Radical enzyme that catalyzes the transformation of (2R)-methylmalonyl-CoA to succinyl-CoA. Is involved in the ethylmalonyl-CoA pathway for acetyl-CoA assimilation required for R.sphaeroides growth on acetate as sole carbon source. (R)-methylmalonyl-CoA = succinyl-CoA Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 3/3. Homodimer. Strongly up-regulated during growth on acetate as sole carbon source. Belongs to the methylmalonyl-CoA mutase family.