Multiple Choice
In the citric acid (Krebs) cycle, how much ATP (or equivalent GTP) is produced directly by substrate-level phosphorylation per turn of the cycle for one 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, which set of products is generated (excluding the regenerated oxaloacetate)?
A
2 , 1 , 1 , and 3
B
3 , 1 , 1 , and 2
C
3 , 1 , 1 , and 1
D
2 , 2 , 2 , and 2
Verified step by step guidance1
Recall that one turn of the citric acid cycle begins with the condensation of acetyl-CoA (2 carbons) with oxaloacetate (4 carbons) to form citrate (6 carbons). The cycle then proceeds through a series of reactions that regenerate oxaloacetate.
Identify the key products formed during one turn of the cycle per acetyl-CoA: these include reduced electron carriers (NADH and FADH2), a high-energy phosphate compound (GTP or ATP), and carbon dioxide (CO2) released as waste.
Count the number of NADH molecules produced: NAD+ is reduced to NADH in three separate steps of the cycle (isocitrate to α-ketoglutarate, α-ketoglutarate to succinyl-CoA, and malate to oxaloacetate), so 3 NADH are generated per turn.
Determine the number of FADH2 molecules produced: FAD is reduced to FADH2 in the conversion of succinate to fumarate, so 1 FADH2 is produced per turn.
Note the number of GTP (or ATP) molecules formed: one GTP (or ATP) is generated by substrate-level phosphorylation during the conversion of succinyl-CoA to succinate, and 2 CO2 molecules are released during the decarboxylation steps converting isocitrate to α-ketoglutarate and α-ketoglutarate to succinyl-CoA.
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