Radical enzyme that catalyzes the transformation of (2R)-ethylmalonyl-CoA to (2S)-methylsuccinyl-CoA. Is involved in the ethylmalonyl-CoA pathway for acetyl-CoA assimilation required for R.sphaeroides growth on acetate as sole carbon source. Is highly specific for its substrate, ethylmalonyl-CoA, and accepts methylmalonyl-CoA only at 0.2% relative activity. (2R)-ethylmalonyl-CoA = (2S)-methylsuccinyl-CoA Optimum pH is 6.5-8.0. Homodimer. Cells lacking this gene are able to grow with carbon substrates that do not require the operation of the (complete) ethylmalonyl-CoA pathway (succinate, propionate/HCO3(-), or acetate plus glyoxylate) but are unable to use acetate or acetoacetate as the sole carbon source. They show undetectable ethylmalonyl-CoA mutase activity when grown with acetate plus glyoxylate, while are still able to convert methylmalonyl-CoA to succinyl-CoA. Belongs to the methylmalonyl-CoA mutase family.