaerobic bacteria use a wide variety of compounds as the terminal electron acceptors in cellular respiration : nitrogenous compounds, such as nitrates and nitrites; sulfur compounds, such as sulfates, sulfites, sulfur dioxide, and elemental sulfur; carbon dioxide; iron compounds; manganese compounds; cobalt compounds; and uranium compounds.

In aerobic organisms , a complex mechanism has been developed to use the oxygen in air as the final electron acceptor. The pyruvate produced by glycolysis is an important intermediary in the conversion of carbohydrates into fatty acids and cholesterol.

Passing the Baton: Substrate Channelling in Respiratory Metabolism

However, this acetyl CoA needs to be transported into cytosol where the synthesis of fatty acids and cholesterol occurs. This cannot occur directly.

Glycolysis - Biochemistry

To obtain cytosolic acetyl-CoA, citrate produced by the condensation of acetyl CoA with oxaloacetate is removed from the citric acid cycle and carried across the inner mitochondrial membrane into the cytosol. The oxaloacetate is returned to mitochondrion as malate and then back into oxaloacetate to transfer more acetyl-CoA out of the mitochondrion. The cytosolic acetyl-CoA can be carboxylated by acetyl-CoA carboxylase into malonyl CoA , the first committed step in the synthesis of fatty acids , or it can be combined with acetoacetyl-CoA to form 3-hydroxymethylglutaryl-CoA HMG-CoA which is the rate limiting step controlling the synthesis of cholesterol.

Pyruvate molecules produced by glycolysis are actively transported across the inner mitochondrial membrane, and into the matrix where they can either be oxidized and combined with coenzyme A to form CO 2 , acetyl-CoA, and NADH, [28] or they can be carboxylated by pyruvate carboxylase to form oxaloacetate. Adding more of any of these intermediates to the mitochondrion therefore means that that additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other.

Hence the addition of oxaloacetate greatly increases the amounts of all the citric acid intermediates, thereby increasing the cycle's capacity to metabolize acetyl CoA, converting its acetate component into CO 2 and water, with the release of enough energy to form 11 ATP and 1 GTP molecule for each additional molecule of acetyl CoA that combines with oxaloacetate in the cycle.

To cataplerotically remove oxaloacetate from the citric cycle, malate can be transported from the mitochondrion into the cytoplasm, decreasing the amount of oxaloacetate that can be regenerated.


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This article concentrates on the catabolic role of glycolysis with regard to converting potential chemical energy to usable chemical energy during the oxidation of glucose to pyruvate. Many of the metabolites in the glycolytic pathway are also used by anabolic pathways, and, as a consequence, flux through the pathway is critical to maintain a supply of carbon skeletons for biosynthesis.

The following metabolic pathways are all strongly reliant on glycolysis as a source of metabolites: and many more. Although gluconeogenesis and glycolysis share many intermediates the one is not functionally a branch or tributary of the other. There are two regulatory steps in both pathways which, when active in the one pathway, are automatically inactive in the other. The two processes can therefore not be simultaneously active. NADH is rarely used for synthetic processes, the notable exception being gluconeogenesis. NADPH is also formed by the pentose phosphate pathway which converts glucose into ribose, which can be used in synthesis of nucleotides and nucleic acids , or it can be catabolized to pyruvate.

Cellular uptake of glucose occurs in response to insulin signals, and glucose is subsequently broken down through glycolysis, lowering blood sugar levels. However, the low insulin levels seen in diabetes result in hyperglycemia, where glucose levels in the blood rise and glucose is not properly taken up by cells. Hepatocytes further contribute to this hyperglycemia through gluconeogenesis.

Glycolysis in hepatocytes controls hepatic glucose production, and when glucose is overproduced by the liver without having a means of being broken down by the body, hyperglycemia results. Glycolytic mutations are generally rare due to importance of the metabolic pathway, this means that the majority of occurring mutations result in an inability for the cell to respire, and therefore cause the death of the cell at an early stage. However, some mutations are seen with one notable example being Pyruvate kinase deficiency , leading to chronic hemolytic anemia.

Malignant tumor cells perform glycolysis at a rate that is ten times faster than their noncancerous tissue counterparts. Thus, these cells rely on anaerobic metabolic processes such as glycolysis for ATP adenosine triphosphate. Some tumor cells overexpress specific glycolytic enzymes which result in higher rates of glycolysis.

The increase in glycolytic activity ultimately counteracts the effects of hypoxia by generating sufficient ATP from this anaerobic pathway. The Warburg hypothesis claims that cancer is primarily caused by dysfunctionality in mitochondrial metabolism, rather than because of the uncontrolled growth of cells. A number of theories have been advanced to explain the Warburg effect. One such theory suggests that the increased glycolysis is a normal protective process of the body and that malignant change could be primarily caused by energy metabolism.

This high glycolysis rate has important medical applications, as high aerobic glycolysis by malignant tumors is utilized clinically to diagnose and monitor treatment responses of cancers by imaging uptake of 2- 18 Fdeoxyglucose FDG a radioactive modified hexokinase substrate with positron emission tomography PET. There is ongoing research to affect mitochondrial metabolism and treat cancer by reducing glycolysis and thus starving cancerous cells in various new ways, including a ketogenic diet. Click on genes, proteins and metabolites below to link to respective articles.

Some of the metabolites in glycolysis have alternative names and nomenclature. In part, this is because some of them are common to other pathways, such as the Calvin cycle. From Wikipedia, the free encyclopedia. Main article: Pyruvate kinase. Metabolism portal Biology portal. Res Microbiol. Mol Syst Biol. Retrieved Journal of the History of Biology. Valencia, Spain.

Bios Journal of Biological Chemistry. A new enzyme with the glycolytic function 6-phosphate 1-phosphotransferase". J Biol Chem. Arch Microbiol. Cengage Learning; 5 edition. Biochemistry 6th ed. New York: Freeman. Biochemistry 3rd ed. Journal of Physiology. In: Biochemistry Fourth ed.

New York: W. Freeman and Company. Biochemistry Fourth ed. Voet; Charlotte W. Pratt Fundamentals of Biochemistry, 2nd Edition. John Wiley and Sons, Inc. Biochem J. Acta Pharmaceutica Sinica B. A new pH-based etiopathogenic perspective and therapeutic approach to an old cancer question".

see

Cancer cell metabolism: implications for therapeutic targets

Nelson, Michael M. Lehninger principles of biochemistry 4th ed. Retrieved September 8, Retrieved December 5, Seminars in Cancer Biology. Anti-Cancer Agents in Medicinal Chemistry. Journal of Child Neurology. Metabolism , catabolism , anabolism.

Carbohydrate metabolism - Wikipedia

Metabolic pathway Metabolic network Primary nutritional groups. Protein synthesis Catabolism.


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  • Pentose phosphate pathway Fructolysis Galactolysis. Glycosylation N-linked O-linked. Photosynthesis Anoxygenic photosynthesis Chemosynthesis Carbon fixation. Xylose metabolism Radiotrophism. Fatty acid degradation Beta oxidation Fatty acid synthesis. Steroid metabolism Sphingolipid metabolism Eicosanoid metabolism Ketosis Reverse cholesterol transport. Amino acid synthesis Urea cycle. Purine metabolism Nucleotide salvage Pyrimidine metabolism. Metal metabolism Iron metabolism Ethanol metabolism. Metabolism map.