A 42-year-old epileptic woman on long-term acetazolamide develops hyperchloremic metabolic acidosis. Acetazolamide exerts its effect by inhibiting an enzyme whose active site contains:

• A A serine residue within a catalytic triad
• B A zinc ion coordinated by histidine residues ✓
• C A pyridoxal phosphate Schiff base
• D A heme iron center

Explanation

Carbonic anhydrase is a metalloenzyme whose active site contains a zinc ion coordinated by three histidine residues; zinc polarizes water to generate a hydroxyl nucleophile that hydrates CO2. Inhibition causes bicarbonate diuresis with sodium and potassium loss, producing hyperchloremic metabolic acidosis. Serine triads characterize hydrolases like chymotrypsin, PLP-dependent enzymes are aminotransferases, and heme iron occurs in cytochromes and catalase, none of which describes carbonic anhydrase.

Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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