Lipid metabolism: oxidation of unsaturated 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 · Lipid metabolism: oxidation of unsaturated fatty acids

Sample item

During beta-oxidation of oleic acid, the cis double bond lies at an odd-numbered carbon and the usual hydratase step cannot proceed. Which additional enzyme repositions the bond?

AThiolaseBAcyl-CoA dehydrogenaseCHydroxyacyl-CoA dehydrogenaseDEnoyl-CoA isomerase

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

The answer and the reasoning

D. Enoyl-CoA isomerase

Enoyl-CoA isomerase converts the cis-3 double bond to a trans-2 double bond, which is the substrate for enoyl-CoA hydratase. Acyl-CoA dehydrogenase is bypassed for that cycle, so slightly less FADH2 is made. Polyunsaturated acids with even-numbered bonds also need 2,4-dienoyl-CoA reductase.

Option by option

A. Thiolase
Thiolase cleaves acetyl-CoA from 3-ketoacyl-CoA and does not change double bonds.
B. Acyl-CoA dehydrogenase
Acyl-CoA dehydrogenase forms the trans-2 bond in saturated acids but is not the enzyme that shifts a cis bond.
C. Hydroxyacyl-CoA dehydrogenase
Hydroxyacyl-CoA dehydrogenase oxidises the 3-hydroxy group and is a regular step.
D. Enoyl-CoA isomerase (correct)
Correct. Enoyl-CoA isomerase shifts the cis-3 bond to the trans-2 position for hydration.

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 D, Enoyl-CoA isomerase. Enoyl-CoA isomerase converts the cis-3 double bond to a trans-2 double bond, which is the substrate for enoyl-CoA hydratase.

Why is "Thiolase" not the answer?

Thiolase cleaves acetyl-CoA from 3-ketoacyl-CoA and does not change double bonds.

Why is "Acyl-CoA dehydrogenase" not the answer?

Acyl-CoA dehydrogenase forms the trans-2 bond in saturated acids but is not the enzyme that shifts a cis bond.

Why is "Hydroxyacyl-CoA dehydrogenase" not the answer?

Hydroxyacyl-CoA dehydrogenase oxidises the 3-hydroxy group and is a regular step.

What concept does this question test?

It tests Lipid metabolism: oxidation of unsaturated fatty acids in Biochemistry. Acyl-CoA dehydrogenase is bypassed for that cycle, so slightly less FADH2 is made. Polyunsaturated acids with even-numbered bonds also need 2,4-dienoyl-CoA reductase.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Lipid metabolism: oxidation of unsaturated 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.