Question

Choose the ONE BEST answer explaining how oxaloacetate and acetyl-CoA levels are balanced to maximize flux through the citrate cycle when energy charge in the cell is lovw Oxaloacetate is required for mitochondrial shuttle systems, and therefore it makes sense to activate pyruvate carboxylase by ATP and inhibit it by malate. None of the answers are correct. The best way to balance the input of carbon into the citrate cycle is to regulate the production of citrate using vitamins like panthothenic acid, which is the cause of beriberi in southeast asia. Pyruvate dehydrogenase is activated by CoA but inhibited by its product acetyl- CoA, whereas pyruvate carboxylase is activated by acetyl-CoA to produce more oxaloacetate for the citrate synthase reaction Under conditions of low energy charge in the cell, it makes sense to stimulate pyruvate decarboxylase so that flux through the citrate cycle is maximal, but when CoA is high, maximize the PDH reaction Increased NADH levels inhibits pyruvate dehydrogenase and thereby restricts flux through the pyruvate carboxylase reaction, this in turn, activates the malate dehydrogenase reaction Acetyl-CoA can be produced by the degradation of fat, and therefore, it is advantageous to stimulate ketogenesis by increasing the amount of oxaloacetate through activation of the isocitrate dehydrogenase reaction Pyruvate carboxylase and pyruvate dehydrogenase both require thiamin pyrophosphate (TPP), which facilitates coordinate regulation of the two enzymatic reactions and increased citrate cycle flux.

Answer 1

BEST ANSWER- Pyruvate dehydrogenase is activated by Coa-sh but inhibited by its product Acetyl-coa,

whereas pyruvate carboxylase is activated by acetyl-coa to produce more oxaloacetate for the citrate synthase reaction.

( Pyruvate dehydrogenase complex (PDC) is a complex of three enzymes that converts pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular reaction)