Enzyme kinetics: 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 · Enzyme kinetics

Sample item

Which pattern describes a competitive inhibitor?

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

The answer and the reasoning

A. Km increases and Vmax is unchanged

A competitive inhibitor binds the active site and competes with substrate. More substrate is needed to reach half-maximal velocity, so apparent Km rises, but a high enough substrate concentration overcomes it, so Vmax is unchanged. Non-competitive inhibitors lower Vmax with Km unchanged.

Option by option

  • A. Km increases and Vmax is unchanged (correct)

    Correct. Substrate can outcompete the inhibitor, so Vmax is reached but at a higher Km.

  • B. Km is unchanged and Vmax decreases

    This is the non-competitive pattern, where inhibitor reduces the amount of working enzyme.

  • C. Both Km and Vmax decrease

    Both falling is the uncompetitive pattern.

  • D. Km decreases and Vmax is unchanged

    A falling Km with unchanged Vmax does not describe a standard inhibitor pattern.

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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Km increases and Vmax is unchanged. A competitive inhibitor binds the active site and competes with substrate.

Why is "Km is unchanged and Vmax decreases" not the answer?

This is the non-competitive pattern, where inhibitor reduces the amount of working enzyme.

Why is "Both Km and Vmax decrease" not the answer?

Both falling is the uncompetitive pattern.

Why is "Km decreases and Vmax is unchanged" not the answer?

A falling Km with unchanged Vmax does not describe a standard inhibitor pattern.

What concept does this question test?

It tests Enzyme kinetics in Biochemistry. More substrate is needed to reach half-maximal velocity, so apparent Km rises, but a high enough substrate concentration overcomes it, so Vmax is unchanged. Non-competitive inhibitors lower Vmax with Km unchanged.

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 Enzyme kinetics?

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.