Density functional studies on herpes simplex virus type 1 thymidine kinase-substrate interactions: The role of Tyr-172 and Met-128 in thymine fixation

The enzyme herpes simplex virus type 1 thymidine kinase (HSV1 TK) salvages thymidine into the DNA metabolism of the virus. In the active site, the thymine ring of the nucleoside binds in a pocket, formed by two residues, Tyr-172 and Met-128, in a sandwich-type orientation. To investigate the nature of the thymine-enzyme pocket interactions, we have carried out density functional theory calculations with gradient-corrected exchange-correlation functionals of models of the thymine-HSV1 TK adduct. Our calculations indicate that the role of Met-128 in the substrate fixation is purely steric and hydrophobic, while the substrate-Tyr-172 interaction is essentially electrostatic in nature. These findings are completely consistent with the available catalytic properties of mutants on the 128 position.