Metal-iron-sulfur cluster

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Arendsen, A.F., Verhagen, M.F.J.M., Wolbert, R.B.G., Pierik, A.J., Stams, A.J.M., Jetten, M.S.M., and Hagen, W.R. 1993. Thedissimilatory sulfite reductase fromDesulfosarcina variabilis is a desulforubidin containing uncoupled metalated siroheme and S = 9/2 iron-sulfur clusters. Biochemistry 32 10323-10330. 

Iron-sulfur clusters, such as those discussed in Chapter 6, cannot be treated using the 16-e or 18-e rules. Other frameworks exist to treat large metal clusters, and these have some utility in treating [Fe4S4]n+ clusters. One method treats the... 

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. 

The fourth state with [Fe4S4]° shown in Table 6.1 was recently described as the most reduced form possible for the Fe-protein s [Fe4S4] cluster.16 Usually, only two oxidation states for a given metal-sulfur cluster are stable. Therefore a stable [Fe4S4]° state in Fe-protein s iron-sulfur cluster (as appears likely from experimental evidence presented in reference 16) would be unique because the cluster would then have three stable oxidation states, [Fe4S4]2+/1+/0. It appears also that the all-ferrous state is only stable in the protein-bound cluster and not for model... 

Core extrusion studies—removal of the iron-sulfur cluster intact from the enzyme surroundings—have been carried out and the iron-cluster types in proteins identified through the process shown in equation 6.10.18 DMS0/H20 is the protein unfolding solvent for this process. By this method, Fe-protein and MoFe-protein metal-sulfur clusters have been removed from the holoenzyme for separate analysis by many instrumental techniques. 

Figure 4. Structures of blnuclear metal clusters, (a) Iron-0X0 cluster In oxyheraerythrln from Themlste dyscrlta L denotes a coordinated hydroperoxide (27). (b) Iron-sulfur cluster In...

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