Catalyzes the reversible interconversion of isobutyryl-CoA and n-butyryl-CoA, and to a lesser extent, of pivalyl-CoA and isovaleryl-CoA, using radical chemistry. 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. Also displays ATPase activity. Is not able to convert 3-hydroxybutyryl-CoA to 2-hydroxyisobutyryl-CoA. Does not exhibit methylmalonyl-CoA mutase (MCM) activity. 2-methylpropanoyl-CoA = butanoyl-CoA 3-methylbutanoyl-CoA = 2,2-dimethylpropanoyl-CoA GTP + H2O = GDP + H(+) + phosphate kcat is 3.1 sec(-1) for ICM activity in the absence of nucleotides, at pH 7.5 and 37 degrees Celsius. Activity might be 4-fold higher at 60 degrees Celsius, the optimal growth temperature for this organism. kcat is 1.96 sec(-1) for ICM activity in the presence of GDP, at pH 7.5 and 37 degrees Celsius. kcat is 1.88 sec(-1) for ICM activity in the presence of GTP, at pH 7.5 and 37 degrees Celsius (PubMed:19864421). kcat is 10 min(-1) for GTPase activity and 19 min(-1) for ATPase activity (at 37 degrees Celsius) (PubMed:22167181). 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.