Phenol hydroxylase from Bacillus thermoglucosidasius A7, a two-protein component monooxygenase with a dual role for FAD

A Two-component Flavin-dependent Monooxygenase Involved in Actinorhodin Biosynthesis in Streptomyces coelicolor

The two-component flavin-dependent monooxygenases belong to an emerging class of enzymes involved in oxidation reactions in a number of metabolic and biosynthetic pathways in microorganisms. One component is a NAD(P)H:flavin oxidoreductase, which provides a reduced flavin to the second component, the proper monooxygenase. There, the reduced flavin activates molecular oxygen for substrate oxidation. Here, we study the flavin reductase ActVB and ActVA-ORF5 gene product, both reported to be involved in the last step of biosynthesis of the natural antibiotic actinorhodin in Streptomyces coelicolor. For the first time we show that ActVA-ORF5 is a FMN-dependent monooxygenase that together with the help of the flavin reductase ActVB catalyzes the oxidation reaction. The mechanism of the transfer of reduced FMN between ActVB and ActVA-ORF5 has been investigated. Dissociation constant values for oxidized and reduced flavin (FMNox and FMNred) with regard to ActVB and ActVA-ORF5 have been determined. The data clearly demonstrate a thermodynamic transfer of FMNred from ActVB to ActVA-ORF5 without involving a particular interaction between the two protein components. In full agreement with these data, we propose a reaction mechanism in which FMNox binds to ActVB, where it is reduced, and the resulting FMNred moves to ActVA-ORF5, where it reacts with O2 to generate a flavinperoxide intermediate. A direct spectroscopic evidence for the formation of such species within ActVA-ORF5 is reported.

Biochemical characterization of StyAB from Pseudomonas sp. strain VLB120 as a two-component flavin-diffusible monooxygenase

Pseudomonas sp. VLB120 uses styrene as a sole source of carbon and energy. The first step in this metabolic pathway is catalyzed by an oxygenase (StyA) and a NADH-flavin oxidoreductase (StyB). Both components have been isolated from wild-type Pseudomonas strain VLB120 as well as from recombinant Escherichia coli. StyA from both sources is a dimer, with a subunit size of 47 kDa, and catalyzes the enantioselective epoxidation of CC double bonds. Styrene is exclusively converted to S-styrene oxide with a specific activity of 2.1 U mg(-1) (k(cat) = 1.6 s(-1)) and K(m) values for styrene of 0.45 +/- 0.05 mM (wild type) and 0.38 +/- 0.09 mM (recombinant). The epoxidation reaction depends on the presence of a NADH-flavin adenine dinucleotide (NADH-FAD) oxidoreductase for the supply of reduced FAD. StyB is a dimer with a molecular mass of 18 kDa and a NADH oxidation activity of 200 U mg(-1) (k(cat) [NADH] = 60 s(-1)). Steady-state kinetics determined for StyB indicate a mechanism of sequential binding of NADH and flavin to StyB. This enzyme reduces FAD as well as flavin mononucleotide and riboflavin. The NADH oxidation activity does not depend on the presence of StyA. During the epoxidation reaction, no formation of a complex of StyA and StyB has been observed, suggesting that electron transport between reductase and oxygenase occurs via a diffusing flavin.

Structural studies on flavin reductase PheA2 reveal binding of NAD in an unusual folded conformation and support novel mechanism of action

The catabolism of toxic phenols in the thermophilic organism Bacillus thermoglucosidasius A7 is initiated by a two-component enzyme system. The smaller flavin reductase PheA2 component catalyzes the NADH-dependent reduction of free FAD according to a ping-pong bisubstrate-biproduct mechanism. The reduced FAD is then used by the larger oxygenase component PheA1 to hydroxylate phenols to the corresponding catechols. We have determined the x-ray structure of PheA2 containing a bound FAD cofactor (2.2 A), which is the first structure of a member of this flavin reductase family. We have also determined the x-ray structure of reduced holo-PheA2 in complex with oxidized NAD (2.1 A). PheA2 is a single domain homodimeric protein with each FAD-containing subunit being organized around a six-stranded beta-sheet and a capping alpha-helix. The tightly bound FAD prosthetic group (K(d) = 10 nm) binds near the dimer interface, and the re face of the FAD isoalloxazine ring is fully exposed to solvent. The addition of NADH to crystalline PheA2 reduced the flavin cofactor, and the NAD product was bound in a wide solvent-accessible groove adopting an unusual folded conformation with ring stacking. This is the first observation of an enzyme that is very likely to react with a folded compact pyridine nucleotide. The PheA2 crystallographic models strongly suggest that reactive exogenous FAD substrate binds in the NADH cleft after release of NAD product. Nanoflow electrospray mass spectrometry data indeed showed that PheA2 is able to bind one FAD cofactor and one FAD substrate. In conclusion, the structural data provide evidence that PheA2 contains a dual binding cleft for NADH and FAD substrate, which alternate during catalysis.