Biophysical characterization of vaccinia virus thymidine kinase substrate utilization

To provide information for the development of new antiviral compounds that inhibit orthopoxviruses, further characterization of the kinetics and thermodynamics that underlie substrate utilization reactions of vaccinia virus thymidine kinase (VVTK) has been undertaken. The kinetics of 2′deoxythymidine phosphorylation by VVTK and the thermodynamics of complex formation between VVTK and the substrate 2′ deoxythymidine were determined using spectroscopic and calorimetric techniques.These studies demonstrated that kinetic parameters for 2′ deoxythymidine phosphorylation by VVTK were 25 μM and 0.2 s−1 for Km and kcat, respectively. The enthalpy change, ΔH, for the enzyme catalyzed reaction is −18.1 kcal/mol. Thermodynamic studies for the formation of the enzyme substrate complex demonstrated a binding affinity (Ka) of 4 × 104 M−1, an enthalpy change for binding (ΔH) of −17.4 kcal/mol, and a reaction stoichiometry of two molecules of substrate binding to each enzyme tetramer. Kinetic and thermodynamic data were in agreement (Ka ≈ 1/Km) and showed similarities to literature values reported for herpes simplex virus thymidine kinase (HSV-TK) and human thymidine kinase 1 (hTK1) with respect to kcat but not with respect to Km. The Km value found for VVTK in this study is nearly two orders of magnitude larger than the values reported for the hTK1 and the HSV TK enzymes.