Figure 1.

The T2Cu site of AcNiR determined by SF-ROX and neutron crystallography. (a) The T2Cu in SF-ROX^OX is ligated by a single water molecule (W1) hydrogen-bonded to the Asp98 residue. Asp98 (Asp_CAT) is visible in two conformations in the proximal position, with the residue rotating around the fixed O^δ1 atom. Asp_CAT is subsequently hydrogen-bonded to the linking water (W2), which is hydrogen-bonded to His255. Water molecules are shown as red spheres. (b) The T2Cu catalytic site determined by neutron crystallography. The protonation states of the T2Cu site residues are clearly seen, along with the orientations of the catalytic D₂O molecule (D1) and the proton-sharing D₂O molecule D2. There is no expected protonation of His255 and Asp98, while a D2 molecule connects His255 and Asp98. The 2F _o − F _c electron-density map is contoured at the 1σ level and is shown as a grey mesh. The 2F _o − F _c nuclear scattering-length density map is contoured at the 1σ level and is shown as a cyan mesh. Atoms are coloured by element, with different colour schemes used for the different chains. The T2Cu is shown as a cyan sphere and D₂O water molecules are shown as red and white sticks. Metal-coordinating bonds are shown as red dotted lines. Selected hydrogen bonds are shown as black dotted lines.