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J Phys Chem B. 2015 Jul 23;119(29):9438-49. doi: 10.1021/jp511523z. Epub 2015 Feb 17.

Photogeneration and Quenching of Tryptophan Radical in Azurin.

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1
Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States.

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.

PMID:
25625660
PMCID:
PMC5092234
DOI:
10.1021/jp511523z
[Indexed for MEDLINE]
Free PMC Article

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