Fig. 1.

Metabolic pathways involving Cobalamin (Cbl) for MMA, tHcy, and Met. The enzyme propionyl-CoA carboxylase converts propionyl-CoA into D-methylmalonyl-CoA, which is biotin dependent, and then is racemized into l-methylmalonyl-CoA and isomerized into succinyl-CoA, a Krebs cycle intermediate. Adenosylcobalamin (AdoCbl) is the cofactor of the methylmalonyl-CoA mutase (MUT) reaction; methylcobalamin (MeCbl) is the cofactor of the methionine synthase (MTR) reaction. Abbreviations used: Cbl cobalamin, OH-Cbl hydroxycobalamin, AdoCbl adenosylcobalamin, MeCbl methylcobalamin, MTR methionine synthase, MTRR methionine synthase reductase, CBS cystathionine β synthase, S-AdoMet S-adenosylmethionine, S-AdoHcy S-adenosylhomocysteine