The reduction and binding of copper ions to the Cys/Trp motif, which is characterized by two cysteines and two tryptophans, in the extracellular N-terminal domain of the copper transporter (Ctr) protein of fungi are investigated using the model peptides of Ctr4 and Ctr5 from Schizosaccharomyces pombe. The Cys/Trp motif of Ctr5 can reduce Cu(II) and ligate Cu(I), which is the same as that of Ctr4 previously reported. Titration of Cu(II) and Cu(I) ions indicates that both the Cys/Trp motifs of Ctr4 and Ctr5 reduce two Cu(II) and bind two Cu(I) per one peptide. However, the coordination structure of the Cu(I)-peptide complex differs between Ctr4 and Ctr5. Cu(I) is bound to the Cys/Trp motif of Ctr5 via cysteine thiolate-Cu(I) bonds and cation-π interaction with tryptophan, as reported for Ctr4, and a histidine residue in the Cys/Trp motif of Ctr5 is suggested to interact with Cu(I) via its Nτ atom. Ctr4 and Ctr5 exhibit a heterotrimeric form within cell membranes and the copper transport mechanism of the Ctr4/Ctr5 heterotrimer is discussed along with quantitative evaluation of the Cu(I)-binding constant of the Cys/Trp motif.
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