Iron-containing proteins and enzymes

Low-spin Fe(II) or Fe(III) is more compact than high-spin iron. Consequently if the geometry of the ligands is fixed e.g. as in porphyrin system or by a rigid protein structure the binding energy and coordination geometry of the metal may be different in its two spin states. 

The quite remarkable chemistry of iron proteins is reflected by the fact that the square planar tetradentate porphyrin (XXXXVI) is coordinated to Fe in many different proteins but these molecules show such diverse behaviour as oxygen transport, electron transport and catalytic reactivity in metalloenzymes. The protein must be responsible, by providing the other ligand(s) and by general environmental (cf. solvent) effects, for the differences in reduction potential and chemical reactivity of complexes of the various iron porphyrin systems. 

Low-spin Fe can fit into the cavity of the tetrapyrrole macrocycle (XXXXVII) but high-spin Fe takes up the unusual square pyramidal stereochemistry such that Fe(III) is 0.3 A and Fe(II) is 0.7 A out of the plane defined by the 4 nitrogens 

Microsomal cytochrome P450 is an iron-dependent oxygenase which reduces one atom of oxygen to water whilst the other is transferred to the substrate. The general outcome of this reaction is hydroxylation of the substrate which may be important for its consequent breakdown or solubilisation if it is toxic. 

Catalase, which catalyses the conversion of hydrogen peroxide to oxygen and water (Eqn. 29), and peroxidases, which catalyse the conversion of alkyl peroxides to alcohol and water with concomitant dehydrogenation of a substrate (Eqn. 30), are Fe(III) haemoproteins. 

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Thus the question arises as to what forms of haem proteins and transition metals are available in vivo that are capable of mediating the formation of damaging initiating or propagating species, since the majority of the iron and haem proteins in the human body are protected in vivo from exerting pro-oxidant activities by their compart-mentalization within their functional locations in the haem and non-haem iron-containing proteins and enzymes. 

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