Final answer:
The enzyme not regulated by NADH in the TCA cycle is succinate dehydrogenase. Unlike other enzymes in the cycle, it functions without direct feedback inhibition from NADH.
Step-by-step explanation:
The enzyme in the TCA cycle that is not regulated by NADH is E) succinate dehydrogenase. While NADH serves as a regulator for several key enzymes in the citric acid cycle, including pyruvate dehydrogenase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase, it does not directly regulate succinate dehydrogenase. Instead, succinate dehydrogenase participates in both the citric acid cycle and the electron transport chain, and its activity is not regulated by feedback inhibition through NADH levels, though the rate of electron transport can be influenced by ATP demand.
When there is an abundance of NADH and ATP, the activity of isocitrate dehydrogenase and α-ketoglutarate dehydrogenase is reduced, slowing the TCA cycle. In contrast, succinate dehydrogenase continues its function within the cycle, facilitating the conversion of succinate into fumarate. This points out the unique regulation mechanisms within the citric acid cycle where only certain enzymes are responsive to the levels of NADH.