A kinetic model is proposed for catalysis by an enzyme that has several special characteristics: (i) it catalyses an acyl-transfer bi-substrate reaction between two identical molecules of substrate, (ii) the substrate is an amphiphilic molecule that can be present in two physical forms, namely monomers and micelles, and (iii) the reaction progresses through an acyl-enzyme-based mechanism and the covalent intermediate can react also with water to yield a secondary hydrolytic reaction. The theoretical kinetic equations for both reactions were deduced according to steady-state assumptions and the theoretical plots were predicted. The experimental kinetics of lysophosphatidylcholine:lysophosphatidylcholine acyltransferase from rabbit lung fitted the proposed equations with great accuracy. Also, kinetics of inhibition by products behaved as expected. It was concluded that the competition between two nucleophiles for the covalent acyl-enzyme intermediate, and not a different enzyme action depending on the physical state of the substrate, is responsible for the differences in kinetic pattern for the two activities of the enzyme. This conclusion, together with the fact that the kinetic equation for the transacylation is quadratic, generates a 'hysteretic' pattern that can provide the basis of self-regulatory properties for enzymes to which this model could be applied.

译文

提出了一种动力学模型,用于通过具有以下几个特殊特征的酶进行催化 :( i) 它催化两个相同的底物分子之间的酰基转移双底物反应,(ii) 底物是两亲分子,可以以两种物理形式存在,即单体和胶束,(iii) 反应通过基于酰基酶的机理进行,共价中间体也可以与水反应产生二次水解反应。根据稳态假设推导了两个反应的理论动力学方程,并预测了理论曲线。溶血磷脂酰胆碱的实验动力学: 来自兔肺的溶血磷脂酰胆碱酰基转移酶非常准确地拟合了所提出的方程。此外,产品抑制的动力学表现与预期的一样。结论是,两个亲核试剂之间对共价酰基酶中间体的竞争,而不是取决于底物物理状态的不同酶作用,是造成酶两种活性动力学模式差异的原因。该结论以及转酰化的动力学方程是二次的事实,产生了 “滞后” 模式,可以为可以应用该模型的酶提供自我调节特性的基础。

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