Active Center of the Iron Enzymes

Iron SOD s are found in prokaryotes and plants Manganese and iron enzymes are phylogenetically older and they show great sequence homologies which are different from those of the Cu2Zn2superoxide dismutases Compared to the latter species, iron and manganese containing SOD s have received much less attention. 

Iron superoxide dismutase is commonly.isolated as dimer or tetramer each subunit bearing a functional metal ion There have been no reports of a strong interaction between the metal centers. The subunits display relative molecular masses around 22,000 daltons. The tervalent state of the metal atom seems to be the ground state. 

The metal content has been a source of much controversy Recently the iron content was determined directly from anomalous scattering measurements on crystals of the native enzyme A symmetrical dimer with one iron per subunit was found. Since each subunit of the purified enzyme contained slightly more than half an iron atom, it was concluded, that the normal state of this enzyme would be two iron atoms per dimer and that some of the metal was lost during purification. Actually, the metal 

The optical spectrum of iron superoxide dismutase is characterized by a broad band near 350 nm that is attributoi to a li nd-to-metal charge transfer band. The relatively high energy for this transition indicates that tyrosine is not a ligand to the Fe(lII). The EPR-spectrum is characteristically rhombic Azide and fluoride are inhibitors of the enzymatic activity. Intriguinly, cyanide does not affect the catalytic action. As in the case with the Cu ZHj enzymes, hydro n peroxide readily destroys the reactivity of iron SOD s, whereas manganese superoxide dismutases remain unaffected 

Currently the X-ray cristallographic analyses of the iron superoxide dismutases from Pseudomonas ovalis and Escherichia coli have become available (Fig. 8)  

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