Figure 3 - full size

 

Figure 3.
Fig. 3. Spatial relationship of glucose substrate to His-54. (Left) Neutron map (1.8-Å resolution) showing His-54 with coordinates of glucose from an x-ray study (1.6-Å resolution; PDB ID code 1XIF) shown by yellow lines. Nuclear density is shown for Asp-57, His-54, and heavy water W1022. (Right) Model with W1022 superimposed on the site of a metal ion-bound cyclic glucose. His-54 NE2 provides a proton to the water molecule (W1022 at 2.67 Å) in the absence of substrate. Presumably, when the cyclic form of the sugar substrate is present, the water molecule is displaced, and a hydrogen bond (2.68 Å) is formed between His-54 NE2 and O5 of the sugar. This result shows a possible mode of protonation required for substrate ring opening. The metal ion, M1, that binds the other end of the cyclic glucose, is shown at the bottom of the diagrams.