PLASTOCYANIN-PEPTIDE INTERACTIONS - EFFECTS OF LYSINE PEPTIDES ON PROTEIN-STRUCTURE AND ELECTRON-TRANSFER CHARACTER

Structural change of plastocyanin (PC) due to the interaction with lys ine peptides (Lysptd's) has been studied by absorption, resonance Rama n, and electrochemical measurements and by measuring the electron tran sfer between PC and cytochrome c (cyt c) in the presence of Lysptd. Ab sorption spectral changes which were observed when Lysptd's up to pent a-lysine were added to PC solution have been ascribed by resonance Ram an studies to the change in the active site Cu-cysteine geometry upon binding of Lysptd to the PC negative patch. The same spectral changes were observed for the PC-cyt c interaction. Electrochemical measuremen ts showed that the redox potential of PC increases upon Lysptd binding , suggesting that Lysptd's induce a structural change in PC through th e copper ligating cysteine residue to make the copper site adapted for facile electron transfer. Lysptd's competitively inhibited the electr on transfer from reduced cyt c to oxidized PC, which indicated that th ey function as models of the PC interacting site of proteins. The effe cts of Lysptd on electron transfer are explained as competitive inhibi tion due to neutralization of the PC negative patch by formation of PC Lysptd complexes. The electron-transfer rate from reduced cyt c to ox idized PC and the inhibiting effect of Lysptd decreased upon decreasin g the net charge of the negative patch by mutation. The structural cha nge of PC was also found to decrease significantly with these mutants. The present observations strongly support that the PC negative patch is the dominant cyt c/f molecular recognition site and open up the pos sibility that charged peptides can be used for studying protein-protei n interactions in a systematic way.