b Cells extracts from Thermus thermophilus HB27 express phosphorolytic activities on purines and pyrimidine nucleosides. Five putative encoding genes were cloned and expressed in Escherichia coli, and the corresponding recombinant proteins were purified and studied. Two of these showed phosphorolytic activities against purine nucleosides, and third one showed phosphorolytic activity against pyrimidine nucleosides in vitro, and the three were named TtPNPI, TtPNPII, and TtPyNP, respectively. The optimal temperature for the activity of the three enzymes was beyond the water boiling point and could not be measured accurately, whereas all of them exhibited a wide plateau of optimal pHs that ranged from 5.0 to 7.0. Analytical ultracentrifugation experiments revealed that TtPNPI was a homohexamer, TtPNPII was a monomer, and TtPyNP was a homodimer. Kinetic constants were determined for the phosphorolysis of the natural substrates of each enzyme. Reaction tests with nucleoside analogues revealed critical positions in the nucleoside for its recognition. Activities with synthetic nucleobase analogues, such as 5-iodouracil or 2,6-diaminopurine, and arabinosides were detected, supporting that these enzymes could be applied for the synthesis of new nucleoside analogs with pharmacological activities.

Several strains of Thermus thermophilus were tested in order to detect purine nucleoside synthase activity using pyrimidine nucleosides as the sugar-donor and adenine or hypoxanthine as bases. High productivity values (t =1 hr) were obtained while completely avoiding adenosine-deaminase degradation of the products. N-2-deoxy-ribosyltransferase activity is described for the first time in hyperthermophilic bacteria.

Nucleosides and their analogs possess several pharmacological activities, so that many strategies have been developed in order to prepare these compounds. The chemical synthesis of these labile and polyfunctional molecules is very complex because many protection–deprotection steps are involved in their preparation. In this sense, biocatalyzed synthesis of nucleosides using either whole cells or enzymes is a greener alternative to conventional methodologies; not only are the number of synthetic steps reduced, but also the waste production is minimized, while the required solvents are environment‐friendly. Some new enzymes, showing different specificities, have become available in recent years. In fact, advances in fermentation, purification, and immobilization techniques have resulted in the production of more stable biocatalysts at significantly reduced cost. Automated screening, new metagenomic techniques, directed evolution, and metabolic engineering have led to the production of novel customized industrial enzymes. For those reasons, there are a large number of patents in Europe and United States of America that are related to these advances. In the present review, we describe many examples used at laboratory scale, and some industrial applications that are not that extensively implemented now will also be commented on. Nevertheless, new scientific developments promise to overcome these drawbacks in the near future.