Catalyzes the reversible interconversion of isobutyryl-CoA and n-butyryl-CoA, using radical chemistry (PubMed:19864421). Also exhibits GTPase activity, associated with its G-protein domain (MeaI) that functions as a chaperone that assists cofactor delivery and proper holo-enzyme assembly (By similarity). Does not exhibit methylmalonyl-CoA mutase (MCM) activity (PubMed:19864421). 2-methylpropanoyl-CoA = butanoyl-CoA GTP + H2O = GDP + H(+) + phosphate Homodimer. Is composed of four functional domains: the N-terminal 5'-deoxyadenosylcobalamin binding region that is homologous to the small subunit of ICM (IcmB), a middle P-loop GTPase domain (MeaI) that likely acts as a chaperone for ICM, a structured linker region involved in dimer formation, and a C-terminal part that is homologous to the large substrate-binding subunit of ICM (IcmA). Belongs to the IcmF family.