HIV protease : Multiple fold inhibition by silver nanoparticles-Spectrofluorimetric, thermodynamic and kinetic analysis

HIV protease, with one binding site, was inhibited by silver nanoparticles (1-2nm) and the inhibition constants (K_i), dissociation constants (K_d) and Stern-Volmer (K_SV) constants were 97nM, 0.294μM and 0.0043nM^-1, respectively, at 298°K. K_d decreased with increasing temperature but K_SV remained the same supporting a static quenching mechanism. Without silver nanoparticles V_max was 0.22μmol/(mlmin) while in their presence V_max decreased to 0.0018μmol/(minml). Thermodynamic analysis under different temperatures gave δH and δS values for the binding reaction of -96.9kJ/mol and -170J/mol/K, respectively. The complex produces negative δG values and implied that not only were van de Waals and/or hydrogen bonding operative during the binding but also the interaction was exothermic and spontaneous. Based on FRET analysis only one tryptophan residue (Trp⁶) within HIVPR was quenched and the distance between this tryptophan and the silver nanoparticle binding site is 1.03nm. Nanoparticles interact with Cys⁹⁵ that is juxtapositioned 1.05nm from the reactive COO^- of active site Asp²⁵ and 1.07nm from the Trp⁶, disrupting, not only its electronegativity but also decreasing the nucleophilic potential of the COO^- towards water and/or substrate thereby inhibiting the mechanism of reaction. © 2014 Taiwan Institute of Chemical Engineers.