As the pH and temperature might influence the activity of PyNP-04, we determined specific activities of the enzyme at pH 7 or 9. To this end, we performed phosphorolysis reactions (Figure ) with the sugar donors uridine (a) and thymidine (b) in a MOPS (pH 7) or glycine buffer (pH 9). PyNP-04 was active under all conditions and, for instance, catalyzed the phosphorolysis of a at 80 °C with activities of approximately 80 U mg -1 (Figure ). We additionally Scheme 2. Biocatalytic synthesis of 2-Seleno pyrimidine nucleosides via transglycosylation reactions using a thermostable pyrimidine nucleoside phosphorylase (PyNP) as biocatalyst.