Iron-sulfur proteins eight irons

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. 

Oxoglutarate can also serve as a starter piece for elongation by the oxoacid pathway. Extension by three carbon atoms yields 2-oxosuberate (Eq. 21-1). This dicarboxylate is converted by reactions shown in Eq. 24-39 into biotin and in archaebacteria into the coenzyme 7-mercaptoheptanoylthreonine phosphate (HTP), Eq. 21-1. Lipoic acid is also synthesized from a fatty acid, the eight-carbon octanoate. A fatty acid synthase system that utilizes a mitochondrial ACP may have as its primary fimction the synthesis of ocfanoate for lipoic acid formation. The mechanism of insertion of the two sulfur atoms to form lipoate (Chapter 15) is imcerfain. If requires an iron-sulfur protein jg probably similar to the corresponding process in the synthesis of biotin (Eq. 24-39)9 93a formation of HTP (Eq. 21-1). One component of the archaebacterial cofactor methano-furan (Chapter 15) is a tetracarboxylic acid that is formed from 2-oxoglufarafe by successive condensations with two malonic acid imits as in fatty acid synthesis. ... 

Many cytochromes in energy-transducing organelles are membrane-bound proteins. Present in mitochondria and in purple photosynthetic bacteria, the cyt bc complex (also called ubiquinol-cytochrome c oxidoreductase, or complex III) catalyzes the electron transfer from ubiquinol to ferricyt c and pumps protons from the matrix to the cytosol.The catalytic core of the cyt bci complex comprises three redox-active subunits they are a cyt b with two ft-type hemes bn and bf), a cyt Cl, and a Reiske iron sulfur protein. While this catalytic core has enzymatic activity in some a proteobacteria like Paracoccus, Rhodospirillum rubrum, and Rb. capsulatus, mitochondrial cyt bci complexes have an additional seven or eight subunits." ... 

Part 1 of the nitrogenase protein contains another interconnected group of Fe-S atoms, this one with eight iron atoms and seven sulfur atoms. This [8Fe-7S] group collects electrons and transmits them to the binding center. Part 2 of nitrogenase contains a third Fe-S group, this one made up of four iron atoms and four sulfur atoms. This part of the enzyme also binds two molecules of ATP. 

Iron-sulfur (Fe-S) proteins function as electron-transfer proteins in many living cells. They are involved in photosynthesis, cell respiration, as well as in nitrogen fixation. Most Fe-S proteins have single-iron (rubredoxins), or two-, three-, or four-iron (ferredoxins), or even seven/eight-iron (nitrogenases) centers. 

The 8-Fe proteins contain two four-iron-sulfur cubes which are separated (center-to-center) by about 12 A. The EPR spectra from the fully reduced proteins are typical for a 4-Fe center with intercluster spin-spin coupling. An ENDOR study on the 8-Fe ferredo-xin from C. pasteurianum in the fully reduced state has been reported by Anderson et al.273. Since the 57Fe-56Fe difference spectra are only poorly resolved, analysis of the ENDOR data turned out to be difficult. All eight iron atoms are assumed to have similar AFe tensors with principal values Afe = 25 MHz, AFe = 29 MHz and AFe = 33 MHz. These coupling parameters suggest that the reducing electrons are delocalized over the four irons of each of the two nearly identical 4-Fe clusters. 

Figure 5 shows the structures of P and P°. Both states of the P cluster contain eight iron and seven sulfur atoms, which are covalently coordinated to the MoFe protein by six cysteinyl ligands, three from the a-subunit, that is, Cys a 62, Cys 0 88, and Cys a 154, and three from the /3-subunit, that is, Cys /370, Cys /395, and Cys /3153. In both cases, Cys a62, Cys O 154, Cys /370, and Cys /3153 coordinate one iron atom each. 

The name ferredoxin was first proposed for a non haem iron-redox-protein (hence its name) isolated from Clostridium pasteurianum and presumably involved in the hydrogen gas evolution from pyruvate by this bacterium (254). The smallest of the known iron-sulfur proteins, 6,000 dalton, the clostridial ferredoxins are, however, the most complex in terms of the iron and inorganic sulfur content 8Fe 8S. They are single chain polypeptides of about 55 residues of which eight are cysteines (Fig. 18). 

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