(a) Stereodiagram of the active site of the rkbPAP-sulfate complex. Fo-Fc electron density for the sulfate group is overlayed. The sulfate group is bound in the second coordination sphere via extensive hydrogen bonding interactions. General legend: Fe(III) is in tan, Zn(II) in grey, carbon in green, oxygen in red, nitrogen in blue and sulphur in orange. Hydrogen bonds and other contacts are shown as dashed lines. (b) Stereodiagram of the superimposition of the active site of the rkbPAP-sulfate complex (green) with the active site of the rat PAP-sulfate complex  (magenta). In the rkbPAP-sulfate complex the bridging hydroxide adopts an elevated position, precluding the binding of a terminal water to Fe(III). Both metals in the rkbPAP-sulfate complex structure are five coordinate.
Schenk et al. BMC Structural Biology 2008 8:6 doi:10.1186/1472-6807-8-6