CONCLUSIONS
The NP-I and NP-II families represent two structurally distinct families of enzymes that catalyze the fundamental biochemical reaction. These families represent the only two folds known to be involved in the phosphorolysis of a wide range of purine and pyrimidine nucleosides previously. Here, we solved multiple crystal structures of another NP family protein ppnP in bacteria. Our structures showed a conserved dimeric cupin fold with a high hydrophobic dimer interface. The conformational analysis suggested that the ppnP proteins may act as a third nucleoside phosphorylase family to catalyze the reactions. Further complex structures are needed to uncover the catalytic scheme for ppnP proteins to various nucleosides.