Fig. 2.

Diiron structure of TAO. (A) Stereo view of the diiron active site and its environment. Diiron and hydroxo atoms are shown as magenta spheres, four glutamate and two histidine residues important for diiron binding as green sticks, neighboring residues within 6 Å of the diiron in yellow, nitrogen in blue, and oxygen in red. (B) Stereo view of the coordinate bonds (solid lines) and hydrogen bonds (dashed lines) of the diiron active site. Sigma-A weighted electron density map calculated from the refined model of the ligand-free TAO with the diiron centers omitted from the phase calculation is also shown. Contour levels are 1.0 σ (blue) and 3.0 σ (orange). H165 forms hydrogen bonds with E123, E162, and N161 and H269 with E162, E213, and N161. N161, which is situated in the center of the hydrogen network, forms additional hydrogen bonds with Y246 and D265. D265 forms hydrogen bonds with W65 and W247.