Mechanism and Catalytic Site Atlas

Thymidine kinases (TK) are key enzymes in the pyrimidine salvage pathway catalyzing the phosphate transfer from ATP to thymidine (dT) in the presence of Mg2+ and thus, yielding thymidine monophosphate (dTMP) and ADP. Herpesviridae, such as Herpes simplex virus type 1, encode for their own, multifunctional TK. Unlike the very specific human cytosolic TK (TK1), it is able to phosphorylate pyrimidine as well as purine analogs and demands less stereochemical restrictions concerning the sugar moiety also accepting acyclic side chains as phosphate acceptors (e.g., aciclovir). Therefore, the difference in substrate specificity of human TK 1 and TKHSV1 is a crucial point in establishing a molecular basis for selective antiviral therapy, featuring TKHSV1 as the center of activation of antiviral drugs such as aciclovir (ACV), penciclovir, and ganciclovir (GCV). First being activated by phosphorylation by viral encoded TK, these nucleoside analogs in their triphosphate form block the viral replication by subsequently terminating DNA elongation at the viral DNA polymerase. In combination with GCV TKHSV1 is an established tool used as a prodrug-activating enzyme, so-called suicide enzyme, in gene therapy of cancer, AIDS, and in controlling graft-versus-host disease by allogenic bone marrow transplant (allo BMT).