1AZY

Thymidine phosphorylases (TP) are key enzymes in nucleoside metabolism. In the presence of inorganic phosphate, they catalyse the reversible phosphorolysis of the glycosidic bond in thymidine- and uridine-2′-deoxyribosides, with the formation of free base and deoxyribose 1-phosphate. In addition to phosphorolysis, they also catalyse the transfer of the deoxyribosyl moiety from one pyrimidine base to another. This transglycosylation reaction plays a key role in the salvage pathway of nucleoside biosyntheses, providing an alternative to the de novo purine and pyrimidine biosynthetic pathways.

There are two families of nucleoside phosphorylases, which differ remarkably in the type of quaternary structure and in polypeptide chain folding. TP belongs to the family II nucleoside phosphorylases which comprises all pyrimidine phosphorylases (PyNPs) except for uridine phosphorylase. All bacterial and mammalian pyrimidine phosphorylases of family II display a dimeric quaternary structure with subunits consisting of a small alpha domain separated by a large cleft from a larger alpha/beta domain. The biologically active form of these enzymes is a dimer.

Mammalian thymidine phosphorylases stimulate tumour growth, and their level is enhanced in tumour cells, where they are involved in angiogenesis and development of metastasis. Owing to their ability to catalyse transglycosylation, some bacterial TPs are widely used in biotechnology for the large-scale production of natural 2′-deoxy-beta-D-ribonucleosides and their analogues containing modifications in the carbohydrate and base fragments. Among these nucleosides are many biologically important compounds with anticancer and antiviral activity. Therefore, the search for compounds which are able to regulate or inhibit TP activity as well as study of the interactions of these compounds with TP is of special interest. Knowledge at the atomic level of the interaction of such compounds with TP and other enzymes of nucleoside metabolism is required for the rational design of new drugs and the engineering of new forms of the enzymes with altered selectivity.