Quaternary structure, conformational variability and global motions of phosphoglucosamine mutase

Phosphoglucosamine mutase (PNGM) is a bacterial enzyme that participates in the peptidoglycan biosynthetic pathway. Recent crystal structures of PNGM from two bacterial pathogens, Bacillus anthracis and Francisella tularensis, have revealed key structural features of this enzyme for the first time. Here, we follow up on several novel findings from the crystallographic studies, including the observation of a structurally conserved interface between polypeptide chains and conformational variability of the C-terminal domain. Small-angle X-ray scattering of B. anthracis PNGM shows that this protein is a dimer in solution. Comparisons of the four independent polypeptide chains from the two structures reveals conserved residues and structural changes involved in the conformational variability, as well as a significant rotation of the C-terminal domain, of nearly 60°, between the most divergent conformers. Furthermore, the fluctuation dynamics of PNGM are examined via normal mode analyses. The most mobile region of the protein is its C-terminal domain, consistent with observations from the crystal structures. Large regions of correlated, collective motions are identified exclusively for the dimeric state of the protein, comprising both contiguous and noncontiguous structural domains. The motions observed in the lowest frequency normal mode of the dimer result in dynamically coupled opening and closing of the two active sites. The global motions identified in this study support the importance of the conformational change of PNGM in function, and suggest that the dimeric state of this protein may confer advantages consistent with its evolutionary conservation.