Krebs cycle knowledge check — Which compound listed below is NOT an intermediate of the citric acid (Krebs) cycle?
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AIsocitrate
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BSuccinate
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CFumarate
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DStearate
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EMalate
Answer
Correct Answer: Stearate
Explanation
Introduction / Context:The citric acid cycle comprises a defined set of intermediates that interconvert as acetyl-CoA is oxidized. Recognizing bona fide intermediates helps avoid confusing TCA compounds with unrelated metabolites such as fatty acids.
Given Data / Assumptions:
- Canonical TCA intermediates include citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
- Stearate is an 18-carbon saturated fatty acid used in lipid metabolism, not the TCA cycle.
Concept / Approach:Compare each option to the established TCA list. Any fatty acid (e.g., stearate) undergoes beta-oxidation to acetyl-CoA but is not itself a TCA intermediate.
Step-by-Step Solution:
Check isocitrate → valid TCA intermediate.Check succinate and fumarate → both valid TCA intermediates.Check stearate → fatty acid; not part of TCA.Check malate → valid TCA intermediate.Verification / Alternative check:Standard metabolic maps place stearate in the lipid metabolism pathway, not in the TCA cycle diagram.
Why Other Options Are Wrong:
a,b,c,e) Each is a recognized TCA intermediate.Common Pitfalls:Confusing substrates that feed into TCA (like acetyl-CoA derived from fatty acids) with the intermediates inside the cycle.
Final Answer:Stearate.