Figure 4. Coordination geometry at the diiron active site of MMOH.

(a) Electron density at the diiron site in H-B. Side chain ligands are shown as sticks in grey (carbon), blue (nitrogen), and red (oxygen). 2F_o-F_c electron density at 1.0 sigma and 5.0 sigma is drawn as a mesh in light blue and magenta, respectively. Views comparing the diiron site in H-B (grey) with that in MMOH_ox (b, PDB code: 1MTY, black) and in MMOH_red (c, PDB code: 1FYZ, yellow) are also presented in (b) and (c), respectively. Upon MMOB binding, Glu 243 undergoes a substantial conformational change, involving simultaneous chelation of Fe2 and bridging to Fe1. Such bidentate coordination of the Glu 243 side chain resembles that in MMOH_red, but distances between the carboxylate oxygen atoms and Fe2 (OE1-Fe2 and OE2-Fe2) are shorter in H-B (1.9 and 2.0 Å) than in MMOH_red (2.4 and 2.4 Å). This result is more consistent with an Fe(III) than an Fe(II) oxidation state for the iron atoms in H-B. Solvent-derived ligands, such as hydroxide ion and water, are not observed in the H-B complex owing to the 2.9 Å resolution.