USA Biology Olympiad (USABO) Practice Exam

What is the net production of NADH and ATP molecules from each pyruvate molecule during pyruvate oxidation and the citric acid cycle when succinate dehydrogenase is inhibited?

1. 1, 2

2. 2, 1

3. 3, 1

4. 4, 0

The correct answer is: 3, 1

In the context of pyruvate oxidation and the citric acid cycle, focusing on the processes followed, each pyruvate molecule is transformed into acetic acid, which then enters the citric acid cycle. During pyruvate oxidation, each pyruvate generates one NADH molecule as it is converted into acetyl-CoA. The process includes the decarboxylation of pyruvate and the reduction of NAD+ to NADH. Therefore, from the pyruvate oxidation, we get 1 NADH. When we consider the citric acid cycle, each acetyl-CoA that enters leads to further production of NADH and ATP/GTP. Normally, for each turn of the citric acid cycle, three NADH are produced, alongside one GTP (which can be considered equivalent to one ATP). However, if succinate dehydrogenase, which catalyzes the conversion of succinate to fumarate, is inhibited, this will affect the production of one NADH from that step. Despite this inhibition, the citric acid cycle will still complete the other steps for each acetyl-CoA, allowing for two additional NADH to be produced. Thus, from one molecule of acetyl-CoA in such