Biomolecules and enzymes: uncompetitive inhibition on a double reciprocal plot: 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 · Biomolecules and enzymes: uncompetitive inhibition on a double reciprocal plot

Sample item

In an enzyme assay, a plot of 1/v against 1/[S] is drawn with and without an inhibitor. The two lines are parallel, and the inhibited line lies above the control. Which inhibitor type and effect on the kinetic constants fit this plot?

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

The answer and the reasoning

D. Uncompetitive; both apparent Km and Vmax fall

An uncompetitive inhibitor binds only the enzyme-substrate complex, so it lowers Vmax and also lowers apparent Km by the same factor. The slope Km/Vmax stays constant, which gives parallel lines on a Lineweaver-Burk plot. Competitive inhibitors change the x-intercept, and noncompetitive inhibitors change the y-intercept.

Option by option

  • A. Competitive; apparent Km rises and Vmax is unchanged

    Competitive inhibition gives lines that meet on the y-axis because Vmax is unchanged.

  • B. Mixed; apparent Km rises and Vmax falls

    Mixed inhibition gives lines that intersect off the axes, not parallel lines.

  • C. Noncompetitive; Vmax falls and Km is unchanged

    Noncompetitive inhibition gives lines that meet on the x-axis because Km is unchanged.

  • D. Uncompetitive; both apparent Km and Vmax fall (correct)

    Correct. Binding to the enzyme-substrate complex lowers Vmax and Km together, so the slope is unchanged and the lines are parallel.

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 D, Uncompetitive; both apparent Km and Vmax fall. An uncompetitive inhibitor binds only the enzyme-substrate complex, so it lowers Vmax and also lowers apparent Km by the same factor.

Why is "Competitive; apparent Km rises and Vmax is unchanged" not the answer?

Competitive inhibition gives lines that meet on the y-axis because Vmax is unchanged.

Why is "Mixed; apparent Km rises and Vmax falls" not the answer?

Mixed inhibition gives lines that intersect off the axes, not parallel lines.

Why is "Noncompetitive; Vmax falls and Km is unchanged" not the answer?

Noncompetitive inhibition gives lines that meet on the x-axis because Km is unchanged.

What concept does this question test?

It tests Biomolecules and enzymes: uncompetitive inhibition on a double reciprocal plot in Biochemistry. The slope Km/Vmax stays constant, which gives parallel lines on a Lineweaver-Burk plot. Competitive inhibitors change the x-intercept, and noncompetitive inhibitors change the y-intercept.

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 Biomolecules and enzymes: uncompetitive inhibition on a double reciprocal plot?

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.