Catalyzes the isomerization of succinyl-CoA to methylmalonyl-CoA during synthesis of propionate from tricarboxylic acid-cycle intermediates. (R)-methylmalonyl-CoA = succinyl-CoA Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 3/3. Heterodimer of an alpha and a beta chain. Was identified as a high-confidence drug target. Belongs to the methylmalonyl-CoA mutase family.