植物体系中甲酯酶的底物专一性的理论研究

酶对天然底物的高度专一性是酶的特点之一. 然而关于酶是如何对底物具有高度专一性以及识别能力, 我们的理解仍然缺乏. 本文以植物体系中发现的一组甲酯酶(MESs)对一些底物[包括水杨酸甲酯(MeSA), 茉莉酮酸甲酯(MeJA)和吲哚-3-乙酸甲酯(MeIAA)]的催化反应为例, 报道了同源建模和理论计算对茉莉酮酸甲酯酶(AtMES10)和水杨酸结合蛋白2(SABP2)的研究结果. 基于简单的锁-钥匙理论(底物与酶结合时不发生基团的碰撞或严重排斥), 以底物对接到酶的活性部位(即底物中—COO的一部分占据可被催化丝氨酸亲核进攻的位置) 为原则, 可以在空间上为酶对底物的专一性提供解释. 模拟结果表明, SABP2可对MeSA有高活性, 对MeJA和MeIAA有低或无活性; AtMES10可对MeJA有高活性, 而对MeSA和MeIAA有低或无活性, 这与实验结果相一致. 因此, 相关酶的结构预测与计算机模拟对了解酶的底物专一性具有重要的意义.

Understanding Substrate Specificity of Related Plant Methylesterases(MESs) from Computational Investigations

One of enzyme’s hallmarks is the high specificity to their natural substrates. But our understanding is still lacking concerning how enzymes could achieve high specificity and substrate discrimination. This is the case for a group of related methylesterases(MESs) identified in plants that catalyze reactions of different substrates, including methyl salicylate(MeSA), methyl jasmonate(MeJA) and methyl indole-3-acetate(MeIAA). In this work, a homology model was built for AtMES10(a MeJA esterase),and this model along with the X-ray structure of SABP2(a MeSA esterase) was used to understand their substrate specificity. It is shown that the specificity may be explained based on the simple Lock-and-Key Model(that is, the active site being complementary in shape to the substrate) along with the requirement that the —COO moiety involved in the reaction occupies a position allowing the nucleophilic attack by the catalytic serine(that is, in a reactive configuration). The active site of SABP2 appears not to be complementary in shape to MeJA, and this may lead to a low activity on MeJA. For AtMES10, certain bulky residues in SABP2 are replaced by relatively small residues, allowing the substrate to bind to the active site and to be catalyzed by the enzyme. The results are consistent with the substrate specificity of these two enzymes observed experimentally. Explanations are also provided for the lack of the activities of AtMES10 and SABP2 on MeIAA and the lack of the activity of AtMES10 on MeSA.