Figure 2.

Water structure in the catalytic pocket and substrate-entry channel. (a) In the structure of oxidized AcNiR determined by SF-ROX, Asp98 (Asp_CAT) has a dual conformation; the usual proximal conformation hydrogen-bonds to two waters (W3 and W4), while the distorted proximal position, which is observed for the first time, hydrogen-bonds directly to the T2Cu water ligand. The waters (W3 and W4) are part of the ordered water network in the substrate-entry channel. Both proximal conformations of Asp_CAT are hydrogen-bonded via O^δ1, with the water W2 linking His255 (His_CAT) to Asp_CAT. 2F _o − F _c electron density is contoured at the 1σ level and is shown as a grey mesh. (b) In the atomic resolution crystal structure (PDB entry 2bw4) the proximal conformation is hydrogen-bonded to the ligated water W1A with an occupancy of 0.8. Water W1B with an occupancy of 0.2 is not shown for simplicity. (c) In the neutron^OX structure, Asp_CAT is in a single proximal conformation. The 2F _o − F _c nuclear scattering-length density map is contoured around selected heavy waters at the 1σ level and is shown as a teal mesh. Atoms are coloured by element, with different colour schemes used for the different chains. The T2Cu is shown as a cyan sphere, D₂O water molecules are shown as red and white sticks and water molecules are shown as small red spheres. Metal-coordinating bonds are shown as red dotted lines. Selected hydrogen bonds are shown as black dotted lines.