Greater binding affinity of trivalent antimony to a CCCH zinc finger domain compared to a CCHC domain of kinetoplastid proteins

Abstract

It has been reported recently that Sb^III competes with Zn^II for its binding to the CCHC zinc finger domain of the HIV-1 NCp7 protein, suggesting that zinc finger proteins may be molecular targets for antimony-based drugs. Here, the interaction of Sb^III with a CCCH zinc finger domain, which is considered to play a crucial role in the biology of kinetoplastid protozoa, has been characterized for the first time. The binding characteristics of Sb^III were compared between a CCCH-type peptide derived from a kinetoplastid protein and two different CCHC-type zinc finger peptides. The formation of 1 : 1 Zn–peptide and Sb–peptide complexes from the different peptides was demonstrated using circular dichroism, UV absorption, fluorescence spectroscopies and ESI-MS. Titration of the Zn–peptide complexes with SbCl₃ was performed at pH 6 and 7, exploiting the intrinsic fluorescence of the peptides. The differential spectral characteristics of the peptides allowed for competition experiments between the different peptides for binding of Zn^II. The present study establishes that Sb^III more effectively displaces Zn^II from the CCCH peptide than CCHC ones, as a result of both the greater stability of the Sb–CCCH complex (compared to Sb–CCHC complexes) and the lower stability of the Zn–CCCH complex (compared to Zn–CCHC complexes). Comparison of the binding characteristics of Sb^III or Zn^II between the CCHC-type peptides with different amino acid sequences supports the model that not only the conserved zinc finger motif, but also the sequence of non-conserved amino acids determines the binding affinity of Sb^III and Zn^II. These data suggest that the interaction of Sb^III with CCCH-type zinc finger proteins may modulate, or even mediate, the pharmacological action of antimonial drugs.