Acyl carrier protein. XVII. Purification and properties of -hydroxyacyl acyl carrier protein dehydrase.

/3-Hydroxyacyl acyl carrier protein (ACP) dehydrase has been purified 2900-fold from extracts of Escherichia coli. The enzyme catalyzes the reversible dehydration of /3-hydroxyacyl-ACP thioesters to yield specifically tram-2-enoylACP products. It is active with frans-2-enoyl-ACP thioesters of chain lengths from 4 through 16 carbon atoms. The enzyme catalyzes the hydration of cis-5frans-2-dodecadienoyl-ACP, an intermediate in unsaturated fatty acid synthesis, as well as the trans-2-enoyl-ACP substrates that are intermediates in saturated fatty acid synthesis. Thus this enzyme can function in the synthesis of both saturated and unsaturated fatty acids. The lowest enzyme activity was noted with frans-2-decenoyl-ACP. The relevance of this finding with respect to the activity of P-hydroxydecanoyl thioester dehydrase, which specifically catalyzes the conversion of fl-hydroxydecanoyl-ACP to cis-3-decenoyl-ACP is discussed as a possible factor in control of saturated versus unsaturated fatty acid synthesis. Attempts to separate or to show distinguishing characteristics of @hydroxyacyl-ACP dehydrase activities with short, medium, or long chain substrates were unsuccessful, and these activities are therefore attributed to a single enzyme.