Enzyme flavin reductase

There is some evidence that the iron-sulfur protein, FhuF, participates in the mobilization of iron from hydroxamate siderophores in E. coli (Muller et ah, 1998 Hantke, K. unpublished observations). However, a reductase activity of FhuF has not been demonstrated. Many siderophore-iron reductases have been shown to be active in vitro and some have been purified. The characterization of these reductases has revealed them to be flavin reductases which obtain the electrons for flavin reduction from NAD(P)H, and whose main functions are in areas other than reduction of ferric iron (e.g. flavin reductase Fre, sulfite reductase). To date, no specialized siderophore-iron reductases have been identified. It has been suggested that the reduced flavins from flavin oxidoreductases are the electron donors for ferric iron reduction (Fontecave et ah, 1994). Recently it has been shown, after a fruitless search for a reducing enzyme, that reduction of Co3+ in cobalamin is achieved by reduced flavin. Also in this case it was suggested that cobalamins and corrinoids are reduced in vivo by flavins which may be generated by the flavin... 

The flavin reductase has been purified by several researchers. This enzyme from R. erythropolis IGTS8 was partially purified by Ohshiro et al., and reported to have an optimum pH and temperature of 6.0 and 35°C, respectively [153], The DszD enzyme from IGTS8 was also purified [53] and reported to be of 25 kDa size however, no kinetic details related to DszD were reported. This enzyme couples with FMN with NADH to produce reduced flavin required for DszC and DszA catalyzed reactions. 

The thermophilic enzyme DszD from Paenibacillus All-2 has been cloned into E. coli and characterized [172], The sequence of this enzyme showed 30% similarity to the major flavin reductase of Vibrio fischeri. The optimum activity was reported to be at 45°C in resting cell cultures and 55°C in cell-free extracts. 

Based on the previous publications, azo dye can be reduced by azoreductase-catalyzed reduction under anaerobic conditions. But still there is a speculation whether bacterial flavin reductases are responsible for the azo reductase activity observed with bacterial cell extracts. In a published report, it is reported that flavin reductases are indeed able to act as azo reductases [24]. Bacteria produce extracellular oxidative enzymes, which are relatively nonspecific enzymes catalyzing the oxidation of a variety of dyes. It was reported that so many diverse groups of bacteria play a role in decolorization. It has been also reported that mixed microbial community could reduce various azo dyes, and members of the y-proteabacteria and sulfate reducing bacteria (SRB) were found to be prominent members of mixed bacterial population by using molecular methods to determine the microbial population dynamics [1],... 

Reduced flavins (FADH2, FMNH2, and riboflavin) generated by flavin-dependent reductases have been hypothesized to reduce azo dyes in a nonspecific chemical reaction, and flavin reductases have been revealed to be indeed anaerobic azoreductases. Other reduced enzyme cofactors, for example, NADH, NADH, NADPH, and an NADPH-generating system, have also been reported to reduce azo dyes. Except for enzyme cofactors, different artificial redox mediating compounds, especially such as quinines, are important redox mediators of azo dye anaerobic reduction (Table 1). 

The straightforward concept for the direct light-driven regeneration of flavin-dependent enzymes has been successfully applied for two representative classes of such enzymes a reductase and a monooxygenase. Therefore, it can be expected that this concept can also be applied to other flavin-dependent enzymes, potentially leading to additional practical catalyst systems for applications in synthetic organic chemistry. 

The hydroxylation reaction is directly effected by an enzyme-hemoprotein, monooxigenase, cytochrome P450 containing protocheme IX. The reduction of the enzyme involves flavin reductases and electron carriers, such as adrenodoxin, rubredoxin, and cytochrome b5. Dioxygen, being a weak one-electron oxidant, is activated after the reduction in the enzyme heme coordination sphere. The various forms of cytochrome P450 from liver microsomes and from Pseudomonas putida have a molecular mass of about 49000. One of the subunits of the enzyme from mitochondria of... 

The nitrite reductase system of Achromobacter fischeri appears to be composed of two separable enzymes (341). The first enzyme is a flavin reductase and utilizes NADH or NADPH to reduce FMN or FAD. The second interacts with the flavin reductase and converts nitrite and hy-droxylaraine to ammonia. The nitrite reductase enzyme has a molecular weight of 95,000 4,000 (Table XVII), contains two heme c per mole, and is inhibited by p-mercuribenzoate, cyanide, and carbon monoxide. 

Yubisui T, Matsuki T, Tanishima K, Takeshita M, and Yoneyama Y (1977) NADPH-flavin reductase in human erythrocytes and the reduction of methemoglobin through flavin by the enzyme. Biochemical and Biophysical Research Communications 76, 174-82. 

One possibihty for minimizing oxidized protein damage is the thiol repair (Fig. 3). This repair system requires either glutathione or the thioredoxin system. The thioredoxin/thioredoxin reductase repair system [10] is able to reduce disulfide bonds. It can dethiolate protein disulfides and thus is an extremely important regulator for redox homeostasis in the cells. Thioredoxin is a smaU ubiquitous protein that contains a pair of cysteines that undergo reversible oxidation and are re-reduced by the enzyme thioredoxine reductase. The thioredoxin reductase transfers electrons from NADPH to thioredoxin via a flavin. 

The biologically important product of the reaction, however, is light. Chromophoric accessory proteins that are expressed with luciferase modulate the color of emitted light. Other proteins including flavin reductases and enzymes that recycle the fatty acid back to the aldehyde are also expressed." " ... 

The ability of the Fe (DPAH)2/02/PhNHNHPh system (where PhNHNHPh is a mimic for flavin reductases) to monooxygenate saturated hydrocarbons closely parallels the chemistry of the methane monooxygenase proteins. However, the enzyme oxygenates 2-Me-butane with an isomer distribution of 82% primary alcohol, 10% secondary, and 8% tertiary. The present model gives a distribution of 21% primary, 29% secondary, and 50% tertiary. Clearly the protein affords a cavity that is selective for -CH3 groups. 

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