(meth'il-glū'tă-kon'il hī'dră-tās), An enzyme that catalyzes the reaction of trans-3-methylglutaconyl-CoA and water to form 3-hydroxy-3-methylglutaconyl-CoA; this enzyme participates in the pathway for l-leucine degradation; a deficiency of this enzyme will result in 3-methylglutaconic aciduria.