Catalyzes the reversible isomerization of methylmalonyl-CoA (MMCoA) (generated from branched-chain amino acid metabolism and degradation of dietary odd chain fatty acids and cholesterol) to succinyl-CoA (3-carboxypropionyl-CoA), a key intermediate of the tricarboxylic acid cycle. (R)-methylmalonyl-CoA = succinyl-CoA During catalysis, accumulation of oxidized inactive cofactor hydroxocobalamin (OH2Cbl) leads to loss of MMUT activity (PubMed:21138732, PubMed:28943303). Interaction with MMAA decreases the rate of OH2Cbl formation and promotes the replacement of OH2Cbl by the active cofactor adenosylcobalamin (AdoCbl), thereby restoring MMUT activity (PubMed:21138732, PubMed:28943303). Inhibited by itaconyl-CoA, a metabolite that inactivates the coenzyme B12 cofactor (PubMed:29056341). Inhibited at high concentration of substrate (PubMed:28943303). Homodimer (PubMed:20876572). Interacts (the apoenzyme form) with MMAA; the interaction is GTP dependent (PubMed:20876572, PubMed:21138732, PubMed:28943303). The disease is caused by variants affecting the gene represented in this entry. Belongs to the methylmalonyl-CoA mutase family. Methylmalonyl coenzyme A mutase entry