Crystal structure of enoyl-coenzyme A (CoA) hydratase at 2.5 angstrom resolution: A spiral fold defines the CoA-binding pocket

The crystal structure of rat liver mitochondrial enoyl-coenzyme A (CoA) hydratase complexed with the potent inhibitor acetoacetyl-CoA has been refined at 2.5 Angstrom resolution. This enzyme catalyses the reversible addition of water to alpha beta-unsaturated enoyl-CoA thioesters, with nearly diffusion-controlled reaction rates for the best substrates, Enoyl-CoA hydratase is a hexamer of six identical subunits of 161 kDa molecular mass for the complex, The hexamer is a dimer of trimers. The monomer is folded into a right-handed spiral of four turns, followed by two small domains which are involved in trimerization, Each turn of the spiral consists of two beta-strands and an alpha-helix. The mechanism for the hydratase/dehydratase reaction follows a syn-stereochemistry, a preference that is opposite to the nonenzymatic reaction. The active-site architecture agrees with this stereochemistry. It confirms the importance of Glu164 as the catalytic acid for providing the alpha-proton during the hydratase reaction. It also shows the importance of Glu144 as the catalytic base for the activation of a water molecule in the hydratase reaction. The comparison of an un-liganded and a liganded active site within the same crystal form shows a water molecule in the unliganded subunit. This water molecule is bound between the two catalytic glutamates and could serve as the activated water during catalysis.