Enzymes: units and specific activity: 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 · Enzymes: units and specific activity

Sample item

A researcher incubates 0.2 mL of a tissue extract (4 mg protein per mL) with substrate. In 6 minutes, 24 micromoles of product are formed. What is the specific activity of the enzyme in the extract?

A0.2 micromoles per minute per mg proteinB20 micromoles per minute per mLC30 micromoles per mg proteinD5 micromoles per minute per mg protein

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

The answer and the reasoning

D. 5 micromoles per minute per mg protein

Rate = 24 micromoles / 6 min = 4 micromoles per minute (4 IU). Protein in the sample = 0.2 mL x 4 mg/mL = 0.8 mg. Specific activity = 4 / 0.8 = 5 micromoles per minute per mg protein, which expresses purity of an enzyme preparation.

Option by option

A. 0.2 micromoles per minute per mg protein
This inverts the ratio (0.8 / 4) and gives a number too low by 25-fold.
B. 20 micromoles per minute per mL
This divides the rate by sample volume (4 / 0.2) and gives volume activity per mL, not specific activity.
C. 30 micromoles per mg protein
This divides total product by protein and omits time, so it is not a rate.
D. 5 micromoles per minute per mg protein (correct)
Correct: 4 micromoles per minute divided by 0.8 mg of protein gives 5 per mg.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, 5 micromoles per minute per mg protein. Rate = 24 micromoles / 6 min = 4 micromoles per minute (4 IU).

Why is "0.2 micromoles per minute per mg protein" not the answer?

This inverts the ratio (0.8 / 4) and gives a number too low by 25-fold.

Why is "20 micromoles per minute per mL" not the answer?

This divides the rate by sample volume (4 / 0.2) and gives volume activity per mL, not specific activity.

Why is "30 micromoles per mg protein" not the answer?

This divides total product by protein and omits time, so it is not a rate.

What concept does this question test?

It tests Enzymes: units and specific activity in Biochemistry. Protein in the sample = 0.2 mL x 4 mg/mL = 0.8 mg. Specific activity = 4 / 0.8 = 5 micromoles per minute per mg protein, which expresses purity of an enzyme preparation.

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 Enzymes: units and specific activity?

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.