Methylmalonyl CoA mutase (MUT aka MCM) (Jansen et al. 1989) utilises adenosylcobalamin (AdoCbl) as a cofactor and catalyzes interchange of a carbonyl-CoA group and a hydrogen atom in conversion of methylmalonyl CoA to form succinyl CoA, a precursor for the citric acid cycle. MUT has a homodimeric structure and is located in the mitochondrial matrix. Defects in MUT cause methylmalonic aciduria, mut type (MMAM; MIM:251000), an often fatal disorder of organic acid metabolism (Worgan et al. 2006).
Methylmalonic aciduria type A protein (MMAA) is thought to act as a chaperone to MUT, the enzyme which utilises adenosylcobalamin (AdoCbl) as a cofactor. MMAA is suggested to play a dual role with regards to MUT protection and reactivation. Some AdoCbl-dependent enzymes undergo suicide inactivation after catalysis due to the oxidative inactivation of Cbl. MMAA is thought to play a protective role to prevent MUT being inactivated in this way. After the catalytic cycle when MUT is inactive, MMAA increases the enzymatic activity of MUT through exchange of the damaged cofactor. Whether this happens via GTP-mediated hydrolysis is unknown at present (Takahashi-Iniguez et al. 2011, Froese et al. 2010). Bacterial AdoCbl-containing enzymes possess reactivating factors which release the inactivated cofactor to allow the resulting apoenzyme to reconstitute into an active form. A bacterial orthologue of MMAA, MeaB, forms a stable complex with MUT and plays a role in its protection and reactivation (Padovani & Banerjee 2006).
Defects in MMAA cause methylmalonic aciduria type cblA (cblA aka methylmalonic aciduria type A or vitamin B12-responsive methylmalonicaciduria of cblA complementation type; MIM:251100). Affected individuals accumulate methylmalonic acid in the blood and urine and are prone to potentially life threatening acidotic crises in infancy or early childhood (Dobson et al. 2002, Lerner-Ellis et al. 2004).