Catalyzes the conversion of 3-methylglutaconyl-CoA to 3-hydroxy-3-methylglutaryl-CoA. (3S)-hydroxy-3-methylglutaryl-CoA = 3-methyl-(2E)-glutaconyl-CoA + H2O Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 3/3. Homohexamer. Belongs to the enoyl-CoA hydratase/isomerase family.