Lignin peroxidase compound III. Mechanism of formation and decomposition

Lignin peroxidase compound III (LiPIII) was prepared via three procedures: (a) ferrous LiP + P₂ (LiPIIIa), (b) ferric LiP + O₂^.- (LiPIIIb), and (c) LiP compound II + excess H₂O₂ followed by treatment with catalase (LiPIIIc). LiPIIIa, b, and c each have a Soret maximum at ~ 414 nm and visible bands at 543 and 578 nm. LiPIIIa, b, and c each slowly reverted to native ferric LiP, releasing stoichiometric amounts of O₂^.- in the process. Electronic absorption spectra of LiPIII reversion to the native enzyme displayed isosbestic points in the visible region at 470, 525, and 597 nm, suggesting a single-step reversion with no intermediates. The LiPIII reversion reactions obeyed first-order kinetics with rate constants of ~ 1.0 x 10^-3 s^-1. In the presence of excess peroxide, at pH 3.0, native LiP, LiPII, and LiPIIIa, b, and c are all converted to a unique oxidized species (LiPIII*) with a spectrum displaying visible bands at 543 and 578 nm, but with a Soret maximum at 419 nm, red-shifted 5 nm from that of LiPIII. LiPIII* is bleached and inactivated in the presence of excess H₂O₂ via a biphasic process. The fast first phase of this bleaching reaction obeys second-order kinetics, with a rate constant of 1.7 x 10¹ M^-1 s^-1. Addition of veratryl alcohol to LiPIII* results in its rapid reversion to the native enzyme, via an apparent one-step reaction that obeys second-order kinetics with a rate constant of 3.5 x 10¹ M^-1 s^-1. Stoichiometric amounts of O₂^.- are released during this reaction. When this reaction was run under conditions that prevented further reactions, HPLC analysis of the products demonstrated that veratryl alcohol was not oxidized. These results suggest that the binding of veratryl alcohol to LiPIII* displaces O₂^.-, thus returning the enzyme to its native state. In contrast, the addition of veratryl alcohol to LiPIII did not affect the rate of spontaneous reversion of LiPIII to the native enzyme.