Top: Crystal structure overview of the wild‐type AB tetramer (PDBID 5BZB), view as in Fig
2A. Left: Difference (F
o–F
c) electron density map (mutant minus wild‐type) contoured at −2.5 σ and drawn as red mesh, plotted on the structure of the wild‐type (E178) protein, highlighting the consequences of the loss of the δ‐carbon atom from residue 178 in the E178D mutant. For reference, the locations of the sodium ion sites present in the wild‐type (E178) structure are depicted as cyan spheres. Both the structure and the ions are displayed as faded backgrounds so that the difference map can be seen more clearly. Right: Final refined 2F
o–F
c electron density map for the E178D mutant (PDBID 4X88), contoured at 1.5 σ and shown as grey mesh, with the sodium ions depicted as cyan spheres.