Ferredoxins, molecular electronic

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. 

Tan, M.L. Dolan, E.A. Ichiye, T., Understanding intramolecular electron transfer in ferredoxin a molecular dynamics study, J. Phys. Chem. B 2004,108, 20435-20441. 

The key enzyme hydrogenase catalyses the reversible reduction of protons to molecular hydrogen. Inhibitor experiments indicate that the ferredoxin PetF functions as natural electron donor linking the hydrogenase to the photosynthetic electron transport chain [Florin et al., 2001],... 

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... 

Bacterial ferredoxins. Bacterial ferredoxin was first described in 1962 by Mortenson et al. (p who found a low-molecular iron protein involved in electron transfer of pyruvate hydrogenase and nitrogenase in C. pasteurianum. Subsequently, a number of ferredoxins have been found lii widely different types of bacteria such as photosynthetic bacteria and N2-fixing bacteria. These bacterial type ferredoxins have molecular... 

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 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... 

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