ABSTRACT
The protein provides five histidines and two carboxylate residues that coordi nate the diiron core (Fig. 29). In the structure of the reduced Hr (deoxy-Hr) one of the irons-Fel-has two terminal histidine ligands and the other-Fe2-has three term inal histidine ligands. The two iron ions are bridged by one glutamate, one aspartate, and one hydroxo group. This results in Fel being five-coordinated and Fe2 being sixcoordinate and leaves one open coordination position on Fel for binding dioxygen. The structure of oxo-Hr shows an identical coordination of the protein ligands as seen in deoxy-Hr, with only minor adjustments of the coordination distances [137]. The dioxygen molecule is bound end-on to Fel with its free oxygen atom hydrogen bonded to the bridging oxo group of the Fe(III)-Fe(III) center. The dioxygen molecule faces a hydrophobic pocket and makes direct van der Waals contacts with residue Leu98. Mutations to the corresponding Leu98 residue in a myohemerythrin have been shown to severely affect the stability of dioxygen binding and easily lead to protein autoxidation [133]. The structure of a Hr-azide complex has also been determined. Azide binds to the diiron core in a similar mode as the dioxygen molecule of oxy-Hr [138].
