Glycolysis and Fermentation By M Shrivastava & Dr S Nayak

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A Review on Glycolysis and Fermentation Meenu Shrivastava Lecturer meenushri30@gmail.com Dr. S.Nayak Principal Bansal College of Pharmacy, Bhopal www.bansalpharmacy.com

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Glycolysis takes place in the cytosol of cells. Glucose enters the Glycolysis pathway by conversion to glucose-6-phosphate . .

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1. Hexokinase catalyzes: Glucose + ATP  glucose-6-P + ADP ATP binds to the enzyme as a complex with Mg ++ .

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2. Phosphoglucose Isomerase catalyzes: glucose-6-P (aldose)  fructose-6-P (ketose) .

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3. Phosphofructokinase catalyzes: fructose-6-P + ATP  fructose-1,6-bisP + ADP .

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4. Aldolase catalyzes: fructose-1,6-bisphosphate  dihydroxyacetone-P + glyceraldehyde-3-P The reaction is an aldol cleavage , the reverse of an aldol condensation. .

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The ketose/aldose conversion involves acid/base catalysis , and is thought to proceed via an enediol intermediate, as with Phosphoglucose Isomerase.

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6. Glyceraldehyde-3-phosphate Dehydrogenase catalyzes: glyceraldehyde-3-P + NAD + + P i  1,3-bisphosphoglycerate + NADH + H +

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Recall that NAD + accepts 2 e  plus one H + (a hydride) in going to its reduced form.

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7. Phosphoglycerate Kinase catalyzes: 1,3-bisphosphoglycerate + ADP  3-phosphoglycerate + ATP

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8. Phosphoglycerate Mutase catalyzes: 3-phosphoglycerate  2-phosphoglycerate Phosphate is shifted from the OH on C3 to the OH on C2.

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9. Enolase catalyzes: 2-phosphoglycerate  phosphoenolpyruvate + H 2 O This dehydration reaction is Mg ++ -dependent .

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10. Pyruvate Kinase catalyzes: phosphoenolpyruvate + ADP  pyruvate + ATP

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Required inorganic cations K + and Mg ++ bind to anionic residues at the active site of Pyruvate Kinase.

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Glycolysis continued. Recall that there are 2 GAP per glucose.

Glycolysis:

Glycolysis Balance sheet for ~P bonds of ATP: How many ATP ~P bonds expended? ________ How many ~P bonds of ATP produced? (Remember there are two 3C fragments from glucose.) ________ Net production of ~P bonds of ATP per glucose: ________ 2 4 2

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Balance sheet for ~P bonds of ATP: 2 ATP expended 4 ATP produced (2 from each of two 3C fragments from glucose) Net production of 2 ~P bonds of ATP per glucose . Glycolysis - total pathway, omitting H + : glucose + 2 NAD + + 2 ADP + 2 P i  2 pyruvate + 2 NADH + 2 ATP In aerobic organisms : pyruvate produced in Glycolysis is oxidized to CO 2 via Krebs Cycle NADH produced in Glycolysis & Krebs Cycle is reoxidized via the respiratory chain, with production of much additional ATP.

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Fermentation: Anaerobic organisms lack a respiratory chain.

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E.g., Lactate Dehydrogenase catalyzes reduction of the keto in pyruvate to a hydroxyl, yielding lactate , as NADH is oxidized to NAD + . Lactate , in addition to being an end-product of fermentation, serves as a mobile form of nutrient energy , & possibly as a signal molecule in mammalian organisms. Cell membranes contain carrier proteins that facilitate transport of lactate.

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Skeletal muscles ferment glucose to lactate during exercise, when the exertion is brief and intense. Lactate released to the blood may be taken up by other tissues, or by skeletal muscle after exercise, and converted via Lactate Dehydrogenase back to pyruvate , which may be oxidized in Krebs Cycle or (in liver) converted to back to glucose via gluconeogenesis

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Lactate serves as a fuel source for cardiac muscle as well as brain neurons . Astrocytes , which surround and protect neurons in the brain, ferment glucose to lactate and release it. Lactate taken up by adjacent neurons is converted to pyruvate that is oxidized via Krebs Cycle.

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Some anaerobic organisms metabolize pyruvate to ethanol , which is excreted as a waste product. NADH is converted to NAD + in the reaction catalyzed by Alcohol Dehydrogenase.

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Glycolysis , omitting H + : glucose + 2 NAD + + 2 ADP + 2 P i  2 pyruvate + 2 NADH + 2 ATP Fermentation , from glucose to lactate: glucose + 2 ADP + 2 P i  2 lactate + 2 ATP Anaerobic catabolism of glucose yields only 2 “high energy” bonds of ATP.

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Inhibition of the Glycolysis enzyme Phosphofructokinase when [ATP] is high prevents breakdown of glucose in a pathway whose main role is to make ATP. It is more useful to the cell to store glucose as glycogen when ATP is plentiful.

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Meenu Shrivastava Lecturer meenushri30@gmail.com Dr. S.Nayak Principal Principal.bcp@gmail.com Bansal College of Pharmacy, Bhopal www.bansalpharmacy.com

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