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 Inhibited by itaconyl-CoA, a metabolite that inactivates the coenzyme B12 cofactor. Homodimer. Interacts (the apoenzyme form) with MMAA; the interaction is GTP dependent. Belongs to the methylmalonyl-CoA mutase family.