Iron hemerythrin

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. 

The best characterized and thus prototypical of the binuclear iron-oxo proteins is hemerythrin (Hr), an oxygen carrier from some classes of marine invertebrates, such as sipunculids. The protein consists of several identical 13 kDa subunits which contain two iron atoms and reversibly bind one molecule of O2. Myohemerythrins contain a single 13 kDa subunit (25,26,27). 

A new class of metalloprotelns containing polynuclear, non-heme oxo-bridged iron complexes has emerged recently. Dinuclear centers are present in hemerythrin (Hr), ribonucleotide reductase (RR), purple acid phosphatases (PAP) and, possibly, methane monooxygenase (MMO) these centers as well as model compounds are reviewed in Chapter 8. 

The electronic spectrum of Fe—O2 in a hemerythrin derivative has been reported. Further references to reactions of iron porphyrins with dioxygen species appear in Section 5.4.3.7.2. 

Binuclear iron(II) complexes in which a hydroxide bridge is supported by the dinucleating bis-carboxylate ligand dibenzofuran-4,6-bis(diphenylacetate), (217), have proved useful models for hemerythrin. The nature of the binuclear iron center in hemerythrin itself, and in other metalloproteins, has been reviewed, the binding of O2, NO, N3, and NCS to the iron of hemerythrin discussed, " and the volume profile for hemerythrin reacting with O2 established. Bulky tolyl-substituted carboxylate ligands, both bridging and terminal, and... 

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-... 

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