Question

Which enzyme catalyzes a reversible reaction in the TCA cycle, and produces NADH in the forward direction?

a. Alpha-ketoglutarate dehydrogenase

b. Malate dehydrogenase

c. Isocitrate dehydrogenase

d. Succinate dehydrogenase

Answer 1

Ans:-) option b. Malate dehydrogenase.

Succinate dehydrogenase catalyse reaction in which succinate is converted into fumarate and there is release of FADH.

Main thing is that there is allosteric regulation in TCA cycle. Means regulation is mainly by product inhibition and substrate availability. As the cycle proceeds ADP amount start reducing which cause accumulation of its precursor NADH which in turn can inhibit a number of dehydrogenase enzymes like succinate dehydrogenase, pyruvate dehydrogenase, isocitrate dehydrogenase, alpha keto glutarate dehydrogenase and also citrate synthase.

NADH is product of all dehydrogenase in citric acid cycle except succinate dehydrogenase.

It is also found that succinyl CoA inhibit alpha keto glutarate dehydrogenase.

But it was also found that ATP inhibits alpha keto glutarate dehydrogenase. A rise in ATP level would inhibit the reaction.