Steric Course of the Allylic Rearrangement Catalyzed by β-Hydroxydecanoylthioester Dehydrase. Mechanistic Implications

Article abstract of DOI:10.1021/ja00335a060

β-Hydroxydecanoylthioester dehydrase, which is the pivotal enzyme in the biosynthesis of unsaturated fatty acids in anaerobic metabolism, catalyses the equilibration of thio esters of (R)-3-hydroxydecanoic acid, (E)-2-decenoic acid, and (Z)-3-decenoic acid.On the basis of evidence available to date, both two-base and one-base mechanisms of action can be envisioned for dehydrase.In an effort to distinguish between these mechanisms, the stereochemical course of the dehydrase-catalyzed allylic rearrangement has been determined.N-Acetylcysteamine (NAC) thio esters of (R)- and (S)-(E)-<4,5,5-2H3>decanoic acid were synthesized and incubated with dehydrase.The product (Z)-3-decenoyl-NAC was derivatized, and 2H NMR analysis showed that the pro-4R hydrogen had been removed. (E)-2-<2-2H>Decenoyl-NAC and unlabeled (E)-2-decenoyl NAC were incubated with dehydrase in ¹H2O and 2H2O, respectively.Analysis of a derivative of the resulting labeled (Z)-3-decenoyl-NAC showed that protonation had occured on the si face at substrate C-2.The overall steric course of the reaction is therefore suprafacial.The significance of this result is discussed in terms of the mechanisms of the "normal" dehydrase-catalysed reactions as well as the "suicide" inactivation of the enzyme.