Mononuclear iron-sulfur clusters

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. 

Achieving fast electron transfer to enzyme active sites need not be complicated. As mentioned above, many redox enzymes incorporate a relay of electron transfer centers that facilitate fast electron transfer between the protein surface and the buried active site. These may be iron-sulfur clusters, heme porphyrin centers, or mononuclear... 

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... 

Suppose we have an isolated system with a single unpaired electron and no hyper-fine interaction. Mononuclear low-spin Fe111 and many iron-sulfur clusters fall in this category (cf. Table 4.2). The only relevant interaction is the electronic Zeeman term, so the spin Hamiltonian is... 

There are many other proteins that contain iron in a form that is neither in haem nor in iron-sulfur clusters. We have already encountered the iron storage and transport proteins, ferritin and transferrin (see Chapter 8). We propose to discuss here two other classes of iron-containing proteins, those with mononuclear non-haem iron centres and those with dinuclear non-haem iron centres. 

Mononuclear anions, halogenocuprate 1) and halogenoargentate(I) ions four-coordinated metal centers, 37 19 three-coordinated metal centers, 37 7-11 Mononuclear iron-sulfur clusters, 47 219 EPR spectra, 47 423-425 EPR spectra, 47 448... 

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... 

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). 

T. Waters, X.-B. Wang and L.-S. Wang, Coord. Chem. Rev., 2007, 251, 474. Review describing the use of electrospray to transfer negatively charged transition metal complexes to the gas phase for photoelectron spectroscopy. Examples discussed include square planar and octahedral halide complexes, metal-metal bonded species, transition metal bis(diihiolene) complexes, and mononuclear and polynuclear iron-sulfur clusters. 

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. ° ... 

Iron-sulfur cluster compounds, 1203 dinuclear, 235 electrochemistry, 236 hexanuclear, 241 mononuclear, 235 tetranuclear, 238 trinuclear, 237 Isomerization iridium catalysts, 1158... 

Arr contains molybdenum, iron, and acid-labile sulfur (14). It appears to be a member of the family of mononuclear molybdenum enzymes (16). Approximately 14 0.4 mol equivalents of iron and 16.4 0.8 mol equivalents of acid-labile sulfur are present, suggesting that Arr contains several iron-sulfur clusters as prosthetic groups. 

All known molybdenum- and tungsten-containing enzymes catalyse reduction-oxidation reactions. The oxidation state of the metal centre can vary between iv, v and vi, hence one- and two-electron transfer reaction steps are possible. In Nature two different ways exist to control the catalytic power and the oxidation state of the metal centre of molybdenum enzymes. One is a mononuclear metal centre, which consists of sulfur and oxygen atoms as coordination sphere around molybdenum and the other is the multinuclear metal centre in which the molybdenum is part of an iron-sulfur cluster, which is only known for bacterial nitroge-nase enzymes. ... 

Shima S, Lyon EJ, Thauer RK, Mienert B, Bill E. Mdssbauer studies of the iron-sulfur cluster-free hydrogenase the electronic state of the mononuclear Fe active site. J Am Chem Soc 2005 127 10430 10435. 

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