Together with HcmB, catalyzes the isomerization of 2-hydroxyisobutyryl-CoA and 3-hydroxybutyryl-CoA. Is specific for 2-hydroxyisobutyryl-CoA and (S)-3-hydroxybutyryl-CoA, and shows only very low activity with (R)-3-hydroxybutyryl-CoA, isobutyryl-CoA and butyryl-CoA (PubMed:22433853, PubMed:25720495). In vitro, can isomerize pivalyl-CoA and isovaleryl-CoA, with much lower efficiency (PubMed:25720495). Plays a central role in the degradation of substrates bearing a tert-butyl moiety, such as the fuel oxygenate methyl tert-butyl ether (MTBE) and its metabolites (PubMed:22433853). 2-hydroxyisobutanoyl-CoA = (3S)-3-hydroxybutanoyl-CoA kcat is 12 min(-1) with (S)-3-hydroxybutyryl-CoA as substrate. kcat is 3.0 min(-1) with 2-hydroxyisobutyryl-CoA as substrate. kcat is 0.20 min(-1) with (R)-3-hydroxybutyryl-CoA as substrate. kcat is 0.20 min(-1) with butyryl-CoA as substrate. kcat is 0.06 min(-1) with isobutyryl-CoA as substrate (PubMed:22433853). kcat is 0.98 min(-1) with pivalyl-CoA as substrate. kcat is 0.68 min(-1) with isovaleryl-CoA as substrate (PubMed:25720495). Optimum pH is 6.6. Optimum temperature is 30 degrees Celsius. Homotetramer composed of two large substrate-binding subunits (HcmA) and two small cobalamin-binding subunits (HcmB). Insertion mutant cannot grow on 2-hydroxyisobutyric acid (2-HIBA). Belongs to the acyl-CoA mutase large subunit family.