A Few ANSWERS to Hour Exam 3: SP09 6. (a) Glyceraldehyde-3-phosphate dehydrogenase is not classified as a kinase because the source of P for the phosphorylation reaction that it catalyzes (glyceraldehyde-3-phosphate to 1,3-bisphosphoglyceric acid) is orthophosphate (inorganic P), not a nucleotide triphosphate. (b) The thermodynamic driving force (i.e., source of Gibbs free energy) for this reaction is oxidation (i.e., loss of electrons) of glyceraldehyde-3-phosphate. The thermodynamic driving force for the hexokinase- and phosphofructokinase-catalyzed reactions is hydrolysis of a high-energy ~P from ATP to form a phosphorylated intermediate; for the phosphoglycerate kinase- and pyruvate kinase-catalyzed reactions, the thermodynamic driving force is hydrolysis of a ~P from a phosphorylated glycolytic intermediate to form ATP. Oxidative decarboxylation is loss of a C atom as carbon dioxide, accompanied by loss of electrons, in a reaction catalyzed by a dehydrogenase. The three enzymes of the TCA cycle that catalyze oxidative decarboxylation reactions are: 1. pyruvate dehydrogenase, which is inhibited by ATP, NADH and acetyl-CoA; 2. isocitrate dehydrogenase, which is inhibited by ATP and NADH, and stimulated by ADP and NAD+; and 3. alpha-ketoglutarate deydrogenase, which is inhibited by ATP, NADH and succinyl-CoA. Phosphofructokinase is inhibited by ATP and citrate (from the TCA cycle); and pyruvate kinase is inhibited by ATP, acetyl-CoA (from beta-oxidation of fatty acids)and alanine (from protein catabolism). Isocitrate lyase catalyzes the conversion of isocitrate to succinate and glyoxalate, which in turn reacts with acetyl-CoA in a condensation reaction catalyzed by malate synthase to form malate.
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