title: 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.
abstract: 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.
authors.affiliation: Department of Forest and Forest Products Sciences, Kyushu University, 6-10-1, Hakozaki, Higashiku, Fukuoka 812-8581, Japan.
authors.forename: Shinya
authors.initials: S
authors.lastname: Sasaki
authors.forename: Daisuke
authors.initials: D
authors.lastname: Nonaka
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Wariishi
authors.forename: Yuji
authors.initials: Y
authors.lastname: Tsutsumi
authors.forename: Ryuichiro
authors.initials: R
authors.lastname: Kondo
mesh_terms: D002412:Cations / Q000737:chemistry; D002473:Cell Wall / Q000201:enzymology*; D006418:Heme / Q000737:chemistry; D008031:Lignin / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D008970:Molecular Weight; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D032107:Populus / Q000201:enzymology* / Q000235:genetics; D016415:Sequence Alignment; D013379:Substrate Specificity; D013499:Surface Properties; D014443:Tyrosine / Q000235:genetics / Q000378:metabolism
chemical_list: D002412:Cations; D014443:Tyrosine; D006418:Heme; D008031:Lignin; D010544:Peroxidases