LIGAND VARIATION AND METAL-ION BINDING-SPECIFICITY IN ZINC FINGER PEPTIDES

Three metal binding peptides with coordination sites CYS2HiS2, CYS3His, and CYS4 have been prepared and their metal binding properties characterized. The peptides are based on a zinc finger consensus sequence and have the sequences ProTyrLysCys4ProGluCys7GlyLysSerPheSerGlnysSerAspLeuValLysXaa20GhiArgTbrYaa24ThrGly (Xaa = Yaa = His; Xaa = His, Yaa = Cys; Xaa = Yaa = Cys). The dissociation constants for the peptide complexes with Co2+, Zn2+, and Cd2+ have been determined via a series of direct and competitive metal ion titrations. The trend in relative affinities of the peptides for Co2+ over Zn2+ can be semiquantitatively accounted for by the decrease in ligand field stabilization energy as imidazole ligands are replaced by thiolates. The affinity for Cd2+ increases by over two orders of magnitude for each thiolate for imidazole substitution, in keeping with hard-soft acid-base effects. Furthermore, the results reveal that the N2S2 coordination site is unique among the sites studied in allowing significant preferential binding of Zn2+ over both first row transition metals and second row elements such as Cd2+.