Catalyzes the reversible conversion of succinyl-CoA to (R)-methylmalonyl-CoA through a radical mechanism (PubMed:9772164). Is involved in the fermentation of pyruvate to propanoate that occurs in Propionibacteria (Probable). (R)-methylmalonyl-CoA = succinyl-CoA kcat is 48 sec(-1) with succinyl-CoA as substrate. Heterodimer of an alpha and a beta chain. Belongs to the methylmalonyl-CoA mutase family.