Substitution at the Iron Atom

The iron atom in haems and haemoproteins is usually five- or six-coordi-ftate, since it can bind ligands at the axial positions. Haems such as tron(II) protoporphyrin IX will readily coordinate neutral bases such as NHj and pyridine, small unsaturated molecules like CO, and some anions. In haemoproteins, at least one of the axial ligands is provided by the polypeptide, and with the exception of some cytochromes, this is the only linkage between the polypeptide and the prosthetic group. Where only We axial position is occupied by the polypeptide, the other is thought to be taken up by a water molecule in ferric haemoproteins. This is readily replaced by other ligands. Ferrous haemoproteins, in the absence of potential ligands such as CO, can remain five-coordinate. 

possibly imidazole and S, cysteine N, imidazole or amine, and possibly H2O 

The probable axial ligands of common haemoproteins are given in Table 2. Even in cytochromes, where the protein may occupy both axial positions, one of the axial ligands can be replaced by anions. The effects of changing the axial field in this way are useful in probing the electronic structure of the prosthetic group. 

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