The glycine cleavage (GCV) system is a large multienzyme complex that belongs to the 2-oxoacid dehydrogenase complex family, which also includes EC 188.8.131.52
, branched-chain alpha-keto acid dehydrogenase system, EC 184.108.40.206
, 2-oxoglutarate dehydrogenase system, EC 220.127.116.11
, pyruvate dehydrogenase system, and EC 18.104.22.168
, acetoin dehydrogenase system. The GCV system catalyses the reversible oxidation of glycine, yielding carbon dioxide, ammonia, 5,10-methylenetetrahydrofolate and a reduced pyridine nucleotide. Tetrahydrofolate serves as a recipient for one-carbon units generated during glycine cleavage to form the methylene group. The GCV system consists of four protein components, the P protein (EC 22.214.171.124
, glycine dehydrogenase (aminomethyl-transferring)), T protein (EC 126.96.36.199
, aminomethyltransferase), L protein (EC 188.8.131.52
, dihydrolipoyl dehydrogenase), and the non-enzyme H protein (lipoyl-carrier protein). The P protein catalyses the pyridoxal phosphate-dependent liberation of CO2 from glycine, leaving a methylamine moiety. The methylamine moiety is transferred to the lipoic acid group of the H protein, which is bound to the P protein prior to decarboxylation of glycine. The T protein catalyses the release of ammonia from the methylamine group and transfers the remaining C1 unit to tetrahydrofolate, forming 5,10-methylenetetrahydrofolate. The L protein then oxidizes the lipoic acid component of the H protein and transfers the electrons to NAD+, forming NADH.