Nitrogenase iron-protein properties

The iron-sulfur cubes that have been detected by x-ray analysis (49) as constituents of the enzymes ferridoxin and high-potential iron protein have been extruded from these enzymes by replacing the sulf-hydryl ligands of the enzymes with simple mercaptans, and these cubes identified with the corresponding synthetic compounds, II (50, 51, 52, 53). The latter have oxidation-reduction properties that closely mimic those of the enzymes. Similarly, an iron-sulfur-molybdenum double cube has been ejected from nitrogenase, and a similar double cube has been synthesized by Holm and his collaborators (54). It remains to be seen whether or not the iron-sulfur-molybdenum double cube can mimic the properties of nitrogenase (55). 

Much effort has been devoted to the synthesis of iron - sulphur and heterometallic clusters with the aim of modelling the spectroscopic, electron - transfer and catalytic properties of clusters within proteins such hydrogenase, the ferredoxins, the high potential iron proteins and, not least, the components of nitrogenase. 

Again, the enzyme urease was found to contain a dimeric nickel(II) center based only on its magnetic properties (eqn [10] with 5=1). In large proteins such as ferritin, strong coupling is seen in a Fe-O-phosphate center, rather like a small piece of rust enclosed in a protein sheath. Likewise, nitrogenases contain a number of linked irons. 

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