Hemerythrin coordination site

Hemerythrin is a respiratory protein isolated from sipun-culids (marine worms). All sipunculids examined have, in the coelomic fluid, erythrocytes loaded with the protein which in most species so far examined is octameric, but sometimes tri-meric (18, 19) and in one instance dimeric and tetrameric (20, 21). From the retractor muscle of Themiste zostericola, the protein has been characterized as a monomer (22). The monomer (23) and the subunits of the trimer (24) and octamer (25) are remarkably similar in tertiary structure, having a M.W. of about 13,500 daltons. Each subunit contains one binuclear iron site. There is no porphyrin ring and the irons are coordinated only to amino acids, some of which, as well as probably an oxy group, form the binding atoms (26). 

Proteins with dinuclear iron centres comprise some well studied representatives like ribonucleotide reductase (RNR), purple acid phosphatase (PAP), methane monooxygenase hydroxylase (MMOH), ruberythrin and hemerythrin. The latter is an oxygen carrier in some sea worms it has been first characterized within this group and has thus laid the foundation to this class of iron coordination motif. Ruberythrin is found in anaerobic sulfate-reducing bacteria. Its name implies that, in addition to a hemerythrin-related diiron site another iron is coordinated in a mononuclear fashion relating to rubredoxin which is an iron-... 

The postulate of an oxo bridge is also supported by EXAFS data,1286 1287 which show that the oxo bridge is present in oxyhemerythrin and metazidohemerythrin, but possibly absent in hemerythrin. This work also shows that the peroxide group in oxyhemerythrin is bound at the azide site in metazidohemerythrin, and that one of the iron centres in deoxyhemerythrin may be five-coordinate. 

Figure 1. Comparison of the coordination spheres of the dinuclear iron sites in oxidized hemerythrin (Hr), ribonucleotide reductase (RNR), uteroferrin (Uf), and methane monooxygenase (MMO).

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... 

P.C. Wilkins and R.G. Wilkins (1987) Coordination Chemistry Reviews, vol. 79, p. 195 - The coordination chemistry of the binuclear iron site in hemerythrin . 

A second relevant aspect here is the consideration of models for the various dimetal species that could be important in the reactions at the ferroxidase centers. The area of di-iron metallosites has received a great deal of attention from the early 1980s since the identification of such sites in hemerythrin and the production of the first coordination chemistry model compounds for these (Chapters 8.12 and 8.13). Subsequently several other systems with this motif have been studied and the ones of most relevance to the ferroxidase centers would appear to be those involved in oxygen activation rather than transport. ... 

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