In conclusion, we serendipitously found that NPs glycosylate X to yield both N7X and N9X in an enzyme-and pH-dependent fashion. N7X is a stable nucleoside, although its formation is thermodynamically disfavored and some purine NPs also catalyzes its hydrolysis. We assume that the formation of N7X among NPs has previously gone unnoticed because i) N7X and N9X are indistinguishable by mass spectrometry, ii) N7X only forms slowly under somewhat unusual conditions (alkaline media and comparably high concentrations of Rib1P and X), and iii) NPs are usually screened in the phosphorolysis direction (obscuring potential glycosylation isomers only occurring in the reverse reaction). However, the synthesis of N7X by wild-type NPs likely carries no physiological relevance since both free X and Rib1P only occur in low micromolar concentrations in vivo (far below the KM values of this slow transformation), which precludes this transformation from happening to any relevant degree. This lack of physiological relevance is likely also responsible for the retention of this promiscuous activity in modern PNPs. Collectively, this case study provides a notable exception to the paradigm that "native" purine nucleosides are N9-ribosides and further illustrates how even highly evolved enzymes are imperfect catalysts.