Search

Rubrerythrin is a non-heme iron dimeric protein isolated from the sulfate-reducing bacterium Desulfovibrio vulgaris. Each monomer has one mononuclear iron center similar to rubredoxin and one dinuclear metal center similar to hemerythrin or ribonucleotide reductase. The 1.88 A X-ray structure of the "as isolated" molecule and a uranyl heavy atom derivative have been solved by molecular replacement techniques. The resulting model of the native "as isolated" molecule, including 164 water molecules, has been refined giving a final R factor of 0.197 (R(free) = 0.255). The structure has the same general protein fold, domain structure, and dimeric interactions as previously found for rubrerythrin [1, 2], but it also has some interesting undetected differences at the metal centers. The refined model of the protein structure has a cis peptide between residues 78 and 79. The Fe-Cys4 center has a previously undetected strong seventh N-H...S hydrogen bond in addition to the six N-H...S bonds usually found in rubredoxin. The dinuclear metal center has a hexacoordinate Fe atom and a tetracoordinate Zn atom. Each metal is coordinated by a GluXXHis polypeptide chain segment. The Zn atom binds at a site distinctly different from that found in the structure of a diiron rubrerythrin. Difference electron density for the uranyl derivative shows an extremely large peak adjacent to and replacing the Zn atom, indicating that this particular site is capable of binding other atoms. This feature/ability may give rise to some of the confusing activities ascribed to this molecule.

Haemerythrin and myohaemerythrin are non-haem iron proteins utilized by several marine invertebrates in transporting and storing oxygen. Haemerythrin is oligomeric [usually octameric, but also trimeric1,2, and tetrameric and dimeric (C. A. Appleby, personal communication)] whereas myohaemerythrin is a monomeric analogue of the haemerythrin subunit. Physico-chemical investigations3–5 of these molecules have shown that (1) each subunit binds two iron atoms by means of amino acid side chains, (2) one molecule of O2 is bound to each binuclear iron centre, and (3) the iron atoms are in the +3 oxidation state in oxy and met forms of the proteins and +2 in deoxy. Models have been proposed for the complexes in haemerythrin and myohaemerythrin on the basis of crystallographic studies of the proteins. A μ-oxo bridged model (Fig. 1a) has been proposed from the results of a crystallographic analysis of metazidomyo-haemerythrin from Themiste zostericola6. Based on a similar analysis of methydroxohaemerythrin from Themiste dyscritum, previously referred to as metaquohaemerythrin7, we proposed a confacial bi-octahedron model with no atom identified as a μ-oxo bridge (Fig. 1b)8. Previously9, we stated that “at least one of the models is incorrect”. In fact, neither completely accounts for the electron density from the present 2.2 A resolution difference map of the complex and surrounding electron density of metazidohaemerythrin from T. dyscritum.