Structural change at the Cu site of plastocyanin on complex formation withlysine peptides

Interaction of plastocyanin (PC) with lysine peptides (Lysptds), models of the PC interacting site of cytochrome f, is studied. A longer absorption ma ximum for the 600-nm CT band and a stronger intensity for the 460-nm absorp tion band of PC were observed on Lysptd binding, suggesting that PC suffers a structural change at its active site from the interaction. These changes have been previously attributed to the changes in the active site Cu-Cys g eometry upon binding of Lysptds to the PC negative patch (Hirota, Hayamizu, Endo, Hibino, Takabe, Kohzuma and Yamauchi, J. Am. Chem. Sec. 1998, 120, 8 177-8183). Changes in the absorption difference spectra were observed from pH 5.0 to 8.9, although the intensities were smaller at pH 5.0, probably du e to the decrease in PC-Lysptd complex formation ability due to protonation of aspartic acid or glutamic acid residues of the PC negative patch. The i ntensities of the changes also decreased on addition of NaCl, which demonst rates that the structural change of PC on interaction with Lysptd is induce d by an electrostatic interaction.