Soon it was shown that TK1 is present in the cytoplasm only in anticipation of cell division (cell cycle-dependent), The two isoenzymes have different reaction kinetics and are inhibited by different inhibitors.The viral thymidine kinases differ completely from the mammalian enzymes both structurally and biochemically and are inhibited by inhibitors that do not inhibit the mammalian enzymes.
Thymidine monophosphate, the product of the reaction catalyzed by thymidine kinase, is in turn phosphorylated to thymidine diphosphate by the enzyme thymidylate kinase and further to thymidine triphosphate by the enzyme nucleoside diphosphate kinase.
The triphosphate is included in a DNA molecule, a reaction catalyzed by a DNA polymerase and a complementary DNA molecule (or an RNA molecule in the case of reverse transcriptase, an enzyme present in retrovirus).
Genes for virus specific thymidine kinases have been identified in Herpes simplex virus, Varicella zoster virus and Epstein-Barr virus.
ADP Thymidine reacts with ATP to give thymidine monophosphate and ADP.
Mutations in the gene for TK2 lead to a myopathic form of mitochondrial DNA depletion syndrome.