【兴大报告 Xing Da Lecture 548】

题  目：Studies of SpnF-Catalyzed [4+2]-Cycloaddition in the Biosynthesis of Spinosyn A

报告人:Prof. Hung-wen (Ben) Liu

Division of Chemical Biology and Medicinal Chemistry, College of Pharmacy, and Department of Chemistry, University of Texas at Austin

时　间：2018年6月1日(星期五)下午2:00

地　点：化学楼A204/206

主请人：雷晓光

报告摘要：

The Diels–Alder reactionis a [4+2]-cycloaddition reaction in which a cyclohexene ring is formed between a 1,3-diene and an electron deficient alkene via a single pericyclic transitionstate. This reaction has been proposed as a key transformation in thebiosynthesis of many cyclohexene-containing secondary metabolites. Although several purified enzymes have been implicated in biotransformations that are consistent with a Diels–Alder reaction, these enzymes typically demonstrate more than one catalytic activity, leaving their specific influence on the cycloaddition step uncertain.

 In our studies of the biosynthesis of spinosyn A, a tetracyclic polyketide-derived insecticide from Saccharopolysporaspinosa, we identified a cyclase, SpnF, that catalyzes a transannular [4+2]-cycloaddition to form the cyclohexene ring in the final product. SpnF is unique, because it is the first enzyme that was characterized to specifically catalyze a [4+2]-cycloaddition without introducing any other changes to its substrate. The same cycloaddition also takes placenonenzymatically, but at a much reduced rate. If the reaction catalyzed by SpnF is a concerted process with a single pericyclic transition state, then SpnF would be the first example of a naturally occurring Diels-Alderase. In order to investigate this possibility, alpha-secondary deuterium kinetic isotope effects were measured at all points of rehybridization in the diene during both the nonenzymatic and SpnF-catalyzed [4+2]-cycloaddition reactions. This was accomplished using regiospecifically deuterated substrates and electrospray ionization, time-of-flight mass spectrometry to follow changes in deuterium enrichment of the substrate as the reaction progressed. This presentation will describe the measurement of these KIEs and offer a mechanistic discussion of their implications for understanding the SpnF catalyzed [4+2]-cycloaddition.