Irreversible oxidation of ferricytochrome c by lignin peroxidase

Lignin peroxidase (LiP) from Phanerochaete chrysosporium catalyzes irreversible oxidative damage to ferricytochrome c (Cc3+) in the presence of H2O2 and 3,4-dimethoxybenzyl (veratryl) alcohol (VA). Atomic absorption analysis and UV/vis spectroscopy indicate that the oxidation of Cc3+ is accompanied by a loss of heme iron from the protein and probably oxidation of the porphyrin ring. At H2O2 concentrations of 7.5 microM or higher, this oxidation of Cc3+ by LiP is strictly dependent on the presence of VA. The latter is not oxidized to veratraldehyde at a significant rate in the presence of either ferrocytochrome c (Cc2+) or Cc3+, indicating it is not stimulating the reactions by specifically reducing LiP compound II. LiP is inactivated rapidly in 100 microM H2O2, and the presence of 500 microM VA protects LiP from this inactivation. Neither 20 microM Cc3+ nor 20 microM VA alone can protect LiP from inactivation; however, 20 microM each of VA and Cc3+ together protect LiP fully. This and other results strongly suggest that VA is acting as a protein-bound redox mediator in the oxidation of Cc3+. SDS-PAGE analysis of the Cc3+ oxidation products demonstrates the formation of some covalently linked dimer of Cc3+ in addition to the oxidized Cc3+ monomer. Amino acid analysis of the dimeric and monomeric products indicates the presence of oxidized Met and Tyr residues. This suggests that Tyr residues on the surface of the protein are oxidized to Tyr radicals during LiP oxidation and that some of these radicals subsequently undergo intermolecular radical coupling, resulting in dimerization of some of the Cc3+ molecules. However, most of the Cc3+ molecules appear to be irreversibly oxidized without dimerization. These results demonstrate that Cc3+ can serve as a useful polymeric model of the lignin substrate in studying the enzymatic mechanism of lignin oxidation and the role of VA in the reaction.