Interaction of plastocyanin with oligopeptides: effect of lysine distribution within the peptide

We synthesized and purified four oligopeptides containing four lysines (KKKK, GKKGGKK, KKGGGKK, and KGKGKGK) as models for the plastocyanin (PC) interacting site of cytochrome f. These peptides competitively inhibited electron transfer between cytochrome c and PC. The inhibitory effect increased as the peptide concentrations were increased. The association constants between PC and the peptides did not differ significantly (3500-5100 M(-1)), although the association constant of PC-KGKGKGK was a little larger than the constants between PC and other peptides. Changes in the absorption spectrum of PC were observed when the peptides were added to the PC solution: peaks and troughs were detected at about 460 and 630 nm and at about 560 and 700 nm, respectively, in the difference absorption spectra between the spectra with and without peptides. These changes were attributed to the structural change at the copper site of PC by interaction with the peptides. The structural change was most significant when tetralysine was used. These results show that binding of the oligopeptide to PC is slightly more efficient when lysines are distributed uniformly within the peptide, whereas the structural change of PC becomes larger when the lysines are close to each other within the peptide.