Mechanism and Catalytic Site Atlas

In Escherichia coli, the enzyme beta-hydroxydecanoyl thiol ester dehydrase is responsible for the key step, where an unsaturated intermediate in the biosynthetic pathway of saturated fatty acids is intercepted and shunted into the pathway leading to unsaturated products. Dehydrase catalyses two reactions on fatty acid thiol esters of acyl carrier protein (ACP): the dehydration of (R)-3-hydroxydecanoyl-ACP to (E)-2-decenoyl-ACP, a reaction that also occurs in the elongation of saturated fatty acids; and the isomerization of (E)-2-decenoyl-ACP to (Z)-3-decenoyl-ACP. (E)-2-decenoyl-ACP can be reduced to decanoyl-ACP, which is elongated to the usual saturated fatty acids; in contrast, the cis (Z) double bond of (Z)-3-decenoyl-ACP is retained through the further cycles of fatty-acid elongation. The isomerisation catalysed by dehydrase is an allylic rearrangement, which is a relatively simple, single-substrate reaction. Both the dehydration and isomerization reactions seem to occur in the same active site.

3-hydroxydecanoyl-[acyl-carrier protein] dehydratase (dehydrase) is required for the biosynthesis of unsaturated fatty acids, by shunting a 10-carbon intermediate from the saturated fatty acid pathway into the unsaturated fatty acid pathway.

Dehydratase catalyses dehydration and isomerisation reactions by a mechanism that does not involve metals or other cofactors, unlike the majority of the enzymes that catalyse similar reactions. The catalytic site is isolated from solution and is predominantly hydrophobic apart from histidine (A HIS 70) and aspartic acid (B ASP 84), which together are proposed to catalyse the reactions. The reactions take place in a bifunctional active site.