Mechanism of Chain Extension Step in Biosynthesis

摘要

The chain extension step in the enzymatic synthesis of fatty acid synthase, involving a formal Claisen condensation of thio esters, has been clarified by theoretical calculations for model systems, using the modified neglect of diatomic overlap and Austin Model 1 parametric self-consistent field molecular orbital procedures. The reaction involves a free carbanion, formed by decarboxylation of a malonate ion. Formation of the carbanion and condensation with the fatty acid thio ester are not concerted. The decarboxylation is strongly endothermic. It is brought about by electrostatic interaction (field effect) with an ammonium ion derived from an adjacent lysine residue, the ions being far enough apart to inhibit proton transfer between them. Proton transfer would lead to an enol that is predicted not be able to undergo the Claisen condensation. The formation of the ammonium ion is considered in terms of the pK sub a of the relevant groups. The bearing of this work on a recent interpretation of the activity and selectivity of enzyme reactions is discussed, and some misunderstandings concerning this interpretation are clarified. Reprints. (aw)