Fatty acid oxidation: odd-chain fatty acids: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Fatty acid oxidation: odd-chain fatty acids

Sample item

The final round of beta-oxidation of an odd-chain fatty acid yields propionyl-CoA, which is converted to succinyl-CoA. Which cofactors are needed, in order of the reactions?

ATetrahydrofolate, then lipoic acidBBiotin for propionyl-CoA carboxylase, then adenosylcobalamin for methylmalonyl-CoA mutaseCThiamine pyrophosphate, then pyridoxal phosphateDAdenosylcobalamin for propionyl-CoA carboxylase, then biotin for methylmalonyl-CoA mutase

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Biotin for propionyl-CoA carboxylase, then adenosylcobalamin for methylmalonyl-CoA mutase

Propionyl-CoA is carboxylated to D-methylmalonyl-CoA by biotin-dependent propionyl-CoA carboxylase, racemised to the L form, and then rearranged to succinyl-CoA by vitamin B12-dependent methylmalonyl-CoA mutase. Because succinyl-CoA can enter gluconeogenesis, the odd-chain fatty acids are the only fatty acids with a small glucogenic contribution.

Option by option

A. Tetrahydrofolate, then lipoic acid
Tetrahydrofolate and lipoic acid are not cofactors for these reactions.
B. Biotin for propionyl-CoA carboxylase, then adenosylcobalamin for methylmalonyl-CoA mutase (correct)
Biotin acts on the carboxylation step and B12 on the mutase step, in that order.
C. Thiamine pyrophosphate, then pyridoxal phosphate
Thiamine and pyridoxal phosphate are not used in this pathway.
D. Adenosylcobalamin for propionyl-CoA carboxylase, then biotin for methylmalonyl-CoA mutase
The order is reversed: the carboxylase needs biotin and the mutase needs B12.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Biotin for propionyl-CoA carboxylase, then adenosylcobalamin for methylmalonyl-CoA mutase. Propionyl-CoA is carboxylated to D-methylmalonyl-CoA by biotin-dependent propionyl-CoA carboxylase, racemised to the L form, and then rearranged to succinyl-CoA by vitamin B12-dependent methylmalonyl-CoA mutase.

Why is "Tetrahydrofolate, then lipoic acid" not the answer?

Tetrahydrofolate and lipoic acid are not cofactors for these reactions.

Why is "Thiamine pyrophosphate, then pyridoxal phosphate" not the answer?

Thiamine and pyridoxal phosphate are not used in this pathway.

Why is "Adenosylcobalamin for propionyl-CoA carboxylase, then biotin for meth…" not the answer?

The order is reversed: the carboxylase needs biotin and the mutase needs B12.

What concept does this question test?

It tests Fatty acid oxidation: odd-chain fatty acids in Biochemistry. Because succinyl-CoA can enter gluconeogenesis, the odd-chain fatty acids are the only fatty acids with a small glucogenic contribution.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Fatty acid oxidation: odd-chain fatty acids?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.