Enzyme kinetics: inhibition types: 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: inhibition types

Sample item

In a Lineweaver-Burk plot, an enzyme without inhibitor has Km 5 mM and Vmax 100. With an inhibitor, the line has double the y-intercept but the same x-intercept. Which inhibition type and values apply?

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

The answer and the reasoning

A. Noncompetitive; Vmax 50, Km 5 mM

The y-intercept is 1/Vmax, so doubling it halves Vmax to 50. The x-intercept is -1/Km, and since it is unchanged Km stays at 5 mM. Reduced Vmax with unchanged Km is the signature of pure noncompetitive inhibition, in which the inhibitor binds a site distinct from the active site.

Option by option

  • A. Noncompetitive; Vmax 50, Km 5 mM (correct)

    Correct. Doubled 1/Vmax means Vmax 50, and an unchanged x-intercept means Km 5 mM.

  • B. Uncompetitive; Vmax 50, Km 2.5 mM

    Uncompetitive inhibition gives parallel lines, with both Vmax and Km reduced.

  • C. Competitive; Vmax 100, Km 10 mM

    Competitive inhibition shifts the x-intercept and leaves the y-intercept unchanged.

  • D. Noncompetitive; Vmax 200, Km 5 mM

    A doubled y-intercept halves Vmax; it does not double it.

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, Noncompetitive; Vmax 50, Km 5 mM. The y-intercept is 1/Vmax, so doubling it halves Vmax to 50.

Why is "Uncompetitive; Vmax 50, Km 2.5 mM" not the answer?

Uncompetitive inhibition gives parallel lines, with both Vmax and Km reduced.

Why is "Competitive; Vmax 100, Km 10 mM" not the answer?

Competitive inhibition shifts the x-intercept and leaves the y-intercept unchanged.

Why is "Noncompetitive; Vmax 200, Km 5 mM" not the answer?

A doubled y-intercept halves Vmax; it does not double it.

What concept does this question test?

It tests Enzyme kinetics: inhibition types in Biochemistry. The x-intercept is -1/Km, and since it is unchanged Km stays at 5 mM. Reduced Vmax with unchanged Km is the signature of pure noncompetitive inhibition, in which the inhibitor binds a site distinct from the active site.

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 Enzyme kinetics: inhibition types?

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.