Bacterial-Type Iron-Sulfur Proteins

The bacterial-type iron-sulfur proteins all contain larger amounts of iron and labile sulfide than the plant-type iron-sulfur proteins best estimates for the iron and labile sulfide content being 8 Fe and 8 S per protein molecule (172, 173) for these ferredoxins from Clostridium and from Chromatium. Although these proteins have large amounts of Fe and S, the molecular weights are less than the molecular weights of the 

The first Mossbauer spectroscopic studies on this class of iron-sulfur proteins were carried out by Blomstrom et al. 174) who reported only spectra taken on the oxidized form of the protein from Clostridium pasteuranium. Evidence in the form of two partially resolved quadrupole pairs was presented to support two distinct environments for the iron in the oxidized form of the protein, the ratio of 5 2 was suggested in keeping with the then thought number of Fe atoms per protein molecule. This assignment of the number of Fe atom per site, of course, rests on the assumption of equal Lamb-Mossbauer recoil-free fractions for the two sites. 

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Iron-sulfur proteins can be classified on the basis of iron and sulfide content into "plant-type iron-sulfur proteins, and "bacterial-type iron-sulfur proteins. Plant-type iron-sulfur proteins contain just two Fe and two inorganic S atoms per protein molecule the name refers to the material first isolated from chloroplasts. The bacterial-type iron-sulfur proteins always contain more than two Fe (and S—) atoms per protein molecule in general there are eight Fe and eight S— atoms per protein molecule. 

Putidaredoxin. Cushman et al. (36) isolated a low molecular iron-sulfur protein from camphor-grown Pseudomonas putida. This protein, putidaredoxin, is similar to the plant type ferredoxins with two irons attached to two acid-labile sulfur atoms (37). It has a molecular weight of 12,000 and shows absorption maxima at 327, 425 and 455 nm. Putidaredoxin functions as an electron transfer component of a methylene hydroxylase system involved in camphor hydroxylation by P. putida. This enzyme system consists of putidaredoxin, flavoprotein and cytochrome P.cQ (38). The electron transport from flavoprotein to cytochrome P.cq is Smilar to that of the mammalian mixed-function oxidase, but requires NADH as a primary electron donor as shown in Fig. 4. In this bacterial mixed-function oxidase system, reduced putidaredoxin donates an electron to substrate-bound cytochrome P. g, and the reduced cytochrome P. g binds to molecular oxygen. One oxygen atom is then used for substrate oxidation, and the other one is reduced to water (39, 40). 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

All iron-sulfur proteins, whether of the plant-type or the bacterial-type have three characteristics in common all contain the acid-labile sulfide in equimolar ratio to iron all show reduction potentials in the range from —240 to —420 mV (E0,pli = 7.0) and when these proteins are chemically-reduced (typically with dithionite), they display an uncommon EPR signal, known as the g = 1.94 signal. The oxidized forms of the proteins are not paramagnetic (159). 

The FcjS -type ferredoxins can be arranged into three distantly related classes based on amino acid sequence homologies bacterial-, plant-, and vertebrate-type . Extensive information on the function and mechanisms of this system has been gained through work on the bacterial P450cam system in Pseudomonas putida, which catalyzes the conversion of rf-camphor to 5-exo-hydroxy-camphor. In P putida, the iron-sulfur protein is putidaredoxin (Pdx), a 106 amino acid residue ferredoxin. For catalysis, two reducing equivalents are sequentially transferred from NADH... 

Another type of iron-containing monpoxygenase was flrst described by Bernhardt et al. ) and contains a two iron-two-acid-labile-sulfur cluster. It was isolated from bacteria and catalyzes the 0-demethylation of 4-methoxybenzoate The corresponding electron transport chain involves NADH, a flavoprotein and a second iron-sulfur protein It seems that many more bacterial monooxygenases belong to this type rather than to the heme-sulfur-containing category. 

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. 

This enzyme [EC 1.14.99.15] catalyzes the reaction of 4-methoxybenzoate with AH2 and dioxygen to produce 4-hydroxybenzoate, formaldehyde. A, and water. The bacterial enzyme consists of a ferredoxin-type protein and an iron-sulfur fiavoprotein (FMN). 4-Ethoxyben-zoate, A-methyl-4-aminobenzoate, and toluate can serve as substrates as well. The fungal enzyme acts best on ver-atrate. 

Ferredoxins. Ferredoxins are proteins which contain two or four iron atoms bound to cysteine and inorganic sulfur atoms as shown in Fig. IB. There are two types of ferredoxins plant type ferredoxins (top) which consist of two iron and two labile sulfur atoms coordinated to four cysteine residues, and bacterial type ferredoxins (bottom) consisting of four iron and four labile sulfur atoms coordinated to four cysteine residues. 

The impetus for the development of iron-sulfur cluster chemistry over the last three decades derives largely from the occurrence of iron-sulfur clusters in an extensive variety of proteins and enzymes.Five cluster types (l)-(5) have been demonstrated by protein crystallography and are shown in Figure 1. Clusters (l)-(3) represent the fundamental set found in proteins (ferredoxins) from a variety of prokaryotic and eucaryotic sources. Rhomb-like cluster (1) is especially prevalent in green plants. Cuboidal cluster (2) and cubane-type cluster (3) are found in bacterial sources... 

Many bacterial ferredoxins have two such clusters, each of which can be reduced to a paramagnetic state. In other proteins, 4Fe4S clusters can be oxidized to a paramagnetic state. The tetranuclear clusters in these two types of proteins are similar in structure, but they functionally shuttle between different reduction states. In both cases, each iron atom is additionally coordinated by four cysteinyl sulfur ligands. 

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