KINETIC ANALYSES OF SITE-SPECIFIC MUTANTS OF CARBOXYLESTERASES

该子项目是利用NIH/NCRR资助的中心赠款提供的资源的许多研究子项目之一。子项目和研究者（PI）可能从另一个NIH来源获得主要资金，因此可以在其他CRISP条目中表示。所列机构为中心，不一定是研究者所在机构。羧酸酯酶（Carboxylesterases，CE）是一类能催化酯化外源物质和内源物质水解的代谢酶。拟除虫菊酯杀虫剂是一种含酯的环境污染物，由于其在农业中的广泛使用，在美国东南部广泛存在。当人类暴露于拟除虫菊酯时，CE是肝脏中负责解毒和清除体内这些化学物质的主要酶。然而，CE识别拟除虫菊酯并催化其水解的机制尚未完全表征。例如，氯菊酯是一种广泛使用的拟除虫菊酯，作为顺式和反式异构体的混合物使用。这些异构体被CE酶立体选择性代谢，例如反式异构体的裂解速度明显快于顺式异构体。 这种选择性的原因是什么？这是该试点项目正在解决的主要问题之一。最近，在枯草芽孢杆菌中发现的羧酸酯酶（称为pnb CE）显示与哺乳动物CE兔肝羧酸酯酶具有81%的氨基酸序列同一性和高度的结构同源性（Wierdl等人，2004年）。重要的是，pnb CE不需要被糖基化以具有酶活性，并且可以在E.杆菌因此，可以快速构建pnb CE cDNA中的序列变体，从而在参与底物识别和催化的关键位置产生具有位点特异性氨基酸修饰的酶。该试验项目的目的是利用野生型和pnb CE突变体研究拟除虫菊酯水解的催化机制。研究的中心假设是：突变羧酸酯酶与野生型酶相比改变了底物结合亲和力和/或催化步骤，这些酶将为羧酸酯酶的催化机制提供见解。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Carboxylesterases (CEs) are promiscuous metabolizing enzymes that catalyze the hydrolysis of esterified xenobiotics and endobiotics. The pyrethroid insecticides are ester-containing environmental pollutants widely found in the southeastern U.S. due to their extensive use in the agricultural industry. When humans are exposed to pyrethroids, CEs are the primary enzymes in the liver responsible for detoxifying and clearing these chemicals from the body. However, the mechanism by which CEs recognize pyrethroids and catalyze their hydrolysis is not completely characterized. For instance, permethrin is a widely used pyrethroid used as a mixture of cis- and trans-isomers. These isomers are stereoselectively metabolized by CE enzymes, for example the trans-isomer is cleaved markedly faster than the cis-isomer. What accounts for this selectivity? This is one of the primary questions being addressed by this pilot project. Recently, a carboxylesterase found in Bacillus subtilis (termed pnb CE) was shown to possess 81% amino acid sequence identity and a high degree of structural homology with a mammalian CE, rabbit liver carboxylesterase (Wierdl et al., 2004). Importantly, pnb CE did not need to be glycosylated to be enzymatically active and expression of large amounts of this enzyme can be done rapidly and conveniently in E. coli. Thus, sequence variants in pnb CE cDNA can be quickly constructed yielding enzymes with site-specific amino acid modifications at key locations involved in substrate recognition and catalysis. The objective of this pilot project will be to use wild type and pnb CE mutants to study the catalytic mechanism of pyrethroid hydrolysis. The central hypothesis for the research is: Mutant carboxylesterases have altered substrate binding affinities and/or catalytic steps compared with wild type enzyme, and these enzymes will provide insight into the catalytic mechanism of carboxylesterases.