Enzyme kinetics: Michaelis-Menten calculation: 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: Michaelis-Menten calculation

Sample item

An enzyme has a Vmax of 100 micromol per minute and a Km of 2 mM. What is the initial velocity at a substrate concentration of 0.5 mM?

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

The answer and the reasoning

D. 20 micromol per minute

The Michaelis-Menten equation is v = Vmax [S] / (Km + [S]). Substituting gives v = 100 x 0.5 / (2 + 0.5) = 50 / 2.5 = 20 micromol per minute. Velocity reaches half of Vmax only when [S] equals Km.

Option by option

  • A. 25 micromol per minute

    25 would be obtained by wrongly using [S] / Km as the fraction of Vmax.

  • B. 50 micromol per minute

    50 would be the velocity only if [S] were equal to Km (2 mM).

  • C. 33 micromol per minute

    33 would be the velocity if [S] were 1 mM (100 x 1 / 3).

  • D. 20 micromol per minute (correct)

    Correct. 100 x 0.5 / 2.5 = 20.

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, 20 micromol per minute. The Michaelis-Menten equation is v = Vmax [S] / (Km + [S]).

Why is "25 micromol per minute" not the answer?

25 would be obtained by wrongly using [S] / Km as the fraction of Vmax.

Why is "50 micromol per minute" not the answer?

50 would be the velocity only if [S] were equal to Km (2 mM).

Why is "33 micromol per minute" not the answer?

33 would be the velocity if [S] were 1 mM (100 x 1 / 3).

What concept does this question test?

It tests Enzyme kinetics: Michaelis-Menten calculation in Biochemistry. Substituting gives v = 100 x 0.5 / (2 + 0.5) = 50 / 2.5 = 20 micromol per minute. Velocity reaches half of Vmax only when [S] equals Km.

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: Michaelis-Menten calculation?

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.