Lignin peroxidase of Phanerochaete chrysosporium: Evidence for an acidic ionization controlling activity

The active site amino acid residues of lignin peroxidase are homologous to those of other peroxidases; however, in contrast to other peroxidases, no pH dependence is observed for the reaction of ferric lignin peroxidase with H₂O₂ to form compound I (Andrawis, A., Johnson, K. A., and Tien, M. (1988) J. Biol. Chem. 263, 1195-1198). Chloride binding is used in the present study to investigate this reaction further. Chloride binds to lignin peroxidase at the same site as cyanide and hydrogen peroxide. This is indicated by the following. 1) Chloride competes with cyanide in binding to lignin peroxidase. 2) Chloride is a competitive inhibitor of lignin peroxidase with respect to H₂O₂. The inhibition constant (K_i) is equal to the dissociation constant (K_d) of chloride at all pH values studied. Chloride binding is pH dependent: chloride binds only to the protonated form of lignin peroxidase. Transient-state kinetic studies demonstrate that chloride inhibits lignin peroxidase compound I formation in a pH-dependent manner with maximum inhibition at low pH. An apparent pK_a was calculated at each chloride concentration; the pK_a increased as the chloride concentration increased. Extrapolation to zero chloride concentration allowed us to estimate the intrinsic pK_a for the ionization in the lignin peroxidase active site. The results reported here provide evidence that an acidic ionizable group (pK_a ≈ 1) at the active site controls both lignin peroxidase compound I formation and chloride binding. We propose that the mechanism for lignin peroxidase compound I formation is similar to that of other peroxidases in that it requires the deprotonated form of an ionizable group near the active site.