Figure 7
The schematic view of the E. coli PNP hexameric molecule, in the apo form, with the three-fold and the two-fold symmetry axes (left, based on PDB 1ECP8), and in the complexes with ligands, phosphate and nucleoside (right). In just one ternary complex of the E. coli PNP with its ligands (PDB 1K9S)9 the three-fold symmetry is retained (upper right), and all three dimers are with one monomer in the open (blue) and one in the closed (green) conformation of the active sites. The binary enzyme-phosphate complex, and the majority of various ternary enzyme-phosphate-nucleoside complexes show only two-fold symmetry (lower right, see also Fig. 1), as one of three dimers is symmetric (rotten green), hence different than two other unsymmetrical dimers (blue-green). The present study shows that this is more likely not a crystallographic artefact as also in solution the three-binding-site model is necessary to properly describe interaction of this enzyme with ligands, hence dimers are probably also functionally not equivalent.
