Interaction of VIVO–8-hydroxyquinoline species with RNase A: the effect of metal ligands in the protein adduct stabilization

Herein, we studied the interaction of bovine pancreatic ribonuclease (RNase A) with [(VO)-O-IV(8-HQ)(2)] - a promising antitrypanosomal, antituberculosis, and antitumor vanadium complex (VC) formed by 8-hydroxyquinolinato (8-HQ), reaching IC50 values lower than cisplatin in HCT116 and A2780 cancer lines (0.99 and 0.96 & mu;M vs. 15.0 and 3.4 & mu;M) and the minimum inhibitory concentration (MIC) of 3.7 & mu;M against strains of Mycobacterium tuberculosis. The system was investigated by spectroscopic (UV-vis absorption spectroscopy, circular dichroism, and electron paramagnetic resonance), computational (docking and DFT calculations), and X-ray crystallographic multi-technique approach. Crystallographic results at pH 5.1 demonstrate that upon interaction with the protein, one of the 8-HQ ligands is replaced by a water molecule and by Glu111 side-chain, resulting in the inhibition of the RNase A activity. EPR data confirm the formation of this adduct, with V-IV coordinated by 8-HQ and by the couples (Glu-COO-; H2O) at pH 5.1, (His-N; H2O) at pH 7.4, and (His-N; OH-) or (Ser/Thr-O-, H2O) at pH 8.4. Computational data unveil the reasons behind the ligand exchange, not observed for similar complexes like [(VO)-O-IV(picolinato)(2)] and not predicted by thermodynamic considerations. Overall, our results show that the VC-protein binding is defined not only by the thermodynamic stability of VC but also by other factors, such as the structure and steric requirements of the metal ligands, with important implications in the drug delivery strategies.