The Rational of Catalytic Activity of Herpes Simplex Virus Thymidine Kinase

Most antiherpes therapies exploit the large substrate acceptance of herpes simplex virus type 1 thymidine kinase (TKHSV1) relative to the human isoenzyme. The enzyme selectively phosphorylates nucleoside analogs that can either inhibit viral DNA polymerase or cause toxic effects when incorporated into viral DNA. To relate structural properties of TKHSV1ligands to their chemical reactivity we have carried out ab initio quantum chemistry calculations within the density functional theory framework in combination with biochemical studies. Calculations have focused on a set of ligands carrying a representative set of the large spectrum of sugar-mimicking moieties and for which structural information of the TKHSV1-ligand complex is available. The kcat values of these ligands have been measured under the same experimental conditions using an UV spectrophotometric assay. The calculations point to the crucial role of electric dipole moment of ligands and its interaction with the negatively charged residue Glu225. A striking correlation is found between the energetics associated with this interaction and the kcat values measured under homogeneous conditions. This finding uncovers a fundamental aspect of the mechanism governing substrate diversity and catalytic turnover and thus represents a significant step toward the rational design of novel and powerful prodrugs for antiviral and TKHSV1-linked suicide gene therapies.