Flavodoxin reductase

Both GcpE and LytB must be associated with a reducing system to convert the oxidized [Fc4S4] + cluster into the active reduced [Fc4S4] + form (Fig. 3), which can be performed with the isolated enzymes from E. coli either by the biologic system flavodoxin/flavodoxin reductase/NADPH, by the semiquinone radical of 5-deazaflavin, or by dithionite. In cyanobacteria, and in plant chloroplasts where flavodoxin is absent, this reducing... 

The two-domain, structural motif in FNR represents a common structural feature in a large class of enzymes that catalyze electron transfer between a nicotinamide dinucleotide molecule and a one-electron carrier. Beside the photosynthetic electron-transfer enzyme, others non-photosynthetic ones include flavodoxin reductase, sulfite reductase, nitrate reductase, cytochrome reductase, and NADPH-cyto-chrome P450 reductase. FNR belongs to the group of so-called dehydrogenases-electron transferases, i.e., flavoproteins that catalyze electron transfer from two, one-electron donor molecules to a single two-electron acceptor molecule. 

This very interesting reaction involves the formal insertion of a sulfur atom into two unactivated CH bonds. The purified protein contains a [2Fe-2S] cluster [131]. A defined system, consisting of biotin synthase, flavodoxin, flavodoxin reductase, fructose 1,6-bisphosphate, cysteine, DTT, NADPH, ferrous ion, and SAM, capable of catalyzing the conversion of dethiobiotin to biotin has been characterized [132-134]. This system is still incomplete and gives a maximum of two moles of biotin per mole of biotin synthase. 

For the reductive cleavage of SAM into methionine and the deoxyadenosyl radical (DOA ), an electron has to be transferred from a reduced [Fe-S] cluster. The reducing system consisting of NADPH, flavodoxine, and flavodoxine reductase has been identified in E. This allowed to set up an in vitro assay, which besides the... 

Figure 15 Cleavage of S-adenosyl-L-methionine by enzymes within the radicai SAM superfamiiy. The reductive cleavage reaction generates a 5 -deoxyadenosyl 5 -radical and L-methionine, a spectator in the reaction. The physiological electron donor is the flavodoxin/flavodoxin reductase reducing system, with electrons deriving ultimately from NADPH. Artificial electron donors such as sodium dithionite and 5-deazaflavin plus light can also effect reduction in in vitro activity determinations.

Figure 13.2. A proposal for a simplified classification of CYPs with reference to either use of a ferredoxin or alternative electron transport mechanism. Class la— typical bacterial system supported by ferredoxin and ferredoxin reductase, for example, CYPlOl Class Ib—Rhodococcus sp. CYP fusion protein containing a ferredoxin domain Class Ic—Methylococcus capsulatus CYPS 1-ferredoxin fusion . Class 11a— typical eukaryotic CYP/NADPH-cytochrome P450 reductase system Class lib—a fusion protein of a CYP and flavoprotein reductase, for example, P450gM 3 Class IIc—P450 j containing separate flavodoxin and flavodoxin reductase partners . Class fll—standalone functional CYPs, for example, P450jj. ...

The other two proteins appeared to be auxilliary proteins involved in the activation of the small component of the reductase. They were identified as the NADPH flavodoxin reductase and flavodoxin [35, 36]. Ferredoxin could not substitute for flavodoxin. 

Hall DA, Vander Kooi CW et al (2001) Mapping the interactions between flavodoxin and its physiological partners flavodoxin reductase and cobalamin-dependent methionine synthase. Proc Natl Acad Sci USA 98(17) 9521-9526... 

Jenkins CM, Waterman MR (1994) Flavodoxin and NADPH-flavodoxin reductase from Escherichia coli support bovine cytochrome P450C17 hydroxylase activities. J Biol Chem 269 27401-27408... 

Jenkins CM, Waterman MR (1998) NADPH-flavodoxin reductase and flavodoxin from Escherichia coli characteristics as a soluble microsomal P450 reductase. Biochemistry 37 6106-6113... 

FdR ferredoxin/flavodoxin reductase, Fdx OFOR 2-oxoacid-ferredoxin oxidoreductase. 

The cytochrome P450 reductases from class II are thought to be a fusion of a FAD-containing reductase (such as ferredoxin or flavodoxin reductase) and flavodoxin. This family of enzymes includes the flavoprotein subunit of sulfite reductase and the flavoprotein domains of natural fusion proteins such as nitric oxide synthase and P450PM3. The pathway of electron transfer in these enzymes is NADPH -> FAD FMN. The FMN serves as a one-electron donor to cytochrome c. 

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