Provision of ligands to the metal

In the hemoglobin family the heme group is anchored in a cleft in the globin chain by an imidazole ligand from a histidine residue (the proximal histidine). The other (distal) side of the heme plane is more or less open to accommodate a small sixth ligand (see Figure 4.2).
 

For hemerythrin and hemocyanin the requirements of the protein chain are more severe. In contrast to the hemoglobin family, all but two of the ligands are provided by the protein chain, and in addition the metal ions are encapsulated as a pair. The exogenous ligands for hemerythrin are a fL-(hydr)oxo moiety and dioxygen  or anions (depending on oxidation state); for hemocyanin, theidentity of the second exogenous ligand, if there is one at all, is still unclear.
 

Although hemerythrin has a distinctive cofacial bioctahedral structure (Figure 4.10)46-48 that would appear to be very difficult to assemble in the absence of the protein, it turns out that with a variety of tridentate ligands the (fL-OXO)bis(fLcarboxylato) diiron(III) core may be assembled rather easily.82,156,157 Thus, this core appears to be a thermodynamically very stable structural motif. Such a synthesis has been termed "self-assembly" and appears to be a common phenomenon in biological systems. 158 The low-temperature assembly of bis-copper( II)-fL-peroxo complexes (models for oxyhemocyanin) from mononuclear copper(l) compounds provides other examples of this phenomenon.