碳酸酐酶II催化二氧化碳水合反应的活化参数。

Activation parameters for the carbonic anhydrase II-catalyzed hydration of CO2.

作者信息

Ghannam A F, Tsen W, Rowlett R S

出版信息

J Biol Chem. 1986 Jan 25;261(3):1164-9.

Abstract

We have determined the activation parameters of kcat and kcat/Km for the carbonic anhydrase II-catalyzed hydration of CO2. The enthalpy and entropy of activation for kcat is 7860 +/- 120 cal mol-1 and -3.99 +/- 0.42 cal mol-1 K-1, respectively, for the human enzyme. Results for the bovine enzyme were statistically indistinguishable from those of the human enzyme. The entropy of activation of kcat for the human enzyme was further decomposed into partially compensating electrostatic(es) (delta Ses = +15.1 cal mol-1 K-1) and nonelectrostatic(nes) (delta Snes = -19.1 cal mol-1 K-1) terms. Computer simulations of a formal kinetic mechanism for carbonic anhydrase II-catalyzed CO2 hydration show that 82% of the temperature effect on kcat can be attributed to the temperature effect on the intramolecular proton transfer step. The reported activation parameters are consistent with a substantial enzyme or active site solvent conformational change in the transition state of the intramolecular proton transfer step, and is consistent with the mechanism of proton transfer proposed by Venkatasubban and Silverman (Venkatasubban, K. S., and Silverman, D. N. (1980) Biochemistry 19, 4984-4989).

摘要

我们已经确定了碳酸酐酶II催化CO₂水合反应的kcat和kcat/Km的活化参数。对于人源酶，kcat的活化焓和活化熵分别为7860±120 cal mol⁻¹和 -3.99±0.42 cal mol⁻¹ K⁻¹。牛源酶的结果在统计学上与人源酶的结果没有差异。人源酶kcat的活化熵进一步分解为部分补偿的静电（es）（δSes = +15.1 cal mol⁻¹ K⁻¹）和非静电（nes）（δSnes = -19.1 cal mol⁻¹ K⁻¹）项。碳酸酐酶II催化CO₂水合反应的形式动力学机制的计算机模拟表明，对kcat的温度影响的82%可归因于对分子内质子转移步骤的温度影响。所报道的活化参数与分子内质子转移步骤过渡态中酶或活性位点溶剂的大量构象变化一致，并且与Venkatasubban和Silverman提出的质子转移机制一致（Venkatasubban, K. S., and Silverman, D. N. (1980) Biochemistry 19, 4984 - 4989）。