Mononuclear iron-sulfur clusters ferredoxins

Mononuclear iron-sulfur clusters (continued) ferredoxins, 47 6-7 spin-lattice relaxation, 47 425-426 spin-spin interactions, 47 462, 465- 7 Mononuclear manganese redox enzymes, see Manganese, redox enzymes, mononuclear Mononuclear polysulfidemetal ring complexes, 31 97-99... 

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. 

In some cases, small biological redox partner proteins such as heme-containing cytochromes, ferredoxins comprising an iron-sulfur cluster, or azurin with a mononuclear Cu site have been used as natural mediators to facilitate fast electron exchange with enzymes. A specific surface site on the redox protein often complements a region on the enzyme surface, and enables selective docking with a short electron tunneling... 

From the biological area, iron-sulfur clusters in biomolecules such as rubredoxin mononuclear Fe-S clusters (Rao et at., 1972), plant-type ferredoxin 2Fe-2S clusters (Johnson, 1975) and bacterial-type ferredoxin 4Fe-2S clusters (Thompson et at., 1974) are readily distinguished from one another by their Mossbauer spectra. The temperature dependence of relaxation effects can provide information about the types of internuclear interaction and can even lead to estimates of the distance between paramagnetic sites, for example, the two 4Fe-4S clusters in ferredoxin in Peptococcus aerogenes (Adman etal., 1973). 

Naphthalene dioxygenase consists of three components, which form an electron transfer chain an NADH-dependent flavoprotein reductase, a ferredoxin containing two [2Fe2S] Rieske iron-sulfur clusters, and a Rieske oxygenase containing both a [2Fe2S] Rieske iron-sulfiir cluster and a mononuclear iron(II) center in the enzyme active site. ° ... 

Although there are three types of active sites in non-heme iron proteins, namely containing one, two, and four iron atoms per center, in the discussion above only tetrameric Fe-S species have been considered. This topic has been treated with some detail because these compounds meet properly the concept of cluster and are therefore clearly within the scope of this book. That is not the case for the di-iron species which form the 2Fe-2S active sites in the ferredoxins nor for the mononuclear center in the rubredoxins. However, in order to get a better understanding of the chemistry of the iron-sulfur proteins, an overview of the chemistry of the 2Fe-2S analogues is also outlined in this Section. 

---