Catalyzes the isomerization of succinyl-CoA to methylmalonyl-CoA during synthesis of propionate from tricarboxylic acid-cycle intermediates. This conversion most likely represents an important source of building blocks for polyketide antibiotic biosynthesis. It is unable to catalyze the conversion of isobutyryl-CoA into N-butyryl-CoA. (R)-methylmalonyl-CoA = succinyl-CoA Heterodimer of an alpha and a beta chain. Belongs to the methylmalonyl-CoA mutase family.