Potential substrate-binding pocket
Ec ppnP protein was suggested to be the enzyme catalyzing the phosphorolysis of diverse nucleosides such as uridine, adenosine, guanosine, cytidine, thymidine, inosine, and xanthosine as substrates6. Thus, we further analyzed the potential substrate-binding region in ppnP proteins. All the four structures we obtained showed a groove with a relative hydrophobic pocket constitute by the bilayer β-sheet (Figure 3A, 3B, 3C,3D ). Detailly, theEc ppnP contains many residues involved in the pocket formation, including Leu81, Met30, Tyr89, Met45, Phe79, Phe37, Phe8, Cys91, Tyr7 as the bottom of the pocket, and Glu43, Ser26, Ala40, Glu41, Gln3, Arg24, Met1 as the wall (Figure 3E ). These residues, together with the ones involved in the dimer formation are all well conserved in many ppnP proteins from different species (Figure 3F ). These results suggested the conservative conformation in bacteria.