Hemerythrin dioxygen coordination

The geometry at the iron centres of the bridged dinuclear unit in hemerythrin in its dioxygen-free deoxy (left) and oxygenated oxy (right) forms, with changes upon dioxygen coordination shown. 

Hemerythrin is the best studied example and high-resolution X-ray data are available for the deoxy (176), oxy (176), met (177), and azido-met (177) forms. Extensive spectroscopic studies have also been carried out on these forms and on the semi-met Fe" Fe form 178). The active site structure of hemerythrin is illustrated in Fig. 40. The two iron atoms are bridged by an oxygen donor (OH or 0 ) and by two carbox-ylates, one aspartate and one gluamate. The remaining protein donors are five histidine residues, three coordinated to one iron and two to the other. In the deoxy form one iron atom is five coordinate and this vacant site can be occupied by dioxygen in oxyhemerythrin or by other... 

Earlier EPR studies were also extended to the nitrosyl adducts of deoxyhe-merythrin since the NO adduct is believed to be a reasonable mimic of the superoxide intermediate expected to be formed upon reaction of hemerythrin wifli O2 [92,93,95,526] The spin-Hamiltonian parameters for the Fe - FeNO unit of NO-deoxyhmerythrin are consistent with antiferromagnetic coupling between the two iron centers, and the coordination of NO to the five-coordinate Fe2, as expected for dioxygen binding. 

There have been advances in the structural and physical properties of diiron complexes with dioxygen ligands. All of the well-defined structures have a, u-l,2-peroxo coordinated between two Fe(III) centers. These systems are not directly related to hemerythrin because of the mode of O2 bonding they are more relevant to the proposed structures in the active sites of ribonucleotide reductase (RNR) [5, 15] and MMOH [5] - two nonheme diiron-containing enzymes that activate dioxygen (vide infra). 

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