Iron-sulfur clusters Fe4S4 centers

To successfully describe the structure and function of nitrogenase, it is important to understand the behavior of the metal-sulfur clusters that are a vital part of this complex enzyme. Metal-sulfur clusters are many, varied, and usually involved in redox processes carried out by the protein in which they constitute prosthetic centers. They may be characterized by the number of iron ions in the prosthetic center that is, rubredoxin (Rd) contains one Fe ion, ferredoxins (Fd) contain two or four Fe ions, and aconitase contains three Fe ions.7 In reference 18, Lippard and Berg present a more detailed description of iron-sulfur clusters only the [Fe4S4] cluster typical of that found in nitrogenase s Fe-protein is discussed in some detail here. The P-cluster and M center of MoFe-protein, which are more complex metal-sulfur complexes, are discussed in Sections 6.5.2. and 6.5.3. 

Figure 15-9 Stereoscopic view of the large domain (residues 1-383) of tri-methylamine dehydrogenase from a methylotrophic bacterium. The helices and 3 strands of the (aP)8 barrel are drawn in heavy lines as are the FMN (center) and the Fe4S4 iron-sulfur cluster at the lower right edge. The a/P loop to which it is bound is drawn with dashed lines. The 733-residue protein also contains two other structural domains. From Lim et al.150 Courtesy of F. S. Mathews.

The electrons from FMNH2 are transferred to the next electron carrier, coenzyme Q, via the iron-sulfur centers of the NADH-CoQ reductase. The iron-sulfur centers consist of iron atoms paired with an equal number of acid-labile sulfur atoms. The respiratory chain iron-sulfur clusters are of the Fe2S2 or Fe4S4 type. The iron atom, present as nonheme iron, undergoes oxidation-reduction cycles (Fe + Fe + -t- e ). In the Fe4S4 complexes, the centers... 

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