Structural perturbation of a C4 zinc-finger module by cis-diamminedichloroplatinum(II): insights into the inhibition of transcription processes by the antitumor drug

Three dimensional structures of a zinc-finger peptide containing 31 amino acid residues, I C E E P T C R N R T R H L P L Q F S R T G P L C P A C M K A (zpp), were determined by nuclear magnetic resonance spectroscopy and were compared with platinum complexes formed by the reaction between cis-diamminedichloroplatinum(II) (cisplatin; cis-DDP) and zpp. The structures of the zpp complex reveal an α-helix-like domain at the amino terminus, extending eight residues from P5 through R12. The helix region is followed by two turns with seven residues starting at H13. By judging the dihedral angles, the first turn resembles an ideal Type I turn, whereas the second turn appears to be a non-ideal Type V turn. A third atypical turn starting at L24 was also detected. Proline serves as the third residue of both the first and third turns. Four cysteine residues, two located at the beginning of the helix and two at the carboxy end, are coordinated to Zn(II), facilitating the formation of a loop. These structural features are consistent with the circular dichroism measurements which indicate the presence of 26% helix and 21% β turns. Cisplatin undergoes facile reaction with zpp complex. The reaction between cisplatin and zpp proceeded in a stepwise manner. A complete deligation of Zn(II) was observed with two equivalent of cis-DDP. The first unit of the platinum complex is bound to the two cysteine residues at the carboxy terminus as revealed by the heteronuclear 1H–15N and NOESY experiments. The second unit is coordinated to the remaining two cysteines at the amino terminus. The structures of these 1:1 and 1:2 adducts are remarkably different from those of the Zn-finger motif. In particular, substantial loss of helical component with the concomitant increase of non-ordered structures was observed. The implications of the loss of structural integrity of zinc-finger domain upon binding to cisplatin are discussed in the context of inhibition of transcription processes leading to apoptosis.