菌株Sphingopyxis sp. DBS4代谢除草剂2,4-滴丙酸手性异构体的分子机制

Dichlorprop (DCPP) is a phenoxy acid herbicide which has two chiral isomers. Since different chiral isomers show different herbicidal activities and ecotoxicological activities, the metabolic mechanism by microbes of different isomers in the environment are of great concern. However, the catabolic mechanisms involved in the microbial degradation of different isomers of DCPP are still insufficient. A strain named Sphingopyxis sp. DBS4 that can simultaneously mineralize R-DCPP and S-DCPP was obtained, a novel dioxygenase for the initial degradation of S-DCPP was preliminarily identified, and no reported conserved gene rdpA for the initial degradation of R-DCPP was found in the genome of strain DBS4. Suggesting that novel DCPP initial degrading genes exist. In this project, we will clone the key genes responsible for R-DCPP and S-DCPP initial degradation, characterize their enzymatic properties, substrate specificity and the catalytic mechanism for different isomers, reveal the complete metabolic pathway and the key genes involved in the downstream pathway of (R, S)-DCPP degradation. The progress of this project will enrich the theory of microbial metabolic mechanism of chiral herbicides, and also provide new enzyme resources for the chiral separation of DCPP.

2,4-滴丙酸(DCPP)是具有R-和S-两种异构体的苯氧羧酸类手性除草剂，由于不同异构体具有不同的除草活性和生态毒理，其不同异构体在环境中由微生物参与的代谢规律受到广泛关注。然而，关于DCPP不同异构体的微生物代谢机制研究仍很不充分。申请人前期分离到一株能同时矿化R-DCPP和S-DCPP的菌株Sphingopyxis sp. DBS4，并从中初步鉴定到一个新的针对S-DCPP起始降解的双加氧酶，且菌株DBS4中未发现针对R-DCPP起始降解的已报道的保守基因rdpA，强烈暗示菌株DBS4中有新的起始降解酶基因。本项目将从菌株DBS4中克隆DCPP两种手性异构体的起始降解酶基因，研究其编码酶的酶学特性、底物特异性及对不同异构体的催化机制，揭示(R,S)-DCPP的完整代谢途径和下游途径中关键酶基因。项目成果不仅能丰富手性除草剂的微生物代谢理论，还能为实现DCPP的手性拆分提供新的酶资源。

手性化合物的不同异构体具有不同的生物活性和生态毒理。除草剂2,4-滴丙酸 (DCPP)是一种典型的手性化合物，拥有(R)-和(S)-两种对映异构体，其不同异构体在环境中由微生物参与的代谢规律被广泛关注。本项目通过对一株能同时矿化(R)-DCPP和(S)-DCPP的菌株Sphingopyxis sp. DBS4进行基因组完成图测序，结合目的蛋白的分级纯化，最终从菌株DBS4的基因组中克隆到两个新的，分别针对(R)-DCPP和(S)-DCPP起始降解的双加氧酶基因rpoA和spoA。酶学特性研究发现，SpoA的最适反应温度和pH值分别为30 ℃，8.0。SpoA对(S)-DCPP，2,4-D 以及(S)-2-甲基-4-氯苯氧丙酸 [(S)-MCPP]的Km 值分别为134.3 ± 7.3 μM，343.5 ± 12.3 μM和176.7 ± 9.1 μM。此外，利用同源模建和分子对接等方法确定氨基酸残基Lys101和Asp119为影响SpoA催化活性的关键氨基酸位点，阐明了SpoA蛋白对(S)-DCPP和(R)-DCPP的催化机制差异。RpoA的最适反应温度和pH值分别为30 ℃，7.5。RpoA对(R)-DCPP，2,4-D和(R)-MCPP的Km 值分别为254.6 ± 12.1 μM、445.7 ± 14.7 μM和226.3 ± 9.8 μM。通过对菌株DBS4基因组的比对分析，克隆到了负责2,4-二氯苯酚 (2,4-DCP)下游代谢的完整基因簇tfdBⅠCⅠDⅠEⅠFⅠ和tfdBⅡCⅡDⅡEⅡFⅡ。因此，最终阐明了菌株DBS4代谢(R)-DCPP和(S)-DCPP的分子机制。即RpoA和SpoA分别催化(R)-DCPP和(S)-DCPP转化为中间产物2,4-DCP，随后2,4-DCP被TfdBⅠCⅠDⅠEⅠFⅠ和TfdBⅡCⅡDⅡEⅡFⅡ催化经2,4-D的经典代谢途径逐步降解，直至开环，并最终进入TCA循环。