THE CRYSTAL-STRUCTURE OF ENOYL-COA HYDRATASE COMPLEXED WITH OCTANOYL-COA REVEALS THE STRUCTURAL ADAPTATIONS REQUIRED FOR BINDING OF A LONG-CHAIN FATTY ACID-COA MOLECULE

The structure of the hexameric rat mitochondrial enoyl-Coenzyme A (CoA ) hydratase, co-crystallised with the inhibitor octanoyl-CoA, has been refined at a resolution of 2.4 Angstrom. Enoyl-CoA hydratase catalyse s the hydration of 2,3-unsaturated enoyl-CoA thioesters. In the crysta l structure only four of the six active sites of the hexamer in the as ymmetric unit are occupied with a ligand molecule, showing an unligand ed and a liganded active site within the same crystal form. While the protein assembly and fold is identical to the previously solved acetoa cetyl-CoA complex, differences are observed close to the fatty acid bi nding pocket due to the different nature of the Ligands. The fatty aci d tail of octanoyl-CoA is bound in an extended conformation. This is p ossible because a high B-factor loop, which separates in the acetoacet yl-CoA complex the binding pocket of the acetoacetyl-CoA fatty acid ta il from the intertrimer space, has moved aside to allow binding of the longer octanoyl-CoA moiety. The movement of this loop opens a tunnel which traverses the complete subunit from the solvent space to the int ertrimer space. The conformation of the catalytic residues is identica l, in both structures as well as in the liganded and the unliganded ac tive sites. Ln the unliganded active sites a water molecule is bound b etween the two catalytic glutamate residues, Glu144 and Glu164. After superposition of a Liganded active site on an unliganded active site t his water molecule is close to the carbon centre that becomes hydroxyl ated in the hydratase reaction. These findings support the idea that t he active site is rigid and that the catalytic residues and the water molecule, as seen in the unliganded active site, are pre-positioned fo r very efficient catalysis. (C) 1998 Academic Press Limited.