title: Photogeneration and Quenching of Tryptophan Radical in Azurin.
abstract: Tryptophan and tyrosine can form radical intermediates that enable long-range, multistep electron transfer (ET) reactions in proteins. This report describes the mechanisms of formation and quenching of a neutral tryptophan radical in azurin, a blue-copper protein that contains native tyrosine (Y108 and Y72) and tryptophan (W48) residues. A long-lived neutral tryptophan radical W48• is formed upon UV-photoexcitation of a zinc(II)-substituted azurin mutant in the presence of an external electron acceptor. The quantum yield of W48• formation (Φ) depends upon the tyrosine residues in the protein. A tyrosine-deficient mutant, Zn(II)Az48W, exhibited a value of Φ = 0.080 with a Co(III) electron acceptor. A nearly identical quantum yield was observed when the electron acceptor was the analogous tyrosine-free, copper(II) mutant; this result for the Zn(II)Az48W:Cu(II)Az48W mixture suggests there is an interprotein ET path. A single tyrosine residue at one of the native positions reduced the quantum yield to 0.062 (Y108) or 0.067 (Y72). Wild-type azurin with two tyrosine residues exhibited a quantum yield of Φ = 0.045. These data indicate that tyrosine is able to quench the tryptophan radical in azurin.
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Bethany C
authors.initials: BC
authors.lastname: Larson
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Pomponio
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Hannah S
authors.initials: HS
authors.lastname: Shafaat
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Rachel H
authors.initials: RH
authors.lastname: Kim
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Brian S
authors.initials: BS
authors.lastname: Leigh
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Tauber
authors.affiliation: Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.
authors.forename: Judy E
authors.initials: JE
authors.lastname: Kim
mesh_terms: D001400:Azurin / Q000737:chemistry* / Q000235:genetics; D003035:Cobalt / Q000737:chemistry; D004583:Electrons; D009154:Mutation; D010782:Photolysis; D011550:Pseudomonas aeruginosa; D013057:Spectrum Analysis; D014364:Tryptophan / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry
chemical_list: D001400:Azurin; D003035:Cobalt; D014443:Tyrosine; D014364:Tryptophan