Mutation Study of Two Thymidine Kinases  : An Attempt to Create an Enzyme that Effectively Activates Anticancer and Antiviral Nucleoside Analogs

The deoxyribonucleoside kinase, thymidine kinase 1 (TK1), is a key enzyme in the salvage pathway, where it catalyzes the first of three phosphate transfers from ATP to thymidine. Besides from their importance in DNA metabolism deoxyribonucleoside kinases are also important for activation of anticancer and antiviral nucleoside pro-drugs.

Humans have four different deoxyribonucleoside kinases, the two cytosolic: TK1 and deoxycytidine kinase (dCK) and the two mitochondrial: thymidine kinase 2 (TK2) and deoxyguanosine kinase (dGK). In insects, a single gene encodes a multi substrate kinase that phosphorylates all the natural deoxyribonucleosides and like insects, C. elegans only contains a single deoxyribonucleoside kinase-like gene. In contrast to the insects, however, the protein encoded by the elegans gene is 46 % identical to human TK1 (HuTK1) and have no homology to the insect kinase. Like HuTK1 the C. elegans kinase (CeTK1) has thymidine as the preferred substrate, but it also displays activity with deoxyguanosine, though with high K_m. A number of point mutations have been introduced in the active site of both the human and elegans TK's in order to change the substrate specificity away from thymidine and towards other deoxynucleosides and nucleoside analogs. One of the HuTK1-mutants gained increased activity with the analog azidothymidine and reduced thymidine activity, and two CeTK1 mutants phosphorylates deoxycytidine, which has not previously been observed for TK1-type kinases. They did not phosphorylate the anticancer analog 1-β-D-arabinofuranosylcytosine (AraC), however. The HuTK1 mutant has been crystallized, and azidothymidine monophosphate has been modelled into the active site.