Cholesterol metabolism: HMG-CoA reductase: 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.

Try it

Biochemistry · Cholesterol metabolism: HMG-CoA reductase

Sample item

In a fasting subject glucagon raises cAMP, and the rate-limiting enzyme of cholesterol synthesis becomes phosphorylated and less active; insulin reverses this change. Which enzyme is being regulated?

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

The answer and the reasoning

A. HMG-CoA reductase

The rate-limiting enzyme is controlled covalently as well as by sterol feedback. Phosphorylation, favoured by glucagon and AMP-activated kinase, inactivates it, while insulin favours the active dephosphorylated state. Late enzymes of the pathway are not regulated this way.

Option by option

  • A. HMG-CoA reductase (correct)

    Correct. Phosphorylation inactivates it in fasting and insulin restores it.

  • B. Farnesyl pyrophosphate synthase

    Farnesyl pyrophosphate synthase makes an isoprenoid precursor after the rate-limiting step.

  • C. Lanosterol synthase

    Lanosterol synthase cyclises squalene epoxide and carries no hormone-sensitive phosphorylation.

  • D. Squalene epoxidase

    Squalene epoxidase is a late enzyme of the pathway and is not hormonally phosphorylated.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, HMG-CoA reductase. The rate-limiting enzyme is controlled covalently as well as by sterol feedback.

Why is "Farnesyl pyrophosphate synthase" not the answer?

Farnesyl pyrophosphate synthase makes an isoprenoid precursor after the rate-limiting step.

Why is "Lanosterol synthase" not the answer?

Lanosterol synthase cyclises squalene epoxide and carries no hormone-sensitive phosphorylation.

Why is "Squalene epoxidase" not the answer?

Squalene epoxidase is a late enzyme of the pathway and is not hormonally phosphorylated.

What concept does this question test?

It tests Cholesterol metabolism: HMG-CoA reductase in Biochemistry. Phosphorylation, favoured by glucagon and AMP-activated kinase, inactivates it, while insulin favours the active dephosphorylated state. Late enzymes of the pathway are not regulated this way.

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 Cholesterol metabolism: HMG-CoA reductase?

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.