The role of calcium in the hydrolysis of the organophosphate paraoxon by human serum A-esterase.
The role of calcium in the hydrolysis of the organophosphate paraoxon by human serum A-esterase.
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DOI：
10.1016/0024-3205(94)00422-o
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发表时间：
1995
期刊：
影响因子：
6.1
通讯作者：
James A. Vitarius;L. Sultatos
中科院分区：
文献类型：
--
作者：
James A. Vitarius;L. Sultatos
人血清A-酯酶是一种钙依赖性酶，其通过有序Uni Bi动力学机制水解有机磷对氧磷。不同浓度的氯化钙与人血清A-酯酶孵育，导致反应的appk 3和appE发生相应的变化，而appk 2不受影响。羧基谷氨酸（CAG）阻止氯化钙改变appk 3，但不能阻止appE.类似的CAG减少钙刺激的对氧磷的非酶水解，以及钙刺激的对氧磷磷酸化的胰凝乳蛋白酶的去磷酸化。这些结果表明，钙在A-酯酶水解对氧磷中起两种作用。首先，钙是必需的，以维持一个活跃的网站。在这种能力下，钙可能直接参与催化反应，或者为了保持活性位点的适当确认，可能需要钙。第二，游离钙（或钙每周与A-酯酶）有利于去除磷酸二乙酯从A-酯酶，可能是通过极化磷酸二乙酯-A-酯酶中间体的P= O键，从而使磷更容易受到氢氧离子的亲核攻击。
Human serum A-esterase is a calcium-dependent enzyme that hydrolyzes the organophosphate paraoxon by an Ordered Uni Bi kinetic mechanism. Incubation of various concentrations of calcium chloride with human serum A-esterase resulted in corresponding changes in appk 3 and appE for the reaction, while appk 2 was unaffected. Carboxyglutamic acid (CAG) prevented calcium chloride from altering appk 3, but not appE. Similary CAG reduced the calcium-stimulated nonenzymatic hydrolysis of paraoxon, as well as the calcium-stimulated de-phosphorylation of chymotrypsin phosphorylated by paraoxon. These results suggest that calcium plays two roles in the hydrolysis of paraoxon by A-esterase. Firstly, calcium is required in order to maintain an active site. In this capacity calcium might participate directly in the catalytic reaction, or it might be required in order to maintain the appropriate confirmation of the active site. And secondly, free calcium (or calcium weekly associated with A-esterase) facilitates the removal of diethyl phosphate from A-esterase, probably by polarizing the P= O bond of the diethyl phosphate-A-esterase intermediate, thereby rendering phosphorus more susceptible to nucleophilic attack by hydroxide ions.