Cubane iron-sulfur

Hail DJ, Rouaoult TA, Harford JB, Kennedy MC, Bondin GA, Beinert H, Klausner RD (1992) Cellular regulation of the iron-responsive element binding protein disassembly of the cubane iron-sulfur cluster results in high-affinity RNA binding. Proc Natl Acad Sci USA 89 11735-11739... 

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. 

Trinuclear Cuboidal and Heterometallic Cubane-Type Iron-Sulfur Clusters New Structural and Reacticity Themes in Chemistry and Biology R. H. Holm... 

Heering, H.A., Bulsink, Y.B.M., Hagen, W.R., and Meyer, T.E. 1995. Reversible superreduction of the cubane [4Fe-4S](3+ 2+ 1+) in the high-potential iron-sulfur protein under non-denaturing conditions EPR spectroscopic and electrochemical studies. European Journal of Biochemistry 232 811-817. 

A brief historical note on the structure of the iron-sulfur clusters in ferredoxins is relevant. After the first analytical results revealed the presence of (nearly) equimolar iron and acid-labile sulfur, it was clear that the metal center in ferredoxins did not resemble any previously characterized cofactor type. The early proposals for the Fe S center structure were based on a linear chain of iron atoms coordinated by bridging cysteines and inorganic sulfur (Blomstrom et al., 1964 Rabino-witz, 1971). While the later crystallographic analyses of HiPIP, PaFd, and model compounds (Herskovitz et al., 1972) demonstrated the cubane-type structure of the 4Fe 4S cluster, the original proposals have turned out to be somewhat prophetic. Linear chains of sulfide-linked irons are observed in 2Fe 2S ferredoxins and in the high-pH form of aconitase. Cysteines linked to several metal atoms are present in metallothionein. The chemistry of iron-sulfur clusters is rich and varied, and undoubtedly many other surprises await in the future. 

Bonding concepts for cubanes, as discussed in Chapter 2.3., predict that the metal polyhedra in the cubanes will contract upon oxidation because of increased metal-metal interaction and vice versa. This has been verified for [CpFeS] 381), [CpFe(CO)]4 167, 177), and [CpCoS]4 361). Furthermore, the ease of oxidation and reduction of several cubane-type clusters 166,167, 361, 381), and the delocalization of electrons in the charged species 48, 176,177, 401) is noticeable. This, together with the prefered formation of iron-sulfur clusters, 381), is borne out by the fact that Nature uses iron-sulfur proteins for redox reactions 207). 

The recent upsurge of interest in iron-sulfur-nitrosyl complexes has been stimulated in part by the reported isolation of [Fe2(SMe)2(NO)4] from natural sources (12), by the obvious resemblances between these complexes and the naturally occurring [2Fe-2S] and [4Fe-4S] clusters of iron sulfur proteins (23, 14), and by the connections between tetrairon-sulfur-nitrosyls and cubane-type clusters (15). Most of the work in this area has been published in the past 5 years or so, and no review has previously been made. However, a number of excellent reviews of the wider aspects of metal-nitrosyl chemistry have appeared (16 19). 

It is instructive, in this context, to compare the case of assembly of both nitrosylated iron sulfur tetranuclear clusters and the related clusters with pendant thiolate groups (13), with the laborious and stepwise initial synthesis (38) of cubane C8H8 this comparison is, of course, simply one between thermodynamically controlled processes, in the case of the iron sulfur systems, and kinetically controlled reactions for C8H8 synthesis. 

Fig. 2.2. Cubane of iron-sulfur cluster (4Fe-4S), prostetic group of cytosolic aconitase...

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