Summary
Thymidine kinase is an enzyme, a phosphotransferase (a kinase): 2'-deoxythymidine kinase, ATP-thymidine 5'-phosphotransferase, EC 2.7.1.21. It can be found in most living cells. It is present in two forms in mammalian cells, TK1 and TK2. Certain viruses also have genetic information for expression of viral thymidine kinases. Thymidine kinase catalyzes the reaction: Thd + ATP → TMP + ADP where Thd is (deoxy)thymidine, ATP is adenosine triphosphate, TMP is (deoxy)thymidine monophosphate and ADP is adenosine diphosphate. Thymidine kinases have a key function in the synthesis of DNA and therefore in cell division, as they are part of the unique reaction chain to introduce thymidine into the DNA. Thymidine is present in the body fluids as a result of degradation of DNA from food and from dead cells. Thymidine kinase is required for the action of many antiviral drugs. It is used to select hybridoma cell lines in production of monoclonal antibodies. In clinical chemistry it is used as a proliferation marker in the diagnosis, control of treatment and follow-up of malignant disease, mainly of hematological malignancies. The incorporation of thymidine in DNA was demonstrated around 1950. Somewhat later, it was shown that this incorporation was preceded by phosphorylation, and, around 1960, the enzyme responsible was purified and characterized. Two different classes of thymidine kinases have been identified and are included in this super family: one family groups together thymidine kinase from herpesvirus as well as cellular thymidylate kinases, the second family groups TK from various sources that include, vertebrates, bacteria, the bacteriophage T4, poxviruses, African swine fever virus (ASFV) and Fish lymphocystis disease virus (FLDV). The major capsid protein of insect iridescent viruses also belongs to this family. The Prosite pattern recognizes only the cellular type of thymidine kinases. Mammals have two isoenzymes, that are chemically very different, TK1 and TK2.
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