Biotin (vitamin B7) is a covalently attached cofactor for carboxylase enzymes. A patient with biotinidase deficiency presents with skin rash, alopecia, lactic acidosis, and organic aciduria. The accumulated metabolites (methylcitrate, 3-hydroxypropionate, 3-methylcrotonylglycine) implicate deficient activity of all EXCEPT which enzyme?

• A Pyruvate carboxylase
• B Propionyl-CoA carboxylase
• C Transcarboxylase ✓
• D 3-Methylcrotonyl-CoA carboxylase

Explanation

Biotinidase deficiency prevents recycling of free biotin from biotinylated peptides, leading to secondary deficiency of all four mammalian biotin-dependent carboxylases: pyruvate carboxylase (lactate/alanine accumulation), propionyl-CoA carboxylase (methylcitrate, 3-hydroxypropionate), 3-methylcrotonyl-CoA carboxylase (3-methylcrotonylglycine), and acetyl-CoA carboxylase. Transcarboxylase is a bacterial enzyme not found in mammals; its absence is irrelevant to biotinidase deficiency. Treatment is lifelong free biotin supplementation.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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