Which cytosolic process regenerates NAD+ during anaerobic glycolysis and why is it necessary?

• A. Pyruvate dehydrogenase converts pyruvate to acetyl-CoA, regenerating NAD+ to sustain glycolysis under anaerobic conditions.
• B. Malate dehydrogenase converts malate to oxaloacetate, regenerating NAD+.
• C. Lactate dehydrogenase converts pyruvate to lactate, regenerating NAD+ to sustain glycolysis under anaerobic conditions.
• D. Isocitrate dehydrogenase regenerates NAD+ during glycolysis.

Answer: (C)

When glycolysis runs without oxygen, the cell must keep NAD+ available for the glyceraldehyde-3-phosphate dehydrogenase step. The way this is achieved in the cytosol is by converting pyruvate to lactate via lactate dehydrogenase. This reaction oxidizes NADH back to NAD+, restoring the pool of NAD+ needed for glycolysis to continue and for ATP to be produced through substrate-level phosphorylation. Without this NAD+ regeneration, glycolysis would stall and no net ATP would be made in anaerobic conditions.

The other options don’t fit the context: pyruvate dehydrogenase uses NAD+ to form NADH and feeds into acetyl-CoA in the mitochondria, which requires oxygen; malate dehydrogenase is involved in shuttle systems and the TCA cycle rather than directly sustaining glycolysis under anaerobic conditions; isocitrate dehydrogenase is a TCA enzyme that generates NADH, not regenerating NAD+ during glycolysis.