A) Iron-sulphur proteins; this group of enzymes contains one or more iron atom ligated by sulphur, or thiolato-sulphur or both. These proteins fulfil a variety of functions ranging from the electron transfer and transport with both redox and non-redox roles, such as succinate dehydrogenase, nitrogenase, and aconitase. The iron can exist in either or both of the oxidation states, +2 or +3.
B) Haem proteins; the haem group (see Section 1.1.4.2.) is a common motif in biological systems and consists of an iron atom co-ordinated in an approximately square planar fashion by the pyrrole nitrogens of the porphyrin ring with one or two further axial ligands. The spin and the redox state of the iron influences the function of haem proteins leading to further subdivisions;
I. Oxygen carriers such as myoglobin and haemoglobin where the iron is in the +2 oxidation state and cycles between a high spin state and a low spin electronic configuration.
II. Oxygen activators such as peroxidases, oxidases and cytochromes P450 where both high and low spin states and oxidation states from +2 to +6 may be important.
III. Electron transfer proteins such as cytochromes a, b, and c where the low spin configurations of iron in the +2 and +3 oxidation states are important.
C) Non-haem and non-Fe/S proteins (metalloenzymes); this type of protein includes superoxide dismutase, dioxygenases, oxygenase, lipoxygenases, and ribonucleotide reductase. Iron may exist in the +2 or +3 oxidation states or both.
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