propionyl-CoA carboxylase

Propionyl-CoA carboxylase catalyzes the ATP-dependent carboxylation of propionyl-CoA to D-methylmalonyl-CoA using biotin as a cofactor, which is then converted by methylmalonyl-CoA epimerase to L-methylmalonyl-CoA, and finally by vitamin B12 (cobalamin)-dependent methylmalonyl-CoA mutase to succinyl-CoA, entering the TCA cycle. This pathway is essential for catabolizing odd-chain fatty acids, cholesterol side chains, and the amino acids valine, isoleucine, methionine, and threonine (mnemonic: VOMIT - Valine, Odd-chain fatty acids, Methionine, Isoleucine, Threonine).

Deficiency of PCC causes propionic acidemia, an autosomal recessive inborn error of metabolism that typically presents in the neonatal period with poor feeding, vomiting, lethargy, hypotonia, and severe anion-gap metabolic acidosis. Laboratory findings include elevated propionic acid and its metabolites, hyperammonemia (due to inhibition of N-acetylglutamate synthase, which is needed for the urea cycle), ketosis, and hypoglycemia. Long-term complications include developmental delay, cardiomyopathy, and pancytopenia.

This condition is distinguished from methylmalonic acidemia (deficiency of methylmalonyl-CoA mutase or B12 metabolism defects), which also causes elevated methylmalonic acid but occurs downstream in the pathway. Treatment involves dietary protein restriction (limiting the precursor amino acids), carnitine supplementation, and management of acute metabolic decompensation. Biotin supplementation may help in some biotin-responsive cases, though most propionic acidemia is not biotin-responsive (unlike some multiple carboxylase deficiencies).

Clinically, this enzyme is high-yield for USMLE Step 1 as it connects biochemistry (biotin-dependent carboxylation reactions), the TCA cycle anaplerosis, and clinical genetics (autosomal recessive organic acidemias). It's often tested alongside pyruvate carboxylase and acetyl-CoA carboxylase as biotin-dependent carboxylases.