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

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Abstract
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 angstroms 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 unliganded 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.