Ferredoxin clostridial

Malkin R, Rabinowitz JC. 1966. The reconstitution of clostridial ferredoxins. Biochem Biophys Res Commun 23 822-7. 

Several models have been proposed for the active center of iron and sulphur in Clostridial ferredoxin in which the cysteine residues in the peptide chain participate in the sulphur bridging. Fig 9 166). Unfortunately X-ray analysis of crystals of these proteins has not been completed. It is difficult to confirm that all the irons are clustered in a single linear array 167, 168). X-ray studies of another non-heme iron protein, the high potential iron protein, hipip, from chromatium, carried out by J. Kraut (personal communication), indicate that the four irons of this molecule may be clustered in a tetrahedral array. 

Fig. 9. A proposed structural configuration for the polynuclear iron-sulphur complex in clostridial ferredoxin.

Carpenter, C. E., Reddy, D. S. A., and Cornforth, D. P. (1987). Inactivation of clostridial ferredoxin and pyruvate-ferredoxin oxidoreductase by sodium nitrite. Appl. Environ. Microbiol. 53, 549-552. 

Pyruvate ferredoxin oxidoreductase. Within Clostridia and other strict anaerobes this enzyme catalyzes reversible decarboxylation of pyruvate (Eq. 15-35). The oxidant used by clostridia is the low-potential iron-sulfur ferredoxin.320 3203 Clostridial ferredoxins contain two Fe-S clusters and are therefore two-electron oxidants. Ferredoxin substitutes for NAD+ in Eq. 15-33 but the Gibbs energy decrease is much less (-16.9 vs - 34.9 kj / mol. for oxidation by NAD+). 

Synthetic iron-sulfur clusters have weakly basic properties273 and accept protons with a pKa of from 3.9 to 7.4. Similarly, one clostridial ferredoxin, in the oxidized form, has a pKa of 7.4 it is shifted to 8.9 in the reduced form 295 If we designate the low-pH oxidized form of such a protein as HOx+ and the reduced form as HRed, we can depict the reduction of each Fe4S4 cluster as follows. 

Lovenberg, Buchanan, and Rabinowitz (65). A sedimentation coefficient of 1.4 0.1 was also found for four other crystalline clostridial ferredoxins. A minimum molecular weight of about 6000 was calculated from amino acid analyses of the four other crystalline clostridial ferredoxins and also for the crystalline ferredoxin from Methanobacillus omelianskii (Buchanan and Rabinowitz (33)). 

One of the characteristics of ferredoxin, as inferred from its name, is the presence of iron. Mortenson, Valentine, and Carnahan (75) reported non-heme iron in bacterial ferredoxin and several investigators independently reported non-heme iron in plant ferredoxin (Tagawa and Arnon (99) Fry and San Pietro (46) Horio and Yamashita (58)). Iron was also found in other clostridial ferredoxins (Lovenberg, Buchanan, and Rabinowitz (65)) and in ferredoxin of photosynthetic bacteria (Evans and Buchanan (41) Bachofen and Arnon (12)). Based on a molecular weight of 13,000, the plant ferredoxins which have been investigated contain 2 atoms of iron per molecule, but the iron content of bacterial ferredoxins differs. Clostridial ferredoxins contain 7 atoms of iron (Loven-... 

As was pointed out previously, clostridial ferredoxins are closely related in spectral properties, iron, and inorganic sulfide content, and amino acid composition. The recent work of Tanaka et al. (100) shows that this similarity applies also to the amino acid sequence of these proteins. Preliminary results indicated that C. butyricum ferredoxin, like C. pasteurianum, contains 55 amino acid residues (3 in addition to those in Table 6), the same glutaminyl-glutamic acid carboxyl-terminal sequence, and the same aminoterminal amino acid, alanine. Their results show also the isomorphic replacement of certain amino acids, such as phenylalanine for tyrosine. 

Bayer et al. (7) succeeded in reconstituting clostridial ferredoxin from a, a -dipyridyl-treated apoferredoxin in the presence of ferrous ion without the presence of a labile sulfur source. Based on this observation, they proposed that labile sulfur originates from cysteinyl residues in the presence of iron. Contrary to their finding, Rabinowitz and his coworkers (21, 33) demonstrated that the reconstitution of clostridial ferredoxin from mersaryl-, a, a -dipyridyl-, and trichloroacetic acid-treated apo-ferredoxins absolutely requires the addition of hydrogen sulfide. Consequently, they concluded that cysteine is not the source of labile sulfur . 

The reactivity of clostridial ferredoxin with iron chelating agents and... 

FIGURE 16-6 Structures of Fe — S Protein-Active Sites, (a) Ferredoxin. (From E.-I. Ochiai, Bioinorganic Chemistry, Allyn and Bacon, Boston, 1977, p. 184.) (b) Clostridial ferredoxin. (Reproduced with permission from E. T. Adman, L. C. Sicker, and L. H. Jensen, J. Biol. Chem., 1973, 248, 3987.) (c) A model for the structure of the Fe Sg active unit. (Reproduced with permission from E.-I. Ochiai, Bioinorganic Chemistry, Allyn and Bacon, Boston, 1977, p. 192.)... 

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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