Flavoprotein reductases

Although reduction of chromate Cr to Cr has been observed in a number of bacteria, these are not necessarily associated with chromate resistance. For example, reduction of chromate has been observed with cytochrome Cj in Desulfovibrio vulgaris (Lovley and Phillips 1994), soluble chromate reductase has been purified from Pseudomonas putida (Park et al. 2000), and a membrane-bound reductase has been purified from Enterobacter cloacae (Wang et al. 1990). The flavoprotein reductases from Pseudomonas putida (ChrR) and Escherichia coli (YieF) have been purified and can reduce Cr(VI) to Cr(III) (Ackerley et al. 2004). Whereas ChrR generated a semi-quinone and reactive oxygen species, YieR yielded no semiquinone, and is apparently an obligate four-electron reductant. It could therefore present a suitable enzyme for bioremediation. 

Gomes CM, Vicente JB,Wasserfallen A,Teixeira M. 2000. Spectroscopic studies and characterization of a novel electron-transfer chain from Escherichia coli involving a flavorubredoxin and its flavoprotein reductase partner. Biochemistry 39 16230-7. 

It should be noted that non-heme oxygenases can also degrade aromatics such as biphenyls and naphthalene (Scheme 7.23). A naphthalene dioxygenase consists of a catalytic oxygenase component with a mononuclear iron site, an iron-sulfur flavoprotein reductase and an iron-sulfur ferredoxin transferring electrons from... 

Porter, T. D. (1991). An unusual yet strongly conserved flavoprotein reductase in bacteria and mammals. Trends Biochem. Sci. 16, 154-158. 

One oxygen of 02 is incorporated into the hydroxyl group of the product, whereas the other is incorporated into water. The enzymes usually include a cytochrome P450, an iron-sulfur cluster-containing protein, such as adrenodoxin or putidaredoxin, and a flavoprotein reductase. 

Pseudomonas oleovorans contains P. oleovorans monooxygenase (POM), which is a typical co-hydroxylase for hydroxylation of the terminal methyl of alkanes as well as epoxidation of terminal olefins. The co-hydroxylation system of P. oleovorans was reconstituted from purified components, POM, rubredoxin, and a flavoprotein reductase [122], In the presence of NADH and oxygen, it oxidizes a wide range of aliphatic methyl alkyl sulfides. Enantioselectivities are very much dependent of the length of the alkyl chain of Me-S(0)-R, as exemplified by the following results ... 

Nitro Reduction. Aromatic amines are susceptible to reduction by both bacterial and mammalian nitroreductase systems. Convincing evidence has been presented that this reaction sequence is catalyzed by CYP. It is inhibited by oxygen, although NADPH is still consumed. Earlier workers had suggested a flavoprotein reductase was involved, and it is not clear if this is incorrect or if both mechanisms occur. It is true, however, that high concentration of FAD or FMN will catalyze the nonenzymatic reduction of nitro groups. 

It is generally accepted that liver microsomes contain two electron transport systems each containing a flavoprotein reductase. The paths of electron flow can be outlined as follows ... 

Lu, G., Campbell, W. H., Schneider, G., and Lindqvist, Y., 1994, Crystal structure of the FAD-containing fragment of corn nitrate reductase at 2.5 A resolution relationship to other flavoprotein reductases. Structure 2 8099821. 

The catalytic cycle of each flavoenzyme consists of two distinct processes, the acceptance of redox equivalents from a substrate and the transfer of these equivalents to an acceptor. Accordingly, the catalyzed reactions consist of two half-reactions a reductive half-reaction in which the flavin is reduced and an oxidative half-reaction, in which the reduced flavin is reoxidized. This review summarizes the chemistry of simple flavoprotein reductases, dehydrogenases, (di)thiol oxidoreductases, oxidases, and monooxygenases (Table 1) (5 0) This grouping provides a good appreciation about what type of common mechanisms can be distinguished and what type of substrates can be converted. Information on the chemistry of complex flavoenzymes can be found in the Further Reading section. 

After binding of the substrate to oxidized cytochrome P450, a one-electron transfer from NADPH is catalyzed by a flavoprotein reductase. Molecular oxygen combines with the reduced enzyme-substrate complex to form a ternary complex that accepts a second electron, possibly derived from NADH via cytochrome bs. One oxygen atom is transferred from the activated oxygen to the substrate to form the oxidized product. Water and oxidized cytochrome P450 also result. 

Kadiyala and Spain 1998), 4-nitrophenol degradation involves initial hydroxylation to 4-nitrocatechol followed by elimination of nitrite with the formation of benzene-l,2,4-triol before ring fission to P-ketoadipate. In B. sphaericus, the first two reactions are carried out by a single two-component enzyme composed of a flavoprotein reductase and an oxygenase. 

Naphthalene dioxygenase consists of three components, which form an electron transfer chain an NADH-dependent flavoprotein reductase, a ferredoxin containing two [2Fe2S] Rieske iron-sulfur clusters, and a Rieske oxygenase containing both a [2Fe2S] Rieske iron-sulfiir cluster and a mononuclear iron(II) center in the enzyme active site. ° ... 

Shames, S. L., Fairlamb, A. H., Cerami, A. and Walsh, C. T. (1986) Purification and characterization of trypanothione reductase from Crithidia fasciculata, a newly discovered member of the family of disulfide-containing flavoprotein reductases. Biochemistry 25 3519-3526. 

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. ...

---