Flavodoxins reduced

According to recent data, the property of dithionite as an electron donor for nitrogenase is different from that of the natural donor flavodoxin (Burgess and Lowe, 1996). Flavodoxin from Azotobacter vinelandii has the redox potential equal to -0.515 V for the reversible transition between the semiquinone and hydroquinone forms of flavodoxin. Unlike dithionite, flavodoxin can reversibly reduce the [Fe4S4]+l cluster Av2 by one electron to the [Fe4S4]° state in which all iron ions exist in the ferrous form. It is assumed that, under natural conditions, two electrons can transfer from Av2 to Avl. Flavodoxin reduces both Av2 bound to Avl and free Av2 in a solution. The apparent rate constants of these reactions are 400 s 1 and > 1000 s"1, respectively (Duyvis et al. 1998). 

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

With incorporation of one atom of oxygen 1.19 Acting on reduced flavodoxin as donor... 

Nitrogenase (ferredoxin) [EC 1.18.6.1] catalyzes the reaction of three reduced ferredoxin molecules with protons, N2, and n ATP molecules to produce three oxidized ferredoxin molecules, two ammonia molecules, n ADP molecules, and n orthophosphate molecules where n is between 12 and 18. This iron-sulfur system also uses either molybdenum or vanadium ions. (2) Nitrogenase (flavodoxin) [EC 1.19.6.1] catalyzes the reaction of six reduced flavodoxin molecules with protons, N2, and n ATP molecules to produce six oxidized flavodoxin molecules, two ammonia molecules, n ADP molecules, and n orthophosphate molecules. This system uses iron-sulfur and molybdenum ions. 

C(8) to be parallel with the nitrogen orbitals, thus providing evidence for the planarity of flavin semiquinones 2. More recent X-ray crystallographic data on the Cl. MP flavodoxin provide confirmatory data on the planarity of the iso-alloxazine ring in its one-electron reduced form. 

All known flavodoxins have low molecular weights (14,500-23,000 g mole" ) and contain a single polypeptide chain and a single bound FMN. Upon the addition of one reducing equivalent, the FMN adds one electron and one proton to form the neutral semiquinone. This species is blue in color (as contrasted to the yellow color of the oxidized form) and has broad absorption bands in the visible between 400 and 700 nm. Addition of a second equivalent leads to a one-electron reduction to the hydroquinone, which is pale yellow in color and has weak bands at around 450 and 365 nm. 

Fig. 19-5), reduced flavodoxin, and perhaps other sources playing a role. In at least one species, the ultimate source of electrons to reduce ferredoxin is pyruvate (Fig. 22-2). 

If an enzyme binds a flavin radical much more tightly than the fully oxidized or reduced forms, reduction of the flavoprotein will take place in two one-electron steps. In such proteins the values of E° for the two steps may be widely separated. The best known examples are the small, low-potential electron-carrying proteins known as flavodoxins.266 269a These proteins, which carry electrons between pairs of other redox proteins, have a variety of functions in anaerobic and photosynthetic bacteria, cyanobacteria, and green algae. Their functions are similar to those of the ferredoxins, iron-sulfur proteins that are considered in Chapter 16. 

The mechanism of the cleavage of the pyruvate in Eq. 15-37 is not obvious. Thiamin diphosphate is not involved, and free C02 is not formed. The first identified intermediate is an acetyl-enzyme containing a thioester linkage to a cysteine side chain. This is cleaved by reaction with CoA-SH to give the final product. A clue came when it was found by Knappe and coworkers that the active enzyme, which is rapidly inactivated by oxygen, contains a long-lived free radical.326 Under anaerobic conditions cells convert the inactive form E to the active form Ea by an enzymatic reaction with S-adenosylmethionine and reduced flavodoxin Fd(red) as shown in Eq. 15-38.327-329 A deactivase reverses the process.330... 

Flavin adenine dinucleotide. See FAD Flavin adenine diphosphate. See FAD Flavin coenzymes 766,780 - 795 modified 788, 789 reduced 794 Flavin radicals 792 color of 794 formation constant 794 Flavocytochrome b2 782, 794, 847 Flavodoxins 793, 799, 800 Flavoprotein(s) 513, 788... 

The soluble electron carriers released from the reaction centers into the cytoplasm of bacteria or into the stroma of chloroplasts are reduced single-electron carriers. Bacterial ferredoxin with two Fe4S4 clusters is formed by bacteria if enough iron is present. In its absence flavodoxin (Chapter 15), which may carry either one or two electrons, is used. In chloroplasts the carrier is the soluble chloroplast ferredoxin (Fig. 16-16,C), which contains one Fe2S2 center. Reduced ferredoxin transfers electrons to NADP+ (Eq. 15-28) via ferredoxin NADP oxidoreductase, a flavoprotein of known three-dimensional structure.367 369... 

Not only are two molecules of ATP hydrolyzed to pump each electron, but the Fe-protein must receive electrons from a powerful (low E°) reductant such as reduced ferredoxin, reduced flavodoxin, or dithionite. Klebsiella pneumoniae contains a pyruvate flavodoxin oxidoreductase (Eq. 15-35) that reduces either flavodoxin or ferredoxin to provide the low potential electron donor.29 30 In some bacteria, e.g., the strictly aerobic Azotobacter, NADPH is the electron donor for reduction of N2. The Fe-protein is thought to accept electrons from a chain that includes at least the ordinary bacterial ferredoxin (Fd) and a special one-electron-accepting azotoflavin, a flavoprotein that is somewhat larger than the flavodoxins (Chapter 15) and appears to play a specific role in N2 fixation.31 In Clostridium and Rhizobium reduced ferredoxins generated by cleavage of pyruvate reduce nitrogenase directly.32... 

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