Hemerythrin structure

The hemerythrin of Golfingia gouldii consists of eight subunits, each of which contains two iron atoms, in a protein with molecular weight 108,000. Spectral and magnetic data point to an oxo-bridged structure around the non-heme iron atom (99). Protein B2 of ribotide reductase of E. coli has some properties in common with hemerythrin presumably a protein corresponding to that of E. coli reduces ribotides in animal tissues, a conclusion based on probes with inhibitors. 

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

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). 

The binuclear iron unit consisting of a (p,-oxo(or hydroxo))bis(p.-carboxylato)diiron core is a potential common structural feature of the active sites of hemerythrin, ribonucleotide reductase, and the purple acid phosphatases. Synthetic complexes having such a binuclear core have recently been prepared their characterization has greatly facilitated the comparison of the active sites of the various proteins. The extent of structural analogy among the different forms of the proteins is discussed in light of their spectroscopic and magnetic properties. It is clear that this binuclear core represents yet another stractural motif with the versatility to participate in different protein functions. 

Heck, R. F., 711 Heme, and dioxygen. 895-897 Hemerythrin. 908 Hemoglobin physiology of, 900-902 structure of, 902-908 Hermann-Mauguin system. 

Figure 16-20 (A) The active site of hemerythrin showing the two iron atoms (green) and their ligands which include the (X oxo bridge and two bridging car-boxylate groups. From Lukat et al.193 The active site is between four parallel helices as shown in Fig. 2-22. (B) Stereoscopic view of the backbone structure of a A9 stearoyl-acyl carrier protein desaturase which also contains a diiron center.

Earlier X-ray structures have given conflicting data on the nature of the iron binuclear site1280-1282 for metazido- and methydroxo-hemerythrin, but the situation has been clarified by a determination at 2.2 A resolution of the structure of metazidohemerythrin from Themiste dyscritum.1283 All three structures are given in Figure 55, while the subunit arrangement is shown schematically in Figure 56. 

Figure 55 Structures for the binuclear iron centre in hemerythrin...

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