Unwinding of zinc finger domain of DNA polymerase I by cis-diamminedichloroplatinum(ii)

Abstract

Structures of a Zn-peptide containing 35 residues (WLQCDDSTCGIVTRQVSVFGKRCLNDGCTGVMRYK) with four cysteines were studied by NMR, CD, and fluorescence spectroscopy, and their structural perturbations and kinetics upon reaction with cis-diamminedichloroplatinum(II) were followed. The secondary structures of the Zn-peptide are comprised of a highly distorted α-helix, β-turn, and an antiparallel β-sheet. The antiparallel β-sheet is located at the C-terminus, while the severely distorted α-helix is at the N-terminus. The reaction of Zn-peptide with cisplatin revealed severe structural perturbations due to successive coordination with all four cysteine residues. The primary reaction proceeded with the formation of two intermediates. Spectroscopic properties and the rate constant (2.2 ± 0.3 M⁻¹ s⁻¹) for the formation of the first intermediate support its composition as a Pt–Zn-peptide precursor adduct, held together by hydrogen bonds and comprising a conformationally relaxed peptide due to the unwinding of N-terminus. Subsequently, the precursor adduct undergoes two consecutive aquation processes (k₂ = 3.3 ± 0. 4 × 10⁻⁴ s⁻¹ and k₃ = 3.0 ± 0.3 × 10⁻⁴ s⁻¹) to form a second intermediate due to the coordination to a cysteine residue and then to the formation of a bis-cysteine platinum complex. Finally, a secondary product is formed through a slow reaction due to the dissociation of zinc from the peptide and deligation of coordinated ammonia from the platinum atom to form a Pt-peptide complex.