Answers Test 3 Spring 2010 8. An oxidative decarboxylation reaction is an oxidation reaction (loss of electrons) in which a carboxylate group is removed, forming carbon dioxide. There are three oxidative carboxylation reactions in the tricarboxylic acid cycle that are catalyzed by pyruvate dehydrogenase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. These are all inhibited allosterically by high concentrations of ATP and NADH, and NAD+ and ADP are allosteric activators of isocitrate dehydrogenase. In addition, pyruvate dehydrogenase is inhibited allosterically by acetyl-CoA, and alpha-ketoglutarate dehydrogenase is inhibited allosterically by succinyl-CoA. 9. Plants, invertebrate animals and microorganisms are able to achieve a net synthesis of oxaloacetate via the glyoxalate cycle. Isocitrate is cleaved to succinate and glyoxalate in a reaction catalyzed by isocitrate lyase. Malate synthase then catalyzes a condensation reaction in which glyoxalate and acetyl-CoA are converted to malate. The decarboxylation reactions of the citric acid cycle are thus circumvented, resulting in a net production of 4-carbon oxaloacetate instead of a net loss of 2 C as carbon dioxide. 10. The c, gamma and epsilon subunits of ATP synthase form a 'rotor' that turns as a result of proton flux from the intermembrane space through alpha-helices in the c subunit. The rotating shaft mediates the energy exchange between proton flux at Fo and ATP synthesis in the F1, where conformational changes in three substrate sites in the beta subunits lead to binding of ADP + Pi (L), synthesis (T) and release (O) of ATP. 11. AMP is a positive allosteric modifier (activator) of both PFK-1 and pyruvate kinase that is also a positive allosteric modifier of glycogen phosphorylase b in skeletal muscle. Glucose-6-phosphate is a negative allosteric modifier (inhibitor)of hexokinase, and ATP is a negative allosteric modifier of both PFK-1 and pyruvate kinase; both glucose-6-phosphate and ATP are negative allosteric modifiers of glycogen phosphorylase b in skeletal muscle.
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