Metalloenzymes: carbonic anhydrase: 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 · Metalloenzymes: carbonic anhydrase

Sample item

A trekker taking acetazolamide for altitude sickness develops tingling and a normal anion gap metabolic acidosis. Which zinc enzyme reaction is inhibited?

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

The answer and the reasoning

A. Reversible hydration of CO2 to carbonic acid, so renal bicarbonate reabsorption falls

Carbonic anhydrase is a zinc metalloenzyme that interconverts CO2 and H2O with H2CO3, which dissociates to H+ and bicarbonate. In the proximal tubule it enables bicarbonate reabsorption, so inhibition causes bicarbonaturia and hyperchloraemic (normal anion gap) acidosis. The metabolic acidosis stimulates ventilation, which is why it helps at altitude.

Option by option

  • A. Reversible hydration of CO2 to carbonic acid, so renal bicarbonate reabsorption falls (correct)

    Correct. Inhibiting carbonic anhydrase causes bicarbonate loss and non-anion gap acidosis.

  • B. Decarboxylation of oxaloacetate to pyruvate, so renal bicarbonate is retained

    Oxaloacetate decarboxylation does not involve zinc and is not the target of acetazolamide.

  • C. Oxidation of lactate to pyruvate, so lactic acid accumulates

    Lactate dehydrogenase is not inhibited by acetazolamide, and lactic acidosis gives a high anion gap.

  • D. Conversion of angiotensin I to angiotensin II, so renal bicarbonate falls

    Angiotensin converting enzyme is also a zinc enzyme, but acetazolamide does not inhibit 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, Reversible hydration of CO2 to carbonic acid, so renal bicarbonate reabsorption falls. Carbonic anhydrase is a zinc metalloenzyme that interconverts CO2 and H2O with H2CO3, which dissociates to H+ and bicarbonate.

Why is "Decarboxylation of oxaloacetate to pyruvate, so renal bicarbonate is…" not the answer?

Oxaloacetate decarboxylation does not involve zinc and is not the target of acetazolamide.

Why is "Oxidation of lactate to pyruvate, so lactic acid accumulates" not the answer?

Lactate dehydrogenase is not inhibited by acetazolamide, and lactic acidosis gives a high anion gap.

Why is "Conversion of angiotensin I to angiotensin II, so renal bicarbonate f…" not the answer?

Angiotensin converting enzyme is also a zinc enzyme, but acetazolamide does not inhibit it.

What concept does this question test?

It tests Metalloenzymes: carbonic anhydrase in Biochemistry. In the proximal tubule it enables bicarbonate reabsorption, so inhibition causes bicarbonaturia and hyperchloraemic (normal anion gap) acidosis. The metabolic acidosis stimulates ventilation, which is why it helps at altitude.

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 Metalloenzymes: carbonic anhydrase?

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.