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.