title: Evidence from spin-trapping for a transient radical on tryptophan residue 171 of lignin peroxidase.
abstract: The heme enzyme lignin peroxidase contains a unique Cbeta-hydroxylated tryptophan residue (Trp171) on the surface of the enzyme. Mutagenetic substitution of Trp171 abolishes completely the veratryl alcohol oxidation activity of the enzyme. This led us to surmise that Trp171 may be involved in electron transfer from natural substrates to the heme cofactor. Here we present evidence for the formation of a transient radical on Trp171 using spin-trapping in combination with peptide mapping. The spin-trap methyl nitroso propane forms a covalent adduct with Trp171 in the presence of hydrogen peroxide which can be detected by its characteristic visible absorbance spectrum. A very similar chromophore can be obtained in a small molecular model system from N-acetyl tryptophanamide, the spin-trap, and a single-electron abstracting system. The precise site the spin-trap is attached to could be identified in a crystal structure of spin-trap/hydrogen peroxide-treated enzyme as the C6 atom of the indole ring of Trp171. These results indicate that Trp171 is redox-active and that it forms an indole radical by transfer of an electron to the heme of compound I and/or II. Apart from cytochrome c peroxidase and DNA photolyase, lignin peroxidase appears to be the third enzyme only which utilizes a tryptophan residue as an integral part of its redox catalysis.
authors.affiliation: Laboratory of Biochemistry, Swiss Federal Institute of Technology Zürich (ETH), Universitätsstrasse 16, Zürich, CH-8092, Switzerland. blodig2biokemi.su.se
authors.forename: W
authors.initials: W
authors.lastname: Blodig
authors.forename: A T
authors.initials: AT
authors.lastname: Smith
authors.forename: K
authors.initials: K
authors.lastname: Winterhalter
authors.forename: K
authors.initials: K
authors.lastname: Piontek
mesh_terms: D019943:Amino Acid Substitution; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D006861:Hydrogen Peroxide / Q000494:pharmacology; D006900:Hydroxylation; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000737:chemistry; D010544:Peroxidases / Q000737:chemistry* / Q000378:metabolism; D013053:Spectrophotometry; D013113:Spin Labels; D014364:Tryptophan
chemical_list: D007527:Isoenzymes; D010446:Peptide Fragments; D013113:Spin Labels; D014364:Tryptophan; D006861:Hydrogen Peroxide; D010544:Peroxidases; C042858:lignin peroxidase