Lignin peroxidase compounds II and III. Spectral and kinetic characterization of reactions with peroxides

Stopped-flow techniques were used to investigate the kinetics of the reaction of lignin peroxidase compounds II and III (LiPII and LiPIII) within peroxides. Rate data were obtained from single-turnover experiments under pseudo-first-order conditions. LiPII reacts with H₂O₂ or peracetic acid (AcOOH) to form a modified LiPIII, designated as LiPIII* via a biphasic reaction. During the first phase, LiPIII is formed as an intermediate. Kinetic analysis also indicates a LiPII-peroxide complex. The first-order dissociation rate constants for the reaction of LiPII with H₂O₂ and AcOOH are 7.9 ± 0.5 and 4.9 ± 0.6 s^-1, respectively. The rate of the H₂O₂ reaction is approximately 500 times the rate of the comparable reaction with horseradish peroxidase, suggesting it is physiologically significant. The activation energy for the formation of LiPIII is 23 kJ mol^-1. During the second phase, the intermediate LiPIII is converted to LiPIII* confirmed by analyzing the reaction of exogenously prepared LiPIII with peroxides. The second-order rate constants for the reaction of LiPIII with H₂O₂ and AcOOH are (3.7 ± 0.2) x 10² M^-1 s^-1 (2.9 ± 0.2) x 10² M^-1 s^-1, respectively. The conversion of LiPIII to LiPIII* is reversible; the first-order rate constant for the reverse reaction is ~(6.6 ± 0.6) x 10² s^-1. The rates of both LiPIII and LiPIII* formation decrease markedly above pH 4.0. The pH dependence of these reactions is controlled by a heme-linked ionizable group of pK(a) ~ 4.2.