Hemerythrin iron liganding

Fig. 10. The primary structure of Golfingia gouldii hemerythrin (189). The amino acid sequence of the Dendrostomum pyroides pigment differs in having Gly, Ala, Glu, and Arg at positions 9, 76, 79 and 82, respectively, and possibly an extra Lys between positions 107 and 111 (196). It should be noted that monomeric Golfingia hemerythrin is heterogeneous, sequence differences with that shown above having been established at positions 79 and 96 where Thr and Ala substitute for Gly and Ser, respectively (193, 194). Heavier circles distinguish potential iron ligand residues as described in the text, however, some of these are unlikely candidates...

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... 

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.

The emphasis on the study of hemoproteins and the iron-sulfur proteins often distracts attention from other iron proteins where the iron is bound directly by the protein. A number of these proteins involve dimeric iron centres in which there is a bridging oxo group. These are found in hemerythrin (Section 62.1.12.3.7), the ribonucleotide reductases, uteroferrin and purple acid phosphatase. Another feature is the existence of a number of proteins in which the iron is bound by tyrosine ligands, such as the catechol dioxygenases (Section 62.1.12.10.1), uteroferrin and purple acid phosphatase, while a tyrosine radical is involved in ribonucleotide reductase. The catecholate siderophores also involve phenolic ligands (Section 62.1.11). Other relevant examples are transferrin and ferritin (Section 62.1.11). These iron proteins also often involve carboxylate and phosphate ligands. These proteins will be discussed in this section except for those relevant to other sections, as noted above. 

Fig. 9. Models for the central core complexes of native octameric hemerythrin (182). Besides being complexed to the protein through amino acid side chains, each iron atom is coordinated to an external, variable ligand X which can be Cl , Bv, F-, CN , NCS-, N3, etc., when bridging the two Fe(III) centers or H2O, OH , F , etc., when two ligands bind. B stands for either OH or an amino acid side chain derived from the protein...

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

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