Biochem #7. A) List the reactions that generate NADH, and list the reactions tha
ID: 227371 • Letter: B
Question
Biochem #7.
A) List the reactions that generate NADH, and list the reactions that regenerate NAD+ during both anaerobic and aerobic glycolysis. Show how and explain why the reactions that generate NADH and those that use NADH are coupled. B) List the three thermodynamically irreversible steps in glycolysis. How are these steps bypassed in gluconeogenesis? What is gluconeogenesis? Identify the metabolic and hormonal regulators that control the activity of these enzymes. Are these reactions truly irreversible?
Explanation / Answer
Aerobic glycolysis: Pyruvate is the end product of aerobic glycolysis. There is only one reaction (conversion of glyceraldehyde 3-phosphate into 1,3- biphosphoglycerate) in which NADH is produced in aerobic glycolysis. There is no regeneration of NAD+ in aerobic glycolysis.
Anaerobic glycolysis: In anaerobic glycolysis pyruvate converts into lactate. Hence the end product of anaerobic glycolysis is lactate. During conversion of pyruvate to lactate NADH is consumed. Thus, in one reaction (conversion of glyceraldehyde 3-phosphate into 1,3- biphosphoglycerate) of anaerobic glycolysis NADH are generated and in one reaction (conversion of pyruvate to lactate) NAD+ are regenerated.
The NADH utilized in conversion of pyruvate to lactate (regenerate NAD+) is obtained from the reaction catalysed by glyceraldehyde 3-phospate dehydrogenase that is conversion of glyceraldehyde 3-phosphate into 1,3- biphosphoglycerate (generate NADH). Hence these both reactions are coupled.
Three thermodynamically irreversible reactions are: 1. conversion of glucose into glucose 6-phosphate. 2. conversion of fructose 6-phosphate into fructose 1,6-biphosphate and 3. conversion of phosphoenolpyruvate into pyruvate.
Gluconeogenesis closely resembles the reversed pathwy of glycolysis, although it is not the complete reversal of glycolysis. Essentially, 3 reactions of glycolysis are irreversible. The other 7 reactions are common for both glycolysis and gluconeogenesis. The three irreversible steps of glycolysis are catalysed by the enzymes, namely hexokinase, phosphofructokinase and pyruvate kinase. These three stages bypassed by alternate enzymes specific to gluconeogenesis.
Gluconeogenesis: Gluconeogenesis is the process of formation of glucose from pyruvate.
Regulation of enzymes: Hexokinase is inhibited by glucose 6-phosphate, phosphofructokinase is an allosteric enzyme regulated by allosteric effectors. ATP, citrate, H+ ions are the most important allosteric inhibitors, whereas fructose 2,6- biphosphate, ADP, AMP and Pi are the allosteric activators. Pyruvate kinase is regulated by cAMP. The hormone glucagone inhibits hepatic glycolysis.
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