Spectroscopic Elucidation of the Inhibitory Mechanism of Cys2His2 Zinc Finger Transcription Factors by Cobalt(III) Schiff Base Complexes

Transcription factors are key regulators in both normal and pathological cell processes. Affecting the activity of these proteins is a promising strategy for understanding gene regulation and developing effective therapeutics. Co-III Schiff base complexes ([Co(acacen)(L)(2)](+) where L=labile axial ligands) have been shown to be potent inhibitors of a number of zinc metalloproteins including Cys(2)His(2) zinc finger transcription factors. Inhibition by [Co(acacen)(L)(2)](+) of the target protein is believed to occur through a dissociative exchange of the labile axial ligands for histidine (His) residues essential for function. Here, we report a series of spectroscopic investigations with model peptides of zinc fingers that elucidate the interaction between [Co(acacen)(L)(2)](+) complexes and zinc finger transcription factors. Observed changes in NMR chemical shifts and 2D H-1-(HNOESY)-H-1 NMR spectra demonstrate the preference of [Co(acacen)(L)(2)](+) complexes to coordinate His residues over other amino acids. The conformation of [Co(acacen)(L)(2)](+) upon His coordination was characterized by (HNMR)-H-1 spectroscopy, near-UV CD, and electronic absorption. These studies reveal that the resulting His-coordinated [Co(acacen)(L)(2)](+) complex possesses an octahedral structure. The effects of [Co(acacen)(L)(2)](+) complexes on the zinc-finger structure were assessed by the degree of hydrogen bonding (probed by 2D NMR spectroscopy) and secondary-structure profiles measured by far-UV CD. These structural studies demonstrate the ability of [Co(acacen)(L)(2)](+) complexes to disrupt the structure of zinc fingers, resulting in primarily random-coil conformations. A mechanism is described wherein [Co(acacen)(L)(2)](+) complexes inhibit zinc finger transcription factor activity through selectively coordinating His residues in the zinc finger by dissociative ligand exchange and disrupting the structural motif required for gene regulation.