The three-dimensional structure of the enzyme B-hydroxydecanoyl thioester dehydrase from E. coli will be determined by X-ray crystallography. This enzyme is essential for biosynthesis of unsaturated fatty acids in E. coli by catalyzing both dehydration and double-bond isomerization reactions. The enzyme is known for its property of suicide inhibition by thiol esters of 3-decynoic acid. This specific, irreversible, covalent modification of the enzyme will be exploited to provide heavy-atom labels for the crystallographic structure determination. The crystal structure will be solved by use of anomalous dispersion due to heavy-atoms incorporated in the inhibitor compounds. Very strongly diffracting crystals of the subject enzyme indicate that the structure determination can proceed to very high resolution. Accordingly, crystallographic studies at ultra-low temperature will be pursued in order to define structural details to the fullest extent possible. The results of the structure determination are expected to explain the enzyme's high degree of specificity for substrate acyl, to verify that a single active site is responsible for catalysis of the two enzymatic reactions, to point out possible similarities to other proteins that bind hydrophobic molecules, and to provide additional data on atomic mobility in protein molecules.
