β-Hydroxydecanoyl Thioester Dehydrase

The fl-hydroxydecanoyl thioester dehydrase of Escherichia coli has a high degree of chain length specificity, catalyzing the dehydration of p-hydroxydecanoyl-N-acetylcysteamine at more than 10 times the rate of the corresponding CS and Cl2 compounds.The enzyme acts on different thioesters of P-hydroxydecanoic acid at the following relative rates: acyl carrier protein, 6; N-acetylcysteamine, 1; and pantetheine, 0.6.In each case, both a,p-and /3 ,y-unsaturated products are formed.The debydrase also catalyzes the interconversion and hydration of the isomeric decenoyl thioesters.The same equilibrium mixture is obtained when either P-hydroxydecanoyl-, tram-a, /3-decenoyl-, or cis+, y-decenoyl-N-acetylcysteamine is used as substrate.Experiments are described which attempt to delineate the pathway of interconversion of the different substrates, and which argue for a single, multifunctional enzyme catalyzing the transformations.In accord with the proposed pathway for the synthesis of long chain acids in E. coli, the bacterial fatty acid synthetase catalyzes the chain elongation of CY ,fi-decenoyl acyl carrier protein to mainly saturated acids, and of /I-hydroxydecanoyl and p, y-decenoyl acyl carrier protein, to mixtures of long chain saturated and unsaturated acids.Explanations are offered for the apparent paradox that the isolated dehydrase converts fl-hydroxydecanoyl thioesters primarily to a! ,pdecenoate, whereas the fatty acid synthetase, of which the dehydrase is a component, catalyzes the formation of predominantly unsaturated acids, the elongation products of @, y-decenoate.In certain bacteria, the biosynthesis of unsaturated fatty acids is closely connected with chain elongation (l-3).