Which statement about the role of NAD+/NADH and FAD/FADH2 is true regarding metabolic flux when the NAD+/NADH ratio is high?

Which statement about the role of NAD+/NADH and FAD/FADH2 is true regarding metabolic flux when the NAD+/NADH ratio is high?

• A. High NADH inhibits dehydrogenase steps and redirects flux.
• B. It has no effect on metabolic flux.
• C. High NAD+/NADH supports oxidation and flux through catabolic pathways.
• D. It only affects anabolic processes.

Answer: (C)

The main idea is that the redox state of the NAD+/NADH pair acts as a gate for dehydrogenase reactions. When NAD+ is plentiful relative to NADH, these enzymes can readily accept electrons from metabolic substrates, so glycolysis, the TCA cycle, and beta-oxidation can proceed efficiently. This pushes flux toward oxidation of fuels to generate ATP and NADH that will be reoxidized in the electron transport chain. FAD/FADH2 participate in similar oxidative steps, but the key driver here is having a high NAD+/NADH ratio to support ongoing catabolic oxidation. If NADH were high (low NAD+/NADH), dehydrogenase steps would slow, diverting flux away from oxidation. Hence, a high NAD+/NADH ratio promotes oxidation and flux through catabolic pathways.

The main idea is that the redox state of the NAD+/NADH pair acts as a gate for dehydrogenase reactions. When NAD+ is plentiful relative to NADH, these enzymes can readily accept electrons from metabolic substrates, so glycolysis, the TCA cycle, and beta-oxidation can proceed efficiently. This pushes flux toward oxidation of fuels to generate ATP and NADH that will be reoxidized in the electron transport chain. FAD/FADH2 participate in similar oxidative steps, but the key driver here is having a high NAD+/NADH ratio to support ongoing catabolic oxidation. If NADH were high (low NAD+/NADH), dehydrogenase steps would slow, diverting flux away from oxidation. Hence, a high NAD+/NADH ratio promotes oxidation and flux through catabolic pathways.