What coenzymes are reduced during the citric acid cycle?

In the citric acid cycle, the main coenzymes that are reduced are NAD+ and FAD. During various steps of the cycle, NAD+ is converted to NADH, and FAD is converted to FADH2. These reductions occur during specific reactions where electrons are transferred from substrates to these coenzymes, facilitating the production of energy-rich molecules.

NAD+ is reduced in steps where it accepts electrons, particularly during the isocitrate and α-ketoglutarate dehydrogenase reactions. Similarly, FAD is reduced in the succinate-to-fumarate step. Both NADH and FADH2 ultimately serve to shuttle electrons to the electron transport chain, contributing to ATP production.

Although ATP and ADP are crucial in energy metabolism, they do not undergo reduction in the cycle. Coenzyme A (CoA) and acetyl groups are involved in the formation of acetyl-CoA and in various reactions but also do not experience reduction themselves in the context of the cycle. Therefore, the correct identification of reduced coenzymes during the citric acid cycle points directly to NAD+ and FAD, highlighting their roles as electron carriers.