Iron Biochemistry
Iron Biochemistry
Iron is the most abundant transition metal in living organisms. It possesses incomplete filled orbitals and has two relatively stable oxidation states, namely iron(II) (d6) and iron(III) (d5). The redox potential between these two states is such that oxidation processes centred on the iron can be coupled to metabolic reactions. Both ferrous and ferric forms can form salts with common anions. At pH values approaching neutral, the maximum solubility of iron(III) is limited to 10-18M due to its Ksp value of 10 -39 M4 .Thus ferric ions are quickly hydrolysed to insoluble polymeric hydroxides. In contrast iron(II) is readily soluble (Ksp=10 -17 M3) corresponding to a maximum concentration close to 1mM at pH 7. Although ferrous salts are quite stable in the solid state, they are readily oxidised to the ferric form in solution in the presence of oxygen. It is thus obvious that under physiological conditions, neither iron(II) nor iron(III) are likely to exist in aerobic aqueous solutions as the hexaaqua co-ordinated iron complexes, [Fe(H2O)6] 2+ and [Fe(H2O)6] 3+, to any appreciate extent (Aisen et al., 1978). Only by the use of suitable ligands is it possible to keep iron(III) in aqueous solution under neutral and basic conditions. The low aqueous solubility of iron(III) at neutral pH has required the development of specialised methods for organisms to acquire the metal from their surroundings. Many micro-organisms have evolved specific iron(III) sequestering agents, renamed siderophores (Hider, 1984), in order to maintain the iron(III) in soluble form so as to facilitate the acquisition of the exogenous metal.
The availability of free co-ordinated sites around an iron ion is potentially damaging due to the ability of iron to participate in redox cycling reactions, where univalent oxidation/reduction by species such as hydrogen peroxide and superoxide radical, which are produced during oxidative stress, can occur. More reactive species such as hydroxyl radical can be produced at the co-ordination sphere of iron, such radicals cause oxidative damage to cells and tissues (Halliwell and Gutteridge, 1989). Consequently, evolution has ensured that iron normally remains bound to carrier molecules such as transferrin, haem and other metalloproteins, thus preventing generation of oxygen free radicals whilst permitting the biologically useful features of iron chemistry to be manipulated. Iron-containing metalloproteins fall into three main groups:
1. Those that form reversible complexes with iron, the main function of which is iron transport and storage.
2. Those characterised by their ability to bind oxygen
reversibly.
3. Metallo-enzymes in which iron performs a catalytic role.
Iron metabolism in man
Plasma iron turnover
Iron containing proteins
Haemoglobin
Hot links:
[Back to Top][Main Page][Thesis ][Curriculum Vitae][ Sites of Interest]
[Contact me at 
moridani@yahoo.com]