Biotin (vitamin B7) functions as a carboxyl carrier. A patient on prolonged raw egg-white diet develops alopecia, dermatitis, and neuropsychiatric symptoms. The enzyme most critically dependent on biotin whose deficiency produces lactic acidosis in this context is:
- A Pyruvate dehydrogenase
- B Pyruvate carboxylase ✓
- C Alpha-ketoglutarate dehydrogenase
- D Succinate dehydrogenase
Explanation
Biotin is the prosthetic group for four carboxylases: pyruvate carboxylase (PC), acetyl-CoA carboxylase, propionyl-CoA carboxylase, and 3-methylcrotonyl-CoA carboxylase. PC catalyses pyruvate → oxaloacetate (first step of gluconeogenesis; also replenishes TCA cycle intermediates). Biotin deficiency/PC deficiency leads to pyruvate accumulation and lactic acidosis since pyruvate cannot enter gluconeogenesis efficiently. Avidin in raw egg whites binds biotin with extreme affinity (Kd ~10^-15 M), preventing absorption. Pyruvate dehydrogenase uses lipoate and thiamine (B1), not biotin.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.