Romero P J, de Meis L
Departamento de Bioquimica, Universidade Federal do Rio de Janeiro, Brazil.
J Biol Chem. 1989 May 15;264(14):7869-73.
The observed equilibrium constants for hydrolysis (Kobs) of a phosphoester and a phosphoanhydride bond were measured under a variety of conditions likely to alter the interactions of reactants and products with water. These included increasing the pH of the medium from 5.0 to 10.0, increasing the MgCl2 concentration form 0 to 200 mM, and decreasing the water activity of the medium by adding either dimethyl sulfoxide (50%, v/v) or polyethylene glycol 6,000-8,000 (50%, w/v). The Kobs for phosphoesters such as phosphoserine, glucose phosphate, glycerol phosphate, and ethylene glycol phosphate varied little over this wide range of conditions, the extreme values of Kobs being 12 and 200 M. In contrast, the Kobs for the phosphoanhydride bond of pyrophosphate varied from a value greater than 20,000 to 0.1 M. In totally aqueous media at a pH between 7.0 and 8.0 and in the presence of 0.5-1.0 mM MgCl2, the energy of hydrolysis of pyrophosphate was 1.2-4.0 kcal/mol greater than that of phosphoserine. However, when the water activity was decreased by adding polyethylene glycol to the medium within the same pH and MgCl2 concentration range, the energy of hydrolysis of phosphoserine became 2.0-2.5 kcal/mol greater than that of pyrophosphate. The results suggest that for phosphoesters, the solvation energies of reactants and products, unlike the case of phosphoanhydride bonds, are not the major factors in determining the energy of hydrolysis.
在各种可能改变反应物和产物与水相互作用的条件下，测定了磷酸酯和磷酸酐键水解的观测平衡常数（Kobs）。这些条件包括将介质的pH从5.0提高到10.0，将MgCl2浓度从0提高到200 mM，以及通过添加二甲基亚砜（50%，v/v）或聚乙二醇6000 - 8000（50%，w/v）来降低介质的水活度。在如此广泛的条件范围内，磷酸丝氨酸、葡萄糖磷酸、甘油磷酸和乙二醇磷酸等磷酸酯的Kobs变化很小，Kobs的极值为12和200 M。相比之下，焦磷酸磷酸酐键的Kobs从大于20000变化到0.1 M。在pH为7.0至8.0的完全水性介质中，以及在存在0.5 - 1.0 mM MgCl2的情况下，焦磷酸的水解能比磷酸丝氨酸的水解能高1.2 - 4.0 kcal/mol。然而，当在相同的pH和MgCl2浓度范围内向介质中添加聚乙二醇以降低水活度时，磷酸丝氨酸的水解能比焦磷酸的水解能高2.0 - 2.5 kcal/mol。结果表明，对于磷酸酯而言，与磷酸酐键的情况不同，反应物和产物的溶剂化能不是决定水解能的主要因素。
