Multiple Choice
In one turn of the citric acid (Krebs) cycle per acetyl-CoA (excluding ATP from oxidative phosphorylation), what is the direct energy yield in terms of reduced cofactors and substrate-level phosphorylation?
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Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism
Citric Acid Cycle 1
Multiple Choice
In the citric acid (Krebs) cycle, how many ATP molecules are produced directly (as substrate-level phosphorylation) per turn of the cycle for each acetyl-CoA that enters?
A
1 ATP equivalent (produced as GTP)
B
2 ATP
C
4 ATP
D
3 ATP
Verified step by step guidance1
Understand that the citric acid cycle (Krebs cycle) produces energy-rich molecules through a series of enzymatic reactions starting with acetyl-CoA.
Identify that ATP can be produced directly by substrate-level phosphorylation during the cycle, which means the direct synthesis of ATP (or its equivalent) without involving the electron transport chain.
Recall that in the citric acid cycle, the direct substrate-level phosphorylation step occurs when succinyl-CoA is converted to succinate, catalyzed by succinyl-CoA synthetase, producing GTP (guanosine triphosphate).
Recognize that GTP is energetically equivalent to ATP because it can be readily converted to ATP by nucleoside diphosphate kinase, so 1 GTP equals 1 ATP equivalent per cycle turn per acetyl-CoA.
Conclude that for each acetyl-CoA entering the cycle, exactly one ATP equivalent is produced directly by substrate-level phosphorylation.
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