In the TCA cycle, which molecule is produced directly and can be converted to ATP?

In the TCA cycle, which molecule is produced directly and can be converted to ATP?

• A. NADH
• B. FADH2
• C. GTP
• D. None (no direct ATP produced)

Answer: (C)

The key point is that one step in the TCA cycle directly makes a nucleotide triphosphate: succinyl-CoA is converted to succinate, and a phosphate group is transferred to GDP to form GTP. This is a substrate-level phosphorylation event, producing GTP right in the cycle. GTP can then be readily converted to ATP by a kinase, so it serves as an immediate ATP source. The other products, NADH and FADH2, are reduced cofactors that feed the electron transport chain to eventually generate ATP, but they are not ATP themselves. So the molecule produced directly and convertible to ATP is GTP.

The key point is that one step in the TCA cycle directly makes a nucleotide triphosphate: succinyl-CoA is converted to succinate, and a phosphate group is transferred to GDP to form GTP. This is a substrate-level phosphorylation event, producing GTP right in the cycle. GTP can then be readily converted to ATP by a kinase, so it serves as an immediate ATP source. The other products, NADH and FADH2, are reduced cofactors that feed the electron transport chain to eventually generate ATP, but they are not ATP themselves. So the molecule produced directly and convertible to ATP is GTP.