Gazaryan I G, Ouporov I V, Chubar T A, Fechina V A, Mareeva E A, Lagrimini L M
Department of Chemical Enzymology, School of Chemistry, Lomonosov Moscow State University, Moscow, 119899, Russia.
Biochemistry (Mosc). 1998 May;63(5):600-6.
The effect of extremely acidic pH on the stability of tobacco peroxidase and lignin peroxidase holoenzymes has been studied. Stabilization of tobacco peroxidase holoenzyme in the presence of calcium cations at pH < 2 and stabilization of lignin peroxidase at pH > 2 in the presence of veratryl alcohol have been shown. The dependence of the reaction rate constant for hydrogen peroxide interaction with tobacco peroxidase on pH suggests that the reaction rate is under control of a group with pK of 2.5. A tobacco peroxidase model structure has been created by means of homology modeling on the basis of the tobacco peroxidase sequence and the coordinates of peanut peroxidase crystal structure. The model structure demonstrates the presence of the negatively charged Glu-141 at the entrance to the active site and its electrostatic repulsion from heme propionates and triad of Asp-76, -79, and -80 residues. The results on tobacco holoperoxidase stabilization at pH 1.8 in the presence of calcium cations and drop in reaction rate constant for the enzyme interaction with hydrogen peroxide are explained by a hypothetical formation of ionic bonds between Glu-141 and the triad of aspartic acid residues via calcium cation lowering the accessibility of the active site and stabilizing the holoenzyme.
研究了极低pH值对烟草过氧化物酶和木质素过氧化物酶全酶稳定性的影响。结果表明，在pH < 2时，钙阳离子存在下烟草过氧化物酶全酶得以稳定；在pH > 2时，藜芦醇存在下木质素过氧化物酶得以稳定。过氧化氢与烟草过氧化物酶相互作用的反应速率常数对pH的依赖性表明，反应速率受pK为2.5的基团控制。基于烟草过氧化物酶序列和花生过氧化物酶晶体结构坐标，通过同源建模构建了烟草过氧化物酶模型结构。该模型结构表明，活性位点入口处存在带负电荷的Glu-141，它与血红素丙酸酯以及Asp-76、-79和-80残基三联体存在静电排斥。在钙阳离子存在下，烟草全过氧化物酶在pH 1.8时的稳定以及该酶与过氧化氢相互作用的反应速率常数下降，可通过Glu-141与天冬氨酸残基三联体之间经由钙阳离子形成离子键的假设来解释，这种离子键降低了活性位点的可及性并稳定了全酶。
