Ferredoxin molecular weight

C) cuboidal three-iron-four-sulfide [Fe3-S4] clusters—stable oxidation states are 0 and + 1 and (D) cubane four-iron-four-sulfide [Fe4-S4] clusters—stable oxidation states are + 1 and +2 for ferredoxin-type clusters and +2 and +3 for HIPIP clusters. Electrons can be delocalized, such that the valences of individual iron atoms lie between ferrous and ferric forms. Low-molecular-weight proteins containing the first and the last three types are referred to as rubredoxins (Rd) and ferredoxins (Fd), respectively. The protein ligands are frequently Cys residues, but a number of others are found, notably His, which replaces two of the thiol ligands in the [Fe2-S2] Rieske proteins. In addition to these, discrete Rd... 

Stadtman TC. 1966. Glycine reduction to acetate and ammonia identification of ferredoxin and another low molecular weight acidic protein as components of the reductase system. Arch Biochem Biophys 113 9-19. 

Plant type ferredoxins. Tagawa and Arnon (16) described the isolation of a ferredoxin from spinach chloroplast. This ferredoxin is a protein of 12,000 molecular weight, and consists of 97 amino acids (17). Spinach ferredoxin has abosrbance maxima at 325, 420 and 465 nm (18). Ferredoxins of this type have been isolated from other sources of plants and algae, e.g., alfalfa (19), taro (20), Leuceana glauca (21) and Scenedesmus (22). The prot s of thes erredoxins are similar in their properties to ferredoxin from spinach. 

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 molecular weight of these proteins ranges from 14,000 to 23,000 as shown in Table 2. Organisms which have been reported to produce flavoproteins include several species of bacteria and alga. However, unlike the case with ferredoxins, these proteins have not yet been found in higher plants and animals. 

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

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... 

Both plant and bacterial ferredoxins are small molecules. A summary of the determinations of their molecular weight is presented in Table 4. 

Table 4. A summary of the molecular weight determinations of bacterial and plant ferredoxin...

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)). 

Apella and San Pietro (4) Fry and San Pietro (47). Arnon (5) estimates spinach ferredoxin to have a molecular weight of 13,000 on the basis of iron content. 

Lovenberg, Buchanan, and Rabinowitz 65) showed that the molecular weight of C. pasteurianum ferredoxin is about 6000, based on sedimentation velocity and sedimentation equilibrium ultracentrifugation determinations and on amino acid analysis. The sedimentation coefficient, S2o,w was 1.4, and the partial specific volume, determined according to the method of Hvidt et al. 59) was 0.63, as compared to the value of 0.71 observed for most proteins. Similar investigations showed that ferredoxins from four other clostridia Lovenberg, Buchanan, and Rabinowitz 65)) and from a photosynthetic bacterium (Bachofen and Arnon 12)) also had a molecular weight of about 6000. 

The values for the molecular weight of plant ferredoxins reported by different investigators are not in agreement. Appella and San Pietro 4) reported a molecular weight of 17,000 for spinach ferredoxin, based on an S2o,w of 1.36 and a diffusion coefficient of 6.6 X 10 7 cm2/sec. The partial specific volume was assumed to be 0.71. A calculation by the writer for the minimal molecular weight of spinach ferredoxin from available amino acid composition data Fry and San Pietro (47)) and the assumption of six cysteine residues per mole gave a value of 13,000. Arnon (5)... 

Plant ferredoxin, therefore, appears to be twice the size of bacterial ferredoxin with a molecular weight of 13,000, although a higher value would seem possible from the available data. 

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-... 

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