Multiple Choice
In eukaryotic cells, the Krebs (citric acid) cycle takes place primarily within the ________.
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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 including ATP from oxidative phosphorylation of NADH and FADH2), how many ATP equivalents are produced directly by substrate-level phosphorylation?
A
3 ATP equivalents
B
0 ATP equivalents
C
2 ATP equivalents (as 2 GTP)
D
1 ATP equivalent (as 1 GTP)
Verified step by step guidance1
Identify the step in the citric acid cycle where substrate-level phosphorylation occurs. This is the step where a direct synthesis of a high-energy phosphate compound (ATP or GTP) happens without involving the electron transport chain.
Recall that substrate-level phosphorylation in the citric acid cycle occurs during the conversion of succinyl-CoA to succinate, catalyzed by succinyl-CoA synthetase.
Understand that this reaction produces GTP (guanosine triphosphate), which is energetically equivalent to ATP. The GTP can be readily converted to ATP by nucleoside diphosphate kinase.
Note that no other steps in the citric acid cycle produce ATP or GTP directly; other energy carriers produced are NADH and FADH2, which generate ATP only through oxidative phosphorylation.
Conclude that per acetyl-CoA molecule entering the cycle, exactly one GTP (equivalent to one ATP) is produced directly by substrate-level phosphorylation.
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