Multiple Choice
In the citric acid (Krebs) cycle, how many molecules of ATP (or GTP, ATP-equivalent) are produced directly by substrate-level phosphorylation per turn of the cycle (per acetyl-CoA)?
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Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism
Citric Acid Cycle 1
Multiple Choice
In one turn of the citric acid (Krebs) cycle per acetyl-CoA (not counting oxidative phosphorylation from NADH and FADH2), how many ATP equivalents are produced directly by substrate-level phosphorylation?
A
ATP equivalents
B
ATP equivalents
C
ATP equivalents
D
ATP equivalent (as GTP or ATP)
Verified step by step guidance1
Understand that the citric acid cycle (Krebs cycle) produces energy-rich molecules through several steps, but only one step involves substrate-level phosphorylation, which directly generates ATP or GTP.
Identify the substrate-level phosphorylation step in the cycle: the conversion of succinyl-CoA to succinate catalyzed by succinyl-CoA synthetase, which produces one molecule of GTP (or ATP) per acetyl-CoA molecule entering the cycle.
Recall that other energy carriers produced in the cycle, such as NADH and FADH2, contribute to ATP production only indirectly through oxidative phosphorylation, which is not counted here.
Conclude that for each acetyl-CoA molecule, the citric acid cycle directly produces exactly one ATP equivalent via substrate-level phosphorylation.
Therefore, the number of ATP equivalents produced directly by substrate-level phosphorylation per turn of the citric acid cycle is 1.
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