The comparative analysis of the properties and structures of purine nucleoside phosphorylases from thermophilic bacterium Thermus thermophilus HB27.

Two recombinant purine nucleoside phosphorylases from thermophilic bacterium Thermus thermophilus HB27 encoded by genes TT_C1070 ( Tth PNPI) and TT_C0194 ( Tth PNPII) were purified and characterized. The comparative analysis of their sequences, molecular weight, enzymes specificity and kinetics of the catalyzed reaction were realized. As a result, it was determined that the Tth PNPI is specific to guanosine while the Tth PNPII to adenosine. According to the results of the size exclusion chromatography and SAXS study both enzymes are hexameric molecules. Based on the sequence alignment with homologous purine nucleoside phosphorylases (PNPs), Asn was identified as a purine base recognizing residue in the active site of Tth PNPI and Asp in Tth PNPII. The three-dimensional structure of Tth PNPII was solved at 2.5 Å resolution by molecular replacement method using crystals grown in microgravity. Position of phosphate in the active site cavity is located. The possible arrangement of adenosine and guanosine in Tth PNPII active site cavity is considered using superposition with the structures of homologous trimeric and hexameric PNPs complexed with corresponding substrates. The peculiarities of oligomeric structure of Tth PNPII in comparison with homologous PNPs are described. It is shown that two trimeric molecules of Tth PNPII in the asymmetric part of the unit cell are connected by three two-fold axis into a hexamer with 32-point symmetry. This type of hexameric structure of PNP is found for the first time. The interface area between the subunits in trimeric molecule and between the trimers in Tth PNPII hexamer is described.Communicated by Ramaswamy H. Sarma.