Appraisal of sildenafil binding on the structure and promiscuous esterase activity of native and histidine-modified forms of carbonic anhydrase II.

Sildenafil was investigated for its interaction with the native and modified human carbonic anhydrase II (hCA II). Modification of exposed histidine side chains with diethyl pyrocarbonate decreased esterase activity of the enzyme. The treatment of both native and modified CA with sildenafil revealed slight and moderate enzyme activation profiles, respectively. In addition, in the present study the effects of sildenafil on the structural properties of native and modified hCA II were investigated employing different computer simulation and spectroscopic techniques such as UV-vis, circular dichroism (CD), fluorescence spectroscopy and molecular dynamics. Fluorescence measurements showed that the sildenafil acts as a quencher of the native and modified enzyme fluorescence. Stern-Volmer analyses revealed the existence of one binding site on the native/modified enzyme for sildenafil. The thermodynamic parameters, enthalpy change (∆H) and entropy change (∆S) of drug binding were not also similar, which indicate that different interactions are responsible in CA-drug interaction. Calculation of the protein surface hydrophobicity (PSH), using 1,8-Anilinonaphtalene Sulfonate (ANS), indicated the increment of PSH of native and modified hCA II in the presence of sildenafil. Overall, sildenafil-CA interaction probably induces protein conformational changes and completes reorganization of both hydrogen bond networks within the active site cavity and hydration positions on the protein surface.