Role of Tyr residues on the protein surface of cationic cell-wall-peroxidase (CWPO-C) from poplar: potential oxidation sites for oxidative polymerization of lignin.

It was previously reported that an unique peroxidase isoenzyme, cationic cell-wall-bound peroxidase (CWPO-C), from poplar callus oxidizes sinapyl alcohol, ferrocytochrome c and synthetic lignin polymers, unlike other plant peroxidases. Here, the catalytic mechanism of CWPO-C was investigated using chemical modification and homology modeling. The simulated CWPO-C structure predicts that the entrance to the heme pocket of CWPO-C is the same size as those of other plant peroxidases, suggesting that ferrocytochrome c and synthetic lignin polymers cannot interact with the heme of CWPO-C. Since Trp and Tyr residues are redox-active, such residues located on the protein surface were predicted to be active sites for CWPO-C. Modification of CWPO-C Trp residues did not suppress its oxidation activities toward guaiacol and syringaldazine. On the other hand, modification of CWPO-C Tyr residues using tetranitromethane strongly suppressed its oxidation activities toward syringaldazine and 2,6-dimethoxyphenol by 90%, respectively, and also suppressed its guaiacol oxidation activity to a lesser extent. Ferrocytochrome c was not oxidized by Tyr-modified CWPO-C. These results indicate that the Tyr residues in CWPO-C mediate its oxidation of syringyl compounds and high-molecular-weight substrates. Homology modeling indicates that Tyr-177 and Tyr-74 are located near the heme and exposed on the protein surface of CWPO-C. These results suggest that Tyr residues on the protein surface are considered to be important for the oxidation activities of CWPO-C with a wide range of substrates, and potentially unique oxidation sites for the plant peroxidase family.