Studies on the mechanism of p-hydroxyphenylacetate 3-hydroxylase from Pseudomonas aeruginosa: a system composed of a small flavin reductase and a large flavin-dependent oxygenase

Biosynthesis of 3-Hydroxyphloretin Using Rational Design of 4-Hydroxyphenylacetate 3-Monooxygenase

The compound 3-hydroxyphloretin is a typical dihydrochalcone that can be obtained in plants by the 3-hydroxylation of phloretin. Here, the flavin-dependent two-component monooxygenase (HpaBC) derived from Pseudomonas aeruginosa was used to convert phloretin into 3-hydroxyphloretin. Following molecular docking and sequence alignment, modifications to the substrate pocket and loop of PaHpaBC were rationally designed, and mutant residues were selected. The results showed that the mutant Q212G/F292A/Q376N gave the best yield of 3-hydroxyphloretin and showed improved catalytic efficiency. Under optimal reaction condition, 2.03 g/L of 3-hydroxyphloretin was produced in the whole-cell catalysis experiment. Molecular docking and molecular dynamics simulations were used to analyze mutants and elucidate the potential mechanism. It was found that the increase in 3-hydroxyphloretin yield was due to the improvement in the flexibility of the loop and the expansion of its substrate pocket. This strategy based on loop and substrate pocket modification has significance in the engineering of PaHpaB.

Unifying and versatile features of flavin-dependent monooxygenases: Diverse catalysis by a common C4a-(hydro)peroxyflavin

Flavin-dependent monooxygenases (FDMOs) are known for their remarkable versatility and for their crucial roles in various biological processes and applications. Extensive research has been conducted to explore the structural and functional relationships of FDMOs. The majority of reported FDMOs utilize C4a-(hydro)peroxyflavin as a reactive intermediate to incorporate an oxygen atom into a wide range of compounds. This review discusses and analyzes recent advancements in our understanding of the structural and mechanistic features governing the enzyme functions. State-of-the-art discoveries related to common and distinct structural properties governing the catalytic versatility of the C4a-(hydro)peroxyflavin intermediate in selected FDMOs are discussed. Specifically, mechanisms of hydroxylation, dehalogenation, halogenation, and light-emitting reactions by FDMOs are highlighted. We also provide new analysis based on the structural and mechanistic features of these enzymes to gain insights into how the same intermediate can be harnessed to perform a wide variety of reactions. Challenging questions to obtain further breakthroughs in the understanding of FDMOs are also proposed.

Advances in 4-Hydroxyphenylacetate-3-hydroxylase Monooxygenase

Catechols have important applications in the pharmaceutical, food, cosmetic, and functional material industries. 4-hydroxyphenylacetate-3-hydroxylase (4HPA3H), a two-component enzyme system comprising HpaB (monooxygenase) and HpaC (FAD oxidoreductase), demonstrates significant potential for catechol production because it can be easily expressed, is highly active, and exhibits ortho-hydroxylation activity toward a broad spectrum of phenol substrates. HpaB determines the ortho-hydroxylation efficiency and substrate spectrum of the enzyme; therefore, studying its structure-activity relationship, improving its properties, and developing a robust HpaB-conducting system are of significance and value; indeed, considerable efforts have been made in these areas in recent decades. Here, we review the classification, molecular structure, catalytic mechanism, primary efforts in protein engineering, and industrial applications of HpaB in catechol synthesis. Current trends in the further investigation of HpaB are also discussed.

Modification of the 4-Hydroxyphenylacetate-3-hydroxylase Substrate Pocket to Increase Activity towards Resveratrol

4-Hydroxyphenylacetate-3-hydroxylase (4HPA3H; EC 1.14.14.9) is a heterodimeric flavin-dependent monooxygenase complex that catalyzes the ortho-hydroxylation of resveratrol to produce piceatannol. Piceatannol has various health benefits and valuable applications in food, medicine, and cosmetics. Enhancing the catalytic activity of 4HPA3H toward resveratrol has the potential to benefit piceatannol production. In this study, the critical amino acid residues in the substrate pocket of 4HPA3H that affect its activity toward resveratrol were identified using semi-rational engineering. Two key amino acid sites (I157 and A211) were discovered and the simultaneous "best" mutant I157L/A211D enabled catalytic efficiency (Kcat/Km-resveratrol) to increase by a factor of 4.7-fold. Molecular dynamics simulations indicated that the increased flexibility of the 4HPA3H substrate pocket has the potential to improve the catalytic activity of the enzyme toward resveratrol. On this basis, we produced 3.78 mM piceatannol by using the mutant I157L/A211D whole cells. In this study, we successfully developed a highly active 4HPA3H variant for the hydroxylation of resveratrol to piceatannol.

Caffeic acid production from glucose using metabolically engineered Escherichia coli

Caffeic acid (3,4-dihydroxycinnamic acid) is a precursor for high-valued compounds with anticancer, antiviral activities, and anti-inflammatory making it an important substance in the food additive, cosmetics, and pharmaceutical industries. Here, we developed an engineered Escherichia coli strain capable of directly producing high levels of caffeic acid from glucose. Tyrosine ammonia-lyase from Rhodotorula glutinis (RgTAL) and p-coumaric acid 3-hydroxylase from Saccharothrix espanaensis (SeC3H) were expressed. Next, feedback-resistant chorismate mutase/prephenate dehydrogenase, was introduced to promote l-tyrosine synthesis. This engineered strain CA3 produced 1.58 g/L of caffeic acid from glucose without tyrosine supplemented to the medium. Furthermore, to reduce p-coumaric acid accumulation, 4-hydroxyphenylacetate 3-hydroxylase from Pseudomonas aeruginosa (PaHpaBC) was introduced. Finally, an engineered strain CA8 directly produced 6.17 g/L of caffeic acid from glucose using a jar fermenter. The E. coli developed in this study would be helpful as a chassis strain to produce value-added caffeic acid-derivatives.

Microbial lignin valorization through depolymerization to aromatics conversion

Lignin is the most abundant source of renewable aromatic biopolymers and its valorization presents significant value for biorefinery sustainability, which promotes the utilization of renewable resources. However, it is challenging to fully convert the structurally complex, heterogeneous, and recalcitrant lignin into high-value products. The in-depth research on the lignin degradation mechanism, microbial metabolic pathways, and rational design of new systems using synthetic biology have significantly accelerated the development of lignin valorization. This review summarizes the key enzymes involved in lignin depolymerization, the mechanisms of microbial lignin conversion, and the lignin valorization application with integrated systems and synthetic biology. Current challenges and future strategies to further study lignin biodegradation and the trends of lignin valorization are also discussed.

Comparative Genomic Analysis of Carbofuran-Degrading Sphingomonads Reveals the Carbofuran Catabolism Mechanism in Sphingobium sp. Strain CFD-1

Owing to the extensive use of carbofuran over the past 50 years, bacteria have evolved catabolic pathways to mineralize this insecticide, which plays an important role in eliminating carbofuran residue in the environment. In this study, the cfd gene cluster, responsible for the catabolism of carbofuran phenol, was predicted by comparing sphingomonad genomes.

In silico screening and heterologous expression of soluble dimethyl sulfide monooxygenases of microbial origin in Escherichia coli

Abstract

Sequence-based screening has been widely applied in the discovery of novel microbial enzymes. However, majority of the sequences in the genomic databases were annotated using computational approaches and lacks experimental characterization. Hence, the success in obtaining the functional biocatalysts with improved characteristics requires an efficient screening method that considers a wide array of factors. Recombinant expression of microbial enzymes is often hampered by the undesirable formation of inclusion body. Here, we present a systematic in silico screening method to identify the proteins expressible in soluble form and with the desired biological properties. The screening approach was adopted in the recombinant expression of dimethyl sulfide (DMS) monooxygenase in Escherichia coli. DMS monooxygenase, a two-component enzyme consisting of DmoA and DmoB subunits, was used as a model protein. The success rate of producing soluble and active DmoA is 71% (5 out of 7 genes). Interestingly, the soluble recombinant DmoA enzymes exhibited the NADH:FMN oxidoreductase activity in the absence of DmoB (second subunit), and the cofactor FMN, suggesting that DmoA is also an oxidoreductase. DmoA originated from Janthinobacterium sp. AD80 showed the maximum NADH oxidation activity (maximum reaction rate: 6.6 µM/min; specific activity: 133 µM/min/mg). This novel finding may allow DmoA to be used as an oxidoreductase biocatalyst for various industrial applications. The in silico gene screening methodology established from this study can increase the success rate of producing soluble and functional enzymes while avoiding the laborious trial and error involved in the screening of a large pool of genes available.

Key points

• A systematic gene screening method was demonstrated.

• DmoA is also an oxidoreductase capable of oxidizing NADH and reducing FMN.

• DmoA oxidizes NADH in the absence of external FMN.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-12008-8.

Structural insights into a flavin-dependent dehalogenase HadA explain catalysis and substrate inhibition via quadruple π-stacking

HadA is a flavin-dependent monooxygenase catalyzing hydroxylation plus dehalogenation/denitration, which is useful for biodetoxification and biodetection. In this study, the X-ray structure of wild-type HadA (HadA_WT) co-complexed with reduced FAD (FADH^–) and 4-nitrophenol (4NP) (HadA_WT−FADH^–−4NP) was solved at 2.3-Å resolution, providing the first full package (with flavin and substrate bound) structure of a monooxygenase of this type. Residues Arg101, Gln158, Arg161, Thr193, Asp254, Arg233, and Arg439 constitute a flavin-binding pocket, whereas the 4NP-binding pocket contains the aromatic side chain of Phe206, which provides π-π stacking and also is a part of the hydrophobic pocket formed by Phe155, Phe286, Thr449, and Leu457. Based on site-directed mutagenesis and stopped-flow experiments, Thr193, Asp254, and His290 are important for C4a-hydroperoxyflavin formation with His290, also serving as a catalytic base for hydroxylation. We also identified a novel structural motif of quadruple π-stacking (π-π-π-π) provided by two 4NP and two Phe441 from two subunits. This motif promotes 4NP binding in a nonproductive dead-end complex, which prevents C4a-hydroperoxy-FAD formation when HadA is premixed with aromatic substrates. We also solved the structure of the HadA_Phe441Val−FADH^–−4NP complex at 2.3-Å resolution. Although 4NP can still bind to this variant, the quadruple π-stacking motif was disrupted. All HadA_Phe441 variants lack substrate inhibition behavior, confirming that quadruple π-stacking is a main cause of dead-end complex formation. Moreover, the activities of these HadA_Phe441 variants were improved by ⁓20%, suggesting that insights gained from the flavin-dependent monooxygenases illustrated here should be useful for future improvement of HadA’s biocatalytic applications.

Steady-state kinetic analysis of halogenase-supporting flavin reductases BorF and AbeF reveals different kinetic mechanisms

The short-chain flavin reductases BorF and AbeF reduce FAD to FADH₂, which is then used by flavin-dependent halogenases (BorH and AbeH respectively) to regioselectively chlorinate tryptophan in the biosynthesis of indolotryptoline natural products. Recombinant AbeF and BorF were overexpressed and purified as homodimers from E. coli, and copurified with substoichiometric amounts of FAD, which could be easily removed. AbeF and BorF can reduce FAD, FMN, and riboflavin in vitro and are selective for NADH over NADPH. Initial velocity studies in the presence and absence of inhibitors showed that BorF proceeds by a sequential ordered kinetic mechanism in which FAD binds first, while AbeF follows a random-ordered sequence of substrate binding. Fluorescence quenching experiments verified that NADH does not bind BorF in the absence of FAD, and that both AbeF and BorF bind FAD with higher affinity than FADH₂. pH-rate profiles of BorF and AbeF were bell-shaped with maximum k_cat at pH 7.5, and site-directed mutagenesis of BorF implicated His160 and Arg38 as contributing to the catalytic activity and the pH dependence.

Biochemical Characterization of the Two-Component Flavin-Dependent Monooxygenase Involved in Valanimycin Biosynthesis

The flavin reductase (FRED) and isobutylamine N-hydroxylase (IBAH) from Streptomyces viridifaciens constitute a two-component, flavin-dependent monooxygenase system that catalyzes the first step in valanimycin biosynthesis. FRED is an oxidoreductase that provides the reduced flavin to IBAH, which then catalyzes the hydroxylation of isobutylamine (IBA) to isobutylhydroxylamine (IBHA). In this work, we used several complementary methods to investigate FAD binding, steady-state and rapid reaction kinetics, and enzyme-enzyme interactions in the FRED:IBAH system. The affinity of FRED for FAD_ox is higher than its affinity for FAD_red, consistent with its function as a flavin reductase. Conversely, IBAH binds FAD_red more tightly than FAD_ox, consistent with its role as a monooxygenase. FRED exhibits a strong preference (28-fold) for NADPH over NADH as the electron source for FAD reduction. Isothermal titration calorimetry was used to study the association of FRED and IBAH. In the presence of FAD, either oxidized or reduced, FRED and IBAH associate with a dissociation constant of 7-8 μM. No interaction was observed in the absence of FAD. These results are consistent with the formation of a protein-protein complex for direct transfer of reduced flavin from the reductase to the monooxygenase in this two-component system.

Phenolic hydroxylases

Many flavin-dependent phenolic hydroxylases (monooxygenases) have been extensively investigated. Their crystal structures and reaction mechanisms are well understood. These enzymes belong to groups A and D of the flavin-dependent monooxygenases and can be classified as single-component and two-component flavin-dependent monooxygenases. The insertion of molecular oxygen into the substrates catalyzed by these enzymes is beneficial for modifying the biological properties of phenolic compounds and their derivatives. This chapter provides an in-depth discussion of the structural features of single-component and two-component flavin-dependent phenolic hydroxylases. The reaction mechanisms of selected enzymes, including 3-hydroxy-benzoate 4-hydroxylase (PHBH) and 3-hydroxy-benzoate 6-hydroxylase as representatives of single-component enzymes and 3-hydroxyphenylacetate 4-hydroxylase (HPAH) as a representative of two-component enzymes, are discussed in detail. This chapter comprises the following four main parts: general reaction, structures, reaction mechanisms, and enzyme engineering for biocatalytic applications. Enzymes belonging to the same group catalyze similar reactions but have different unique structural features to control their reactivity to substrates and the formation and stabilization of C4a-hydroperoxyflavin. Protein engineering has been employed to improve the ability to use these enzymes to synthesize valuable compounds. A thorough understanding of the structural and mechanistic features controlling enzyme reactivity is useful for enzyme redesign and enzyme engineering for future biocatalytic applications.

Monooxygenation of aromatic compounds by flavin-dependent monooxygenases

Many flavoenzymes catalyze hydroxylation of aromatic compounds especially phenolic compounds have been isolated and characterized. These enzymes can be classified as either single-component or two-component flavin-dependent hydroxylases (monooxygenases). The hydroxylation reactions catalyzed by the enzymes in this group are useful for modifying the biological properties of phenolic compounds. This review aims to provide an in-depth discussion of the current mechanistic understanding of representative flavin-dependent monooxygenases including 3-hydroxy-benzoate 4-hydroxylase (PHBH, a single-component hydroxylase), 3-hydroxyphenylacetate 4-hydroxylase (HPAH, a two-component hydroxylase), and other monooxygenases which catalyze reactions in addition to hydroxylation, including 2-methyl-3-hydroxypyridine-5-carboxylate oxygenase (MHPCO, a single-component enzyme that catalyzes aromatic-ring cleavage), and HadA monooxygenase (a two-component enzyme that catalyzes additional group elimination reaction). These enzymes have different unique structural features which dictate their reactivity toward various substrates and influence their ability to stabilize flavin intermediates such as C4a-hydroperoxyflavin. Understanding the key catalytic residues and the active site environments important for governing enzyme reactivity will undoubtedly facilitate future work in enzyme engineering or enzyme redesign for the development of biocatalytic methods for the synthesis of valuable compounds.

Design and engineering of whole-cell biocatalytic cascades for the valorization of fatty acids

This review presents the key factors to construct a productive whole-cell biocatalytic cascade exemplified for the biotransformation of renewable fatty acids.

A comparison between the homocyclic aromatic metabolic pathways from plant-derived compounds by bacteria and fungi

Aromatic compounds derived from lignin are of great interest for renewable biotechnical applications. They can serve in many industries e.g. as biochemical building blocks for bioplastics or biofuels, or as antioxidants, flavor agents or food preservatives. In nature, lignin is degraded by microorganisms, which results in the release of homocyclic aromatic compounds. Homocyclic aromatic compounds can also be linked to polysaccharides, tannins and even found freely in plant biomass. As these compounds are often toxic to microbes already at low concentrations, they need to be degraded or converted to less toxic forms. Prior to ring cleavage, the plant- and lignin-derived aromatic compounds are converted to seven central ring-fission intermediates, i.e. catechol, protocatechuic acid, hydroxyquinol, hydroquinone, gentisic acid, gallic acid and pyrogallol through complex aromatic metabolic pathways and used as energy source in the tricarboxylic acid cycle. Over the decades, bacterial aromatic metabolism has been described in great detail. However, the studies on fungal aromatic pathways are scattered over different pathways and species, complicating a comprehensive view of fungal aromatic metabolism. In this review, we depicted the similarities and differences of the reported aromatic metabolic pathways in fungi and bacteria. Although both microorganisms share the main conversion routes, many alternative pathways are observed in fungi. Understanding the microbial aromatic metabolic pathways could lead to metabolic engineering for strain improvement and promote valorization of lignin and related aromatic compounds.

Two-Component FAD-Dependent Monooxygenases: Current Knowledge and Biotechnological Opportunities

Flavoprotein monooxygenases create valuable compounds that are of high interest for the chemical, pharmaceutical, and agrochemical industries, among others. Monooxygenases that use flavin as cofactor are either single- or two-component systems. Here we summarize the current knowledge about two-component flavin adenine dinucleotide (FAD)-dependent monooxygenases and describe their biotechnological relevance. Two-component FAD-dependent monooxygenases catalyze hydroxylation, epoxidation, and halogenation reactions and are physiologically involved in amino acid metabolism, mineralization of aromatic compounds, and biosynthesis of secondary metabolites. The monooxygenase component of these enzymes is strictly dependent on reduced FAD, which is supplied by the reductase component. More and more representatives of two-component FAD-dependent monooxygenases have been discovered and characterized in recent years, which has resulted in the identification of novel physiological roles, functional properties, and a variety of biocatalytic opportunities.

Hydrolase CehA and Monooxygenase CfdC Are Responsible for Carbofuran Degradation in Sphingomonas sp. Strain CDS-1

Carbofuran, a broad-spectrum systemic insecticide, has been extensively used for approximately 50 years. Diverse carbofuran-degrading bacteria have been described, among which sphingomonads have exhibited an extraordinary ability to catabolize carbofuran; other bacteria can only convert carbofuran to carbofuran phenol, while all carbofuran-degrading sphingomonads can degrade both carbofuran and carbofuran phenol. However, the genetic basis of carbofuran catabolism in sphingomonads has not been well elucidated. In this work, we sequenced the draft genome of Sphingomonas sp. strain CDS-1 that can transform both carbofuran and carbofuran phenol but fails to grow on them. On the basis of the hypothesis that the genes involved in carbofuran catabolism are highly conserved among carbofuran-degrading sphingomonads, two such genes, cehA_CDS-1 and cfdC_CDS-1, were predicted from the 84 open reading frames (ORFs) that share ≥95% nucleic acid similarities between strain CDS-1 and another sphingomonad Novosphingobium sp. strain KN65.2 that is able to mineralize the benzene ring of carbofuran. The results of the gene knockout, genetic complementation, heterologous expression, and enzymatic experiments reveal that cehA_CDS-1 and cfdC_CDS-1 are responsible for the conversion of carbofuran and carbofuran phenol, respectively, in strain CDS-1. CehA_CDS-1 hydrolyzes carbofuran to carbofuran phenol. CfdC_CDS-1, a reduced flavin mononucleotide (FMNH₂)- or reduced flavin adenine dinucleotide (FADH₂)-dependent monooxygenase, hydroxylates carbofuran phenol at the benzene ring in the presence of NADH, FMN/FAD, and the reductase CfdX. It is worth noting that we found that carbaryl hydrolase CehA_AC100, which was previously demonstrated to have no activity toward carbofuran, can actually convert carbofuran to carbofuran phenol, albeit with very low activity.IMPORTANCE Due to the extensive use of carbofuran over the past 50 years, bacteria have evolved catabolic pathways to mineralize this insecticide, which plays an important role in eliminating carbofuran residue in the environment. This study revealed the genetic determinants of carbofuran degradation in Sphingomonas sp. strain CDS-1. We speculate that the close homologues cehA and cfdC are highly conserved among other carbofuran-degrading sphingomonads and play the same roles as those described here. These findings deepen our understanding of the microbial degradation mechanism of carbofuran and lay a foundation for the better use of microbes to remediate carbofuran contamination.

Crystal structure of the flavin reductase of Acinetobacter baumannii p-hydroxyphenylacetate 3-hydroxylase (HPAH) and identification of amino acid residues underlying its regulation by aromatic ligands

The first step in the degradation of p-hydroxyphenylacetic acid (HPA) is catalyzed by the two-component enzyme p-hydroxyphenylacetate 3-hydroxylase (HPAH). The two components of Acinetobacter baumannii HPAH are known as C₁ and C₂, respectively. C₁ is a flavin reductase that uses NADH to generate reduced flavin mononucleotide (FMNH^-), which is used by C₂ in the hydroxylation of HPA. Interestingly, although HPA is not directly involved in the reaction catalyzed by C₁, the presence of HPA dramatically increases the FMN reduction rate. Amino acid sequence analysis revealed that C₁ contains two domains: an N-terminal flavin reductase domain, and a C-terminal MarR domain. Although MarR proteins typically function as transcription regulators, the MarR domain of C₁ was found to play an auto-inhibitory role. Here, we report a crystal structure of C₁ and small-angle X-ray scattering (SAXS) studies that revealed that C₁ undergoes a substantial conformational change in the presence of HPA, concomitant with the increase in the rate of flavin reduction. Amino acid residues that are important for HPA binding and regulation of C₁ activity were identified by site-directed mutagenesis. Amino acid sequence similarity analysis revealed several as yet uncharacterized flavin reductases with N- or C-terminal fusions.

Efficient monooxygenase-catalyzed piceatannol production: Application of cyclodextrins for reducing product inhibition

Piceatannol is a rare, costly plant-based stilbene derivative and exhibits various health-enhancing properties. Recently, we demonstrated that piceatannol could be produced from resveratrol through site-selective hydroxylation using Escherichia coli cells expressing the monooxygenase HpaBC. However, piceatannol production ceased at approximately 25 mM, even when sufficient levels of the substrate resveratrol remained in the reaction mixture. In this study, we found that high concentrations (>20-25 mM) of piceatannol significantly inhibited the HpaBC-catalyzed reaction. Cyclodextrins (CDs) reportedly encapsulate various hydrophobic compounds. We found that the addition of β-CD or γ-CD to the reaction mixture reduced the inhibition caused by the product piceatannol. The effects of β-CD on piceatannol production were more pronounced than those of γ-CD at high concentrations of the substrate resveratrol and CDs. The production of piceatannol reached 49 mM (12 g L^-1) in the presence of β-CD, a level twice that achieved in the absence of β-CD. The technique described here might be applicable to the bioproduction of other stilbenes and structurally related compounds.

PqsL uses reduced flavin to produce 2-hydroxylaminobenzoylacetate, a preferred PqsBC substrate in alkyl quinolone biosynthesis in Pseudomonas aeruginosa

Alkyl hydroxyquinoline N-oxides (AQNOs) are antibiotic compounds produced by the opportunistic bacterial pathogen Pseudomonas aeruginosa They are products of the alkyl quinolone (AQ) biosynthetic pathway, which also generates the quorum-sensing molecules 2-heptyl-4(1H)-quinolone (HHQ) and 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS). Although the enzymatic synthesis of HHQ and PQS had been elucidated, the route by which AQNOs are synthesized remained elusive. Here, we report on PqsL, the key enzyme for AQNO production, which structurally resembles class A flavoprotein monooxygenases such as p-hydroxybenzoate 3-hydroxylase (pHBH) and 3-hydroxybenzoate 6-hydroxylase. However, we found that unlike related enzymes, PqsL hydroxylates a primary aromatic amine group, and it does not use NAD(P)H as cosubstrate, but unexpectedly required reduced flavin as electron donor. We also observed that PqsL is active toward 2-aminobenzoylacetate (2-ABA), the central intermediate of the AQ pathway, and forms the unstable compound 2-hydroxylaminobenzoylacetate, which was preferred over 2-ABA as substrate of the downstream enzyme PqsBC. In vitro reconstitution of the PqsL/PqsBC reaction was feasible by using the FAD reductase HpaC, and we noted that the AQ:AQNO ratio is increased in an hpaC-deletion mutant of P. aeruginosa PAO1 compared with the ratio in the WT strain. A structural comparison with pHBH, the model enzyme of class A flavoprotein monooxygenases, revealed that structural features associated with NAD(P)H binding are missing in PqsL. Our study completes the AQNO biosynthetic pathway in P. aeruginosa, indicating that PqsL produces the unstable product 2-hydroxylaminobenzoylacetate from 2-ABA and depends on free reduced flavin as electron donor instead of NAD(P)H.

N-terminus determines activity and specificity of styrene monooxygenase reductases

Styrene monooxygenases (SMOs) are two-enzyme systems that catalyze the enantioselective epoxidation of styrene to (S)-styrene oxide. The FADH₂ co-substrate of the epoxidase component (StyA) is supplied by an NADH-dependent flavin reductase (StyB). The genome of Rhodococcus opacus 1CP encodes two SMO systems. One system, which we define as E1-type, displays homology to the SMO from Pseudomonas taiwanensis VLB120. The other system, originally reported as a fused system (RoStyA2B), is defined as E2-type. Here we found that E1-type RoStyB is inhibited by FMN, while RoStyA2B is known to be active with FMN. To rationalize the observed specificity of RoStyB for FAD, we generated an artificial reductase, designated as RoStyBart, in which the first 22 amino acid residues of RoStyB were joined to the reductase part of RoStyA2B, while the oxygenase part (A2) was removed. RoStyBart mainly purified as apo-protein and mimicked RoStyB in being inhibited by FMN. Pre-incubation with FAD yielded a turnover number at 30°C of 133.9±3.5s^-1, one of the highest rates observed for StyB reductases. RoStyBart holo-enzyme switches to a ping-pong mechanism and fluorescence analysis indicated for unproductive binding of FMN to the second (co-substrate) binding site. In summary, it is shown for the first time that optimization of the N-termini of StyB reductases allows the evolution of their activity and specificity.

The Two-Component Monooxygenase MeaXY Initiates the Downstream Pathway of Chloroacetanilide Herbicide Catabolism in Sphingomonads

Due to the extensive use of chloroacetanilide herbicides over the past 60 years, bacteria have evolved catabolic pathways to mineralize these compounds. In the upstream catabolic pathway, chloroacetanilide herbicides are transformed into the two common metabolites 2-methyl-6-ethylaniline (MEA) and 2,6-diethylaniline (DEA) through N-dealkylation and amide hydrolysis. The pathway downstream of MEA is initiated by the hydroxylation of aromatic rings, followed by its conversion to a substrate for ring cleavage after several steps. Most of the key genes in the pathway have been identified. However, the genes involved in the initial hydroxylation step of MEA are still unknown. As a special aniline derivative, MEA cannot be transformed by the aniline dioxygenases that have been characterized. Sphingobium baderi DE-13 can completely degrade MEA and use it as a sole carbon source for growth. In this work, an MEA degradation-deficient mutant of S. baderi DE-13 was isolated. MEA catabolism genes were predicted through comparative genomic analysis. The results of genetic complementation and heterologous expression demonstrated that the products of meaX and meaY are responsible for the initial step of MEA degradation in S. baderi DE-13. MeaXY is a two-component flavoprotein monooxygenase system that catalyzes the hydroxylation of MEA and DEA using NADH and flavin mononucleotide (FMN) as cofactors. Nuclear magnetic resonance (NMR) analysis confirmed that MeaXY hydroxylates MEA and DEA at the para-position. Transcription of meaX was enhanced remarkably upon induction of MEA or DEA in S. baderi DE-13. Additionally, meaX and meaY were highly conserved among other MEA-degrading sphingomonads. This study fills a gap in our knowledge of the biochemical pathway that carries out mineralization of chloroacetanilide herbicides in sphingomonads.IMPORTANCE Much attention has been paid to the environmental fate of chloroacetanilide herbicides used for the past 60 years. Microbial degradation is considered an important mechanism in the degradation of these compounds. Bacterial degradation of chloroacetanilide herbicides has been investigated in many recent studies. Pure cultures or consortia able to mineralize these herbicides have been obtained. The catabolic pathway has been proposed, and most key genes involved have been identified. However, the genes responsible for the initiation step (from MEA to hydroxylated MEA or from DEA to hydroxylated DEA) of the downstream pathway have not been reported. The present study demonstrates that a two-component flavin-dependent monooxygenase system, MeaXY, catalyzes the para-hydroxylation of MEA or DEA in sphingomonads. Therefore, this work finds a missing link in the biochemical pathway that carries out the mineralization of chloroacetanilide herbicides in sphingomonads. Additionally, the results expand our understanding of the degradation of a special kind of aniline derivative.

Kinetic Mechanism of the Dechlorinating Flavin-dependent Monooxygenase HadA

The accumulation of chlorophenols (CPs) in the environment, due to their wide use as agrochemicals, has become a serious environmental problem. These organic halides can be degraded by aerobic microorganisms, where the initial steps of various biodegradation pathways include an oxidative dechlorinating process in which chloride is replaced by a hydroxyl substituent. Harnessing these dechlorinating processes could provide an opportunity for environmental remediation, but detailed catalytic mechanisms for these enzymes are not yet known. To close this gap, we now report transient kinetics and product analysis of the dechlorinating flavin-dependent monooxygenase, HadA, from the aerobic organism Ralstonia pickettii DTP0602, identifying several mechanistic properties that differ from other enzymes in the same class. We first overexpressed and purified HadA to homogeneity. Analyses of the products from single and multiple turnover reactions demonstrated that HadA prefers 4-CP and 2-CP over CPs with multiple substituents. Stopped-flow and rapid-quench flow experiments of HadA with 4-CP show the involvement of specific intermediates (C4a-hydroperoxy-FAD and C4a-hydroxy-FAD) in the reaction, define rate constants and the order of substrate binding, and demonstrate that the hydroxylation step occurs prior to chloride elimination. The data also identify the non-productive and productive paths of the HadA reactions and demonstrate that product formation is the rate-limiting step. This is the first elucidation of the kinetic mechanism of a two-component flavin-dependent monooxygenase that can catalyze oxidative dechlorination of various CPs, and as such it will serve as the basis for future investigation of enzyme variants that will be useful for applications in detoxifying chemicals hazardous to human health.

Crystal Structures of SgcE6 and SgcC, the Two-Component Monooxygenase That Catalyzes Hydroxylation of a Carrier Protein-Tethered Substrate during the Biosynthesis of the Enediyne Antitumor Antibiotic C-1027 in Streptomyces globisporus

C-1027 is a chromoprotein enediyne antitumor antibiotic produced by Streptomyces globisporus. In the last step of biosynthesis of the (S)-3-chloro-5-hydroxy-β-tyrosine moiety of the C-1027 enediyne chromophore, SgcE6 and SgcC compose a two-component monooxygenase that hydroxylates the C-5 position of (S)-3-chloro-β-tyrosine. This two-component monooxygenase is remarkable for two reasons. (i) SgcE6 specifically reacts with FAD and NADH, and (ii) SgcC is active with only the peptidyl carrier protein (PCP)-tethered substrate. To address the molecular details of substrate specificity, we determined the crystal structures of SgcE6 and SgcC at 1.66 and 2.63 Å resolution, respectively. SgcE6 shares a similar β-barrel fold with the class I HpaC-like flavin reductases. A flexible loop near the active site of SgcE6 plays a role in FAD binding, likely by providing sufficient space to accommodate the AMP moiety of FAD, when compared to that of FMN-utilizing homologues. SgcC shows structural similarity to a few other known FADH₂-dependent monooxygenases and sheds light on some biochemically but not structurally characterized homologues. The crystal structures reported here provide insights into substrate specificity, and comparison with homologues provides a catalytic mechanism of the two-component, FADH₂-dependent monooxygenase (SgcE6 and SgcC) that catalyzes the hydroxylation of a PCP-tethered substrate.

Biochemical properties and crystal structure of the flavin reductase FerA from Paracoccus denitrificans

The Pden_2689 gene encoding FerA, an NADH:flavin oxidoreductase required for growth of Paracoccus denitrificans under iron limitation, was cloned and overexpressed as a C-terminally His6-tagged derivative. The binding of substrates and products was detected and quantified by isothermal titration calorimetry and fluorometric titration. FerA binds FMN and FAD with comparable affinity in an enthalpically driven, entropically opposed process. The reduced flavin is bound more loosely than the oxidized one, which was confirmed by a negative shift in the redox potential of FMN after addition of FerA. Initial velocity and substrate analogs inhibition studies showed that FerA follows a random-ordered sequence of substrate (NADH and FMN) binding. The primary kinetic isotope effects from stereospecifically deuterated nicotinamide nucleotides demonstrated that hydride transfer occurs from the pro-S position and contributes to rate limitation for the overall reaction. The crystal structure of FerA revealed a twisted seven-stranded antiparallel β-barrel similar to that of other short chain flavin reductases. Only minor structural changes around Arg106 took place upon FMN binding. The solution structure FerA derived from small angle X-ray scattering (SAXS) matched the dimer assembly predicted from the crystal structure. Site-directed mutagenesis pinpointed a role of Arg106 and His146 in binding of flavin and NADH, respectively. Pull down experiments performed with cytoplasmic extracts resulted in a negative outcome indicating that FerA might physiologically act without association with other proteins. Rapid kinetics experiments provided evidence for a stabilizing effect of another P. denitrificans protein, the

NAD(P)H

acceptor oxidoreducase FerB, against spontaneous oxidation of the FerA-produced dihydroflavin.

Biocatalytic synthesis of 3,4,5,3′,5′-pentahydroxy-trans-stilbene from piceatannol by two-component flavin-dependent monooxygenase HpaBC

HpaBC monooxygenase was previously reported to hydroxylate resveratrol to piceatannol. In this article, we report a novel catalytic activity of HpaBC for the synthesis of a pentahydroxylated stilbene. When Escherichia coli cells expressing HpaBC were incubated with resveratrol, the resulting piceatannol was further converted to a new product. This product was identified by mass spectrometry and NMR spectroscopy as a 5-hydroxylated piceatannol, 3,4,5,3′,5′-pentahydroxy-trans-stilbene (PHS), which is a reportedly valuable biologically active stilbene derivative. We attempted to produce PHS from piceatannol on a flask scale. After examining the effects of detergents and buffers on PHS production, E. coli cells expressing HpaBC efficiently hydroxylated piceatannol to PHS in a reaction mixture containing 1.5% (v/v) Tween 80 and 100 mM 3-morpholinopropanesulfonic acid-NaOH buffer at pH 7.5. Under the optimized conditions, the whole cells regioselectively hydroxylated piceatannol, and the production of PHS reached 6.9 mM (1.8 g L−1) in 48 h.

Rapid kinetics of dehalogenation promoted by iodotyrosine deiodinase from human thyroid

Reductive dehalogenation such as that catalyzed by iodotyrosine deiodinase (IYD) is highly unusual in aerobic organisms but necessary for iodide salvage from iodotyrosine generated during thyroxine biosynthesis. Equally unusual is the dependence of this process on flavin. Rapid kinetics have now been used to define the basic processes involved in IYD catalysis. Time-dependent quenching of flavin fluorescence was used to monitor halotyrosine association to IYD. The substrates chloro-, bromo-, and iodotyrosine bound with similar rate constants (k_on) ranging from 1.3 × 10⁶ to 1.9 × 10⁶ M^–1 s^–1. Only the inert substrate analogue fluorotyrosine exhibited a significantly (5-fold) slower k_on (0.3 × 10⁶ M^–1 s^–1). All data fit a standard two-state model and indicated that no intermediate complex accumulated during closure of the active site lid induced by substrate. Subsequent halide elimination does not appear to limit reactions of bromo- and iodotyrosine since both fully oxidized the reduced enzyme with nearly equivalent second-order rate constants (7.3 × 10³ and 8.6 × 10³ M^–1 s^–1, respectively) despite the differing strength of their carbon–halogen bonds. In contrast to these substrates, chlorotyrosine reacted with the reduced enzyme approximately 20-fold more slowly and revealed a spectral intermediate that formed at approximately the same rate as the bromo- and iodotyrosine reactions.

A mechanistic study on SMOB-ADP1: an NADH:flavin oxidoreductase of the two-component styrene monooxygenase of Acinetobacter baylyi ADP1

Two styrene monooxygenase types, StyA/StyB and StyA1/StyA2B, have been described each consisting of an epoxidase and a reductase. A gene fusion which led to the chimeric reductase StyA2B and the occurrence in different phyla are major differences. Identification of SMOA/SMOB-ADP1 of Acinetobacter baylyi ADP1 may enlighten the gene fusion event since phylogenetic analysis indicated both proteins to be more related to StyA2B than to StyA/StyB. SMOB-ADP1 is classified like StyB and StyA2B as HpaC-like reductase. Substrate affinity and turnover number of the homo-dimer SMOB-ADP1 were determined for NADH (24 µM, 64 s(-1)) and FAD (4.4 µM, 56 s(-1)). SMOB-ADP1 catalysis follows a random sequential mechanism, and FAD fluorescence is quenched upon binding to SMOB-ADP1 (K d = 1.8 µM), which clearly distinguishes that reductase from StyB of Pseudomonas. In summary, this study confirmes made assumptions and provides phylogenetic and biochemical data for the differentiation of styrene monooxygenase-related flavin reductases.

Flavoprotein monooxygenases for oxidative biocatalysis: recombinant expression in microbial hosts and applications

External flavoprotein monooxygenases comprise a group of flavin-dependent oxidoreductases that catalyze the insertion of one atom of molecular oxygen into an organic substrate and the second atom is reduced to water. These enzymes are involved in a great number of metabolic pathways both in prokaryotes and eukaryotes. Flavoprotein monooxygenases have attracted the attention of researchers for several decades and the advent of recombinant DNA technology caused a great progress in the field. These enzymes are subjected to detailed biochemical and structural characterization and some of them are also regarded as appealing oxidative biocatalysts for the production of fine chemicals and valuable intermediates toward active pharmaceutical ingredients due to their high chemo-, stereo-, and regioselectivity. Here, we review the most representative reactions catalyzed both in vivo and in vitro by prototype flavoprotein monooxygenases, highlighting the strategies employed to produce them recombinantly, to enhance the yield of soluble proteins, and to improve cofactor regeneration in order to obtain versatile biocatalysts. Although we describe the most outstanding features of flavoprotein monooxygenases, we mainly focus on enzymes that were cloned, expressed and used for biocatalysis during the last years.

Flavin dependent monooxygenases

Flavin-dependent monooxygenases catalyze a wide variety of chemo-, regio- and enantioselective oxygenation reactions. As such, they are involved in key biological processes ranging from catabolism, detoxification and biosynthesis, to light emission and axon guidance. Based on fold and function, flavin-dependent monooxygenases can be distributed into eight groups. Groups A and B comprise enzymes that rely on NAD(P)H as external electron donor. Groups C-F are two-protein systems, composed of a monooxygenase and a flavin reductase. Groups G and H comprise internal monooxygenases that reduce the flavin cofactor through substrate oxidation. Recently, many new flavin-dependent monooxygenases have been discovered. In addition to posing basic enzymological questions, these proteins attract attention of pharmaceutical and fine-chemical industries, given their importance as regio- and enantioselective biocatalysts. In this review we present an update of the classification of flavin-dependent monooxygenases and summarize the latest advances in our understanding of their catalytic and structural properties.

The transfer of reduced flavin mononucleotide from LuxG oxidoreductase to luciferase occurs via free diffusion

Bacterial luciferase (LuxAB) is a two-component flavin mononucleotide (FMN)-dependent monooxygenase that catalyzes the oxidation of reduced FMN (FMNH(-)) and a long-chain aliphatic aldehyde by molecular oxygen to generate oxidized FMN, the corresponding aliphatic carboxylic acid, and concomitant emission of light. The LuxAB reaction requires a flavin reductase to generate FMNH(-) to serve as a luciferin in its reaction. However, FMNH(-) is unstable and can react with oxygen to generate H2O2, so that it is important to transfer it efficiently to LuxAB. Recently, LuxG has been identified as a NADH:FMN oxidoreductase that supplies FMNH(-) to luciferase in vivo. In this report, the mode of transfer of FMNH(-) between LuxG from Photobacterium leiognathi TH1 and LuxABs from both P. leiognathi TH1 and Vibrio campbellii (PlLuxAB and VcLuxAB, respectively) was investigated using single-mixing and double-mixing stopped-flow spectrophotometry. The oxygenase component of p-hydroxyphenylacetate hydroxylase (C2) from Acinetobacter baumannii, which has no structural similarity to LuxAB, was used to measure the kinetics of release of FMNH(-) from LuxG. With all FMNH(-) acceptors used (C2, PlLuxAB, and VcLuxAB), the kinetics of FMN reduction on LuxG were the same, showing that LuxG releases FMNH(-) with a rate constant of 4.5-6 s(-1). Our data showed that the kinetics of binding of FMNH(-)to PlLuxAB and VcLuxAB and the subsequent reactions with oxygen were the same with either free FMNH(-) or FMNH(-) generated in situ by LuxG. These results strongly suggest that no complexes between LuxG and the various species are necessary to transfer FMNH(-) to the acceptors. The kinetics of the overall reactions and the individual rate constants correlate well with a free diffusion model for the transfer of FMNH(-) from LuxG to either LuxAB.

Catalytic activity of the two-component flavin-dependent monooxygenase from Pseudomonas aeruginosa toward cinnamic acid derivatives

4-Hydroxyphenylacetate 3-hydroxylases (HPAHs) of the two-component flavin-dependent monooxygenase family are attractive enzymes that possess the catalytic potential to synthesize valuable ortho-diphenol compounds from simple monophenol compounds. In this study, we investigated the catalytic activity of HPAH from Pseudomonas aeruginosa strain PAO1 toward cinnamic acid derivatives. We prepared Escherichia coli cells expressing the hpaB gene encoding the monooxygenase component and the hpaC gene encoding the oxidoreductase component. E. coli cells expressing HpaBC exhibited no or very low oxidation activity toward cinnamic acid, o-coumaric acid, and m-coumaric acid, whereas they rapidly oxidized p-coumaric acid to caffeic acid. Interestingly, after p-coumaric acid was almost completely consumed, the resulting caffeic acid was further oxidized to 3,4,5-trihydroxycinnamic acid. In addition, HpaBC exhibited oxidation activity toward 3-(4-hydroxyphenyl)propanoic acid, ferulic acid, and coniferaldehyde to produce the corresponding ortho-diphenols. We also investigated a flask-scale production of caffeic acid from p-coumaric acid as the model reaction for HpaBC-catalyzed syntheses of hydroxycinnamic acids. Since the initial concentrations of the substrate p-coumaric acid higher than 40 mM markedly inhibited its HpaBC-catalyzed oxidation, the reaction was carried out by repeatedly adding 20 mM of this substrate to the reaction mixture. Furthermore, by using the HpaBC whole-cell catalyst in the presence of glycerol, our experimental setup achieved the high-yield production of caffeic acid, i.e., 56.6 mM (10.2 g/L) within 24 h. These catalytic activities of HpaBC will provide an easy and environment-friendly synthetic approach to hydroxycinnamic acids.

The C-terminal domain of 4-hydroxyphenylacetate 3-hydroxylase from Acinetobacter baumannii is an autoinhibitory domain

p-Hydroxyphenylacetate (HPA) 3-hydroxylase from Acinetobacter baumannii consists of a reductase component (C(1)) and an oxygenase component (C(2)). C(1) catalyzes the reduction of FMN by NADH to provide FMNH(-) as a substrate for C(2). The rate of reduction of flavin is enhanced ∼20-fold by binding HPA. The N-terminal domain of C(1) is homologous to other flavin reductases, whereas the C-terminal domain (residues 192-315) is similar to MarR, a repressor protein involved in bacterial antibiotic resistance. In this study, three forms of truncated C(1) variants and single site mutation variants of residues Arg-21, Phe-216, Arg-217, Ile-246, and Arg-247 were constructed to investigate the role of the C-terminal domain in regulating C(1). In the absence of HPA, the C(1) variant in which residues 179-315 were removed (t178C(1)) was reduced by NADH and released FMNH(-) at the same rates as wild-type enzyme carries out these functions in the presence of HPA. In contrast, variants with residues 231-315 removed behaved similarly to the wild-type enzyme. Thus, residues 179-230 are involved in repressing the production of FMNH(-) in the absence of HPA. These results are consistent with the C-terminal domain in the wild-type enzyme being an autoinhibitory domain that upon binding the effector HPA undergoes conformational changes to allow faster flavin reduction and release. Most of the single site variants investigated had catalytic properties similar to those of the wild-type enzyme except for the F216A variant, which had a rate of reduction that was not stimulated by HPA. F216A could be involved with HPA binding or in the required conformational change for stimulation of flavin reduction by HPA.

Crystallization and preliminary X-ray analysis of the reductase component of p-hydroxyphenylacetate 3-hydroxylase from Acinetobacter baumannii

p-Hydroxyphenylacetate 3-hydroxylase (HPAH) from Acinetobacter baumannii catalyzes the hydroxylation of p-hydroxyphenylacetate (HPA) at the ortho position to yield 3,4-dihydroxyphenylacetate (DHPA). HPAH from A. baumannii is a two-component flavoprotein consisting of a smaller reductase (C(1)) component and a larger oxygenase (C(2)) component. The C(1) component supplies a reduced flavin in its free form to the C(2) counterpart for hydroxylation. In addition, HPA can bind to C(1) and enhance the flavin-reduction rate without becoming hydroxylated. The recombinant C(1) component was purified and crystallized using the microbatch method at 295 K. X-ray diffraction data were collected to 2.3 Å resolution using synchrotron radiation on the BL13B1 beamline at NSRRC, Taiwan. The crystal belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 47.78, b = 59.92, c = 211.85 Å, and contained two molecules of C(1) per asymmetric unit.

Interactions with the substrate phenolic group are essential for hydroxylation by the oxygenase component of p-hydroxyphenylacetate 3-hydroxylase

p-Hydroxyphenylacetate (HPA) 3-hydroxylase is a two-component flavoprotein monooxygenase that catalyzes the hydroxylation of p-hydroxyphenylacetate to form 3,4-dihydroxyphenylacetate. Based on structures of the oxygenase component (C(2)), both His-120 and Ser-146 are located ~2.8 Å from the hydroxyl group of HPA. The variants H120N, H120Q, H120Y, H120D, and H120E can form C4a-hydroperoxy-FMN (a reactive intermediate necessary for hydroxylation) but cannot hydroxylate HPA. The impairment of H120N is not due to substrate binding because the variant can still bind HPA. In contrast, the H120K variant catalyzes hydroxylation with efficiency comparable with that of the wild-type enzyme; the hydroxylation rate constant for H120K is 5.7 ± 0.6 s(-1), and the product conversion ratio is 75%, compared with values of 16 s(-1) and 90% for the wild-type enzyme. H120R can also catalyze hydroxylation, suggesting that a positive charge on residue 120 can substitute for the hydroxylation function of His-120. Because the hydroxylation reaction of wild-type C(2) is pH-independent between pH 6 and 10, the protonation status of key components required for hydroxylation likely remains unchanged in this pH range. His-120 may be positively charged for selective binding to the phenolate form of HPA, i.e. to form the His(δ+)·HPA(δ-) complex, which in turn promotes oxygen atom transfer via an electrophilic aromatic substitution mechanism. Analysis of Ser-146 variants revealed that this residue is necessary for but not directly engaged in hydroxylation. Product formation in S146A is pH-independent and constant at ~70% over a pH range of 6-10, whereas product formation for S146C decreased from ~65% at pH 6.0 to 27% at pH 10.0. These data indicate that the ionization of Cys-146 in the S146C variant has an adverse effect on hydroxylation, possibly by perturbing formation of the His(δ+)·HPA(δ-) complex needed for hydroxylation.

Stabilization of C4a-hydroperoxyflavin in a two-component flavin-dependent monooxygenase is achieved through interactions at flavin N5 and C4a atoms

p-Hydroxyphenylacetate (HPA) 3-hydroxylase is a two-component flavin-dependent monooxygenase. Based on the crystal structure of the oxygenase component (C(2)), His-396 is 4.5 Å from the flavin C4a locus, whereas Ser-171 is 2.9 Å from the flavin N5 locus. We investigated the roles of these two residues in the stability of the C4a-hydroperoxy-FMN intermediate. The results indicated that the rate constant for C4a-hydroperoxy-FMN formation decreased ~30-fold in H396N, 100-fold in H396A, and 300-fold in the H396V mutant, compared with the wild-type enzyme. Lesser effects of the mutations were found for the subsequent step of H(2)O(2) elimination. Studies on pH dependence showed that the rate constant of H(2)O(2) elimination in H396N and H396V increased when pH increased with pK(a) >9.6 and >9.7, respectively, similar to the wild-type enzyme (pK(a) >9.4). These data indicated that His-396 is important for the formation of the C4a-hydroperoxy-FMN intermediate but is not involved in H(2)O(2) elimination. Transient kinetics of the Ser-171 mutants with oxygen showed that the rate constants for the H(2)O(2) elimination in S171A and S171T were ~1400-fold and 8-fold greater than the wild type, respectively. Studies on the pH dependence of S171A with oxygen showed that the rate constant of H(2)O(2) elimination increased with pH rise and exhibited an approximate pK(a) of 8.0. These results indicated that the interaction of the hydroxyl group side chain of Ser-171 and flavin N5 is required for the stabilization of C4a-hydroperoxy-FMN. The double mutant S171A/H396V reacted with oxygen to directly form the oxidized flavin without stabilizing the C4a-hydroperoxy-FMN intermediate, which confirmed the findings based on the single mutation that His-396 was important for formation and Ser-171 for stabilization of the C4a-hydroperoxy-FMN intermediate in C(2).

pH-dependent studies reveal an efficient hydroxylation mechanism of the oxygenase component of p-hydroxyphenylacetate 3-hydroxylase

p-Hydroxyphenylacetate (HPA) 3-hydroxylase (HPAH) catalyzes the hydroxylation of HPA at the ortho-position to yield 3,4-dihydroxyphenylacetate. The enzyme is a flavin-dependent two-component monooxygenase that consists of a reductase component and an oxygenase component (C(2)). C(2) catalyzes the hydroxylation of HPA using oxygen and reduced FMN as co-substrates. To date, the effects of pH on the oxygenation of the two-component monooxygenases have never been reported. Here, we report the reaction kinetics of C(2)·FMNH(-) with oxygen at various pH values investigated by stopped-flow and rapid quenched-flow techniques. In the absence of HPA, the rate constant for the formation of C4a-hydroperoxy-FMN (∼1.1 × 10(6) m(-1)s(-1)) was unaffected at pH 6.2-9.9, which indicated that the pK(a) of the enzyme-bound reduced FMN was less than 6.2. The rate constant for the following H(2)O(2) elimination step increased with higher pH, which is consistent with a pK(a) of >9.4. In the presence of HPA, the rate constants for the formation of C4a-hydroperoxy-FMN (∼4.8 × 10(4) m(-1)s(-1)) and the ensuing hydroxylation step (15-17 s(-1)) were not significantly affected by the pH. In contrast, the following steps of C4a-hydroxy-FMN dehydration to form oxidized FMN occurred through two pathways that were dependent on the pH of the reaction. One pathway, dominant at low pH, allowed the detection of a C4a-hydroxy-FMN intermediate, whereas the pathway dominant at high pH produced oxidized FMN without an apparent accumulation of the intermediate. However, both pathways efficiently catalyzed hydroxylation without generating significant amounts of wasteful H(2)O(2) at pH 6.2-9.9. The decreased accumulation of the intermediate at higher pH was due to the greater rates of C4a-hydroxy-FMN decay caused by the abolishment of substrate inhibition in the dehydration step at high pH.

StyA1 and StyA2B from Rhodococcus opacus 1CP: a multifunctional styrene monooxygenase system

Two-component flavoprotein monooxygenases are emerging biocatalysts that generally consist of a monooxygenase and a reductase component. Here we show that Rhodococcus opacus 1CP encodes a multifunctional enantioselective flavoprotein monooxygenase system composed of a single styrene monooxygenase (SMO) (StyA1) and another styrene monooxygenase fused to an NADH-flavin oxidoreductase (StyA2B). StyA1 and StyA2B convert styrene and chemical analogues to the corresponding epoxides at the expense of FADH2 provided from StyA2B. The StyA1/StyA2B system presents the highest monooxygenase activity in an equimolar ratio of StyA1 and StyA2B, indicating (transient) protein complex formation. StyA1 is also active when FADH2 is supplied by StyB from Pseudomonas sp. VLB120 or PheA2 from Rhodococcus opacus 1CP. However, in both cases the reductase produces an excess of FADH2, resulting in a high waste of NADH. The epoxidation rate of StyA1 heavily depends on the type of reductase. This supports that the FADH2-induced activation of StyA1 requires interprotein communication. We conclude that the StyA1/StyA2B system represents a novel type of multifunctional flavoprotein monooxygenase. Its unique mechanism of cofactor utilization provides new opportunities for biotechnological applications and is highly relevant from a structural and evolutionary point of view.