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To date, NPs from different mesophiles, such as Escherichia coli (–, ) or Bacillus subtilis (), have been used in the synthesis of pharmacologically active compounds (, ). However, mesophilic enzymes exhibit a limited ability to withstand aggressive conditions with respect to pH, organic solvents, or elevated temperatures, which are frequently required to reduce the viscosity of the medium or to increase the solubility and concentration of some substrates (). In these cases, enzymes from thermophilic microorganisms (or thermozymes) are ideal substitutes for mesophilic enzymes. The adaptive features found in thermozymes, such as a highly polar surface, a more compact hydrophobic core, and a reduced number of cavities and external loops, correlate not only with an increased thermostability but also with a wider tolerance to other stresses, such as low pH, organic solvents, and denaturing agents. Another important side effect of their natural thermostability of interest in an industrial process is that many of them are easily purified from the soluble fraction after a heat shock treatment that denatures most of the host proteins of the mesophilic host where they are overproduced. To date, thermophilic NPs from the moderate thermophile Geobacillus stearothermophilus are the only examples that have been applied in the synthesis of nucleosides (, ).
In contrast to the separation of the activities on purines, TtPyNP uses thymine as well as uracil as substrates. This broad specificity is shared among PyNPs from organisms such as G. stearothermophilus () and Haemophilus influenzae (), as is the fact that neither accepts cytosine nucleosides as a substrate. PyNPs are usually divided into three categories: uridine phosphorylases (UP), thymidine phosphorylases (TPs), and general-purpose PyNPs, which accept both uracil and thymine nucleosides. Whereas UPs are generally hexamers (and can also be classified as PNPs), TPs and PyNPs are homodimers. In this sense, TtPyNP follows this rule, showing the same quaternary structure with the PyNP from G. stearothermophilus ().
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Human NPs are therapeutic targets in tumoral cells, where they are overproduced. Treatment of these types of cancer involve strategies, such as the design of inhibitors of human NPs that hinder the ability of tumoral cells to replicate their DNA by limiting the available pool of nucleotides, and the administration of nucleobase analogues, which are innocuous when conjugated with a pentose but cytotoxic once phosphorolyzed by an NP (, , ). Aside from these applications, there are a number of other examples of the use of nucleoside analogues with antirheumatic, antiviral, or antimicrobial properties (, ).
These three enzymes were active in a broad temperature range, even at temperatures bordering the water boiling point, although TtPyNP lost half of the activity after 2 h at 80°C. This relatively low thermostability could be related with the lability of the interaction between monomers of this dimeric enzyme, which would also explain the sensitivity to high ionic strength (A). Alternatively, it could be the consequence of imperfect folding during its synthesis in E. coli. This relatively low thermostability does not seem to be shared by TtPNPI and TtPNPII, which remain active after 5.5 and 9.6 days at 80°C, respectively. In this case, it seems that the hexameric TtPNPI does not disassemble under these conditions.
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Delk AS and Rabinowitz JC (1975) Biosynthesis of ribosylthymine in the transfer RNA of Streptococcus faecalis: a folate‐dependent methylation not involving S‐adenosylmethionine. Proceedings of the National Academy of Sciences of the United States of America 72: 528–530.
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.
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