Iron sulfide clusters

One of the enzymes given in Table 23 is nitrogenase, which is responsible for the fixation of dinitrogen to give ammonia. Molybdenum probably serves as the binding site for N2, and is present in the iron-molybdenum cofactor, which is a molybdenum-iron sulfide cluster. Nitrogenase will be considered in Section 63.1.14, which deals with the nitrogen cycle. 

A number of molybdenum-iron-sulfide clusters have been synthesized as models for the cofactor.1478,1479,1486 At present there is no synthetic model which reproduces all of the spectroscopic properties of the cofactor. The ultimate test of a synthetic cluster would be its incorporation into the MoFe protein. Some clusters are shown in Figure 73. 

As with chromophores, the steric encapsulation of a dendrimer core can be utilized to prevent intermolecular interactions between redox active sites. A number of different redox active core moieties have been investigated, including, iron—sulfide clusters,9394 bis(terpyridine)iron(II) complexes,92 tris-(bipyridine)ruthenium(II) complexes,330 zinc porphyrins,252 oligothienylenevinylenes,331 fullerenes,236,332 ferrocenes,333-336 oligothiophenes,322 oligonaphtha-lenes,337 and 4,4 -bipyridinium.338... 

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. 

Several iron sulfide nitrosyl compounds are known. These have stmctures that in some cases are formally related to the FeS clusters by replacement of thiolate by NO. The compounds include the anions [Fe2S2(NO)4] and [Fe4S2(NO)2] (Roussin s red and black salts, respectively) and the neutral compounds [Fe2S2(NO)4] and [Fe4S4(NO)4]. Roussin s black salt has found use as a NO releasing vasodilator. 

Recently Jensen and co-workers have determined the structure of a clostridial-type ferredoxin obtained from Micrococcus aerogenes (47). One of the two apparently identical iron-sulfur clusters is illustrated in Fig. 2. The structure is compatible with a model with iron and labile sulfide at alternate comers of a cube. This accounts for the equivalence of these moieties in the protein. Another 8-iron-8 labile sulfur ferredoxin, from Clostridium acidiurici, similarly contains two independent iron-sulfur clusters per molecule (48). Strahs and Kraut (49) had earlier discovered... 

Iron-sulfur proteins are a group of enzymes and other electron carriers that contain clusters of iron and sulfide linked directly to amino-acyl side chains, usually cysteines. They are widely distributed in nature. Soon after their discovery. Hall et al. (1971) proposed that they could be used to follow the course of evolution. Studies of genome sequences have revealed that iron-sulfur cluster binding motifs are among the most commonly recognized sequences. 

K), Fe-S cluster assembly (nIfM) and the biosynthesis of the iron molybdenum cofactor, FeMo-co (nifN, B, E, Q, V, H)(5a). It is the last two functions, involving the placement of unusual transition metal sulfide clusters into the nitrogenase proteins, that cause nitrogenase and its components to be appropriately included in this symposium. 

Iron-sulfur clusters (7) occur as prosthetic groups in oxidoreductases, but they are also found in lyases—e.g., aconitase (see p. 136) and other enzymes. Iron-sulfur clusters consist of 2-4 iron ions that are coordinated with cysteine residues of the protein (-SR) and with anorganic sulfide ions (S). Structures of this type are only stable in the interior of proteins. Depending on the number of iron and sulfide ions, distinctions are made between [Fe2S2], [Fe3S4], and [Fe4S4] clusters. These structures are particularly numerous in the respiratory chain (see p. 140), and they are found in all complexes except complex IV. 

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. 

Nitrite reductase and sulfite reductase are enzymes found in choroplasts and in prokaryotes that reduce nitrite to ammonia and sulfite to sulfide (Scott et al., 1978). Sulfite reductase also catalyzes reduction of nitrite at a lower rate. Both enzymes contain a siroheme prosthetic group linked to an iron-sulfur cluster. In siroheme, the porphyrinoid moiety is present in the more reduced chlorin form. Because NO lies between nitrite and ammonia in oxidation state, it is a potential intermediate. 

Chondrules exhibit a bewildering variety of compositions and textures (F ig. 6.1 a,b). Most are composed primarily of olivine and/or pyroxene, commonly with some glass. (For a crash course in mineral names and compositions, see Box 6.1.) If melt solidifies so quickly that its atoms cannot organize into crystalline minerals, it quenches into glass. Iron-nickel metal and iron sulfide occur in many chondrules, often clustered near the peripheries. The textures of... 

Mineralogical interpretations of Halley dust analyses remain controversial. Lawler et al. (1989) saw no clustering of compositions that might suggest crystalline minerals (Fig. 12.5), whereas Fomenkova et al. (1992) identified compositions that were consistent with a number of minerals, including pyroxene, phyllosilicate, carbonate, FeNi metal, iron sulfide, and iron oxide. The characterization of minerals in returned comet dust (see below) supports the identification of some of these primary minerals but calls into question the identification of those formed by alteration. 

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