Overall structure of PnpE. A. The eight PnpE monomers are represented with cartoon in different colors. AC, BF, DG and EH form four homodimmers. The dashed region denotes one dimer AC. B. The secondary structures of PnpE monomer are depicted in rainbow colored cartoon. 17 β –strands and 14 α-helices are labeled. C. Structure of PnpE dimer. The dimer is shown in cartoon model. Molecular A is in blue color and molecular C is shown in three colors with each represents one domain: the oligomerization domain in red color, the cofactor binding domain in green color and the substrate binding domain in yellow color. The dash region denotes the interface areas that mediate dimerization.