EFFECT OF PH ON TOBACCO ANIONIC PEROXIDASE STABILITY AND ITS INTERACTION WITH HYDROGEN-PEROXIDE

The effect of extremely acidic pH on the stability of tobacco peroxida se and lignin peroxidase holoenzymes has been studied. Stabilization o f tobacco peroxidase holoenzyme in the presence of calcium cations at pH < 2 and stabilization of lignin peroxidase at pH > 2 in the presenc e of veratryl alcohol have been shown. The dependence of the reaction rate constant for hydrogen peroxide interaction with tobacco peroxidas e on pH suggests that the reaction rate is under control of a group wi th pK of 2.5. A tobacco peroxidase model structure has been created by means of homology modeling on the basis of the tobacco peroxidase seq uence and the coordinates of peanut peroxidase crystal structure. The model structure demonstrates the presence of the negatively charged Gl u-141 at the entrance to the active site and its electrostatic repulsi on from heme propionates and triad of Asp-76, -79, and -80 residues. T he results on tobacco holoperoxidase stabilization at pH 1.8 in the pr esence of calcium cations and drop in reaction rate constant for the e nzyme interaction with hydrogen peroxide are explained by a hypothetic al formation of ionic bonds between Glu-141 and the triad of aspartic acid residues via calcium cation lowering the accessibility of the act ive site and stabilizing the holoenzyme.