Bacterial ferredoxins molecular weight

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 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...

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-... 

It is not our purpose to survey here the intriguing aspects of the molecular evolution of these proteins but rather to stress, at this stage, the underlying unity of their chemical design. This justifies discussion of the conformational properties of the bacterial and plant ferredoxins under a common approach in spite of their distinctive molecular weights and iron-sulfur contents. The evolutionary relationship should not, however, be overemphasized indeed, the compositional differences provide valuable information regarding the structural factors that determine the conformational state of iron proteins in general. 

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