Molecular cloning and sequence analysis of the gene encoding the H2O-forming NADH oxidase from Streptococcus mutans

High-Yield Synthesis of Enantiopure 1,2-Amino Alcohols from l-Phenylalanine via Linear and Divergent Enzymatic Cascades

Enantiomerically pure 1,2-amino alcohols are important compounds due to their biological activities and wide applications in chemical synthesis. In this work, we present two multienzyme pathways for the conversion of l-phenylalanine into either 2-phenylglycinol or phenylethanolamine in the enantiomerically pure form. Both pathways start with the two-pot sequential four-step conversion of l-phenylalanine into styrene via subsequent deamination, decarboxylation, enantioselective epoxidation, and enantioselective hydrolysis. For instance, after optimization, the multienzyme process could convert 507 mg of l-phenylalanine into (R)-1-phenyl-1,2-diol in an overall isolated yield of 75% and >99% ee. The opposite enantiomer, (S)-1-phenyl-1,2-diol, was also obtained in a 70% yield and 98–99% ee following the same approach. At this stage, two divergent routes were developed to convert the chiral diols into either 2-phenylglycinol or phenylethanolamine. The former route consisted of a one-pot concurrent interconnected two-step cascade in which the diol intermediate was oxidized to 2-hydroxy-acetophenone by an alcohol dehydrogenase and then aminated by a transaminase to give enantiomerically pure 2-phenylglycinol. Notably, the addition of an alanine dehydrogenase enabled the connection of the two steps and made the overall process redox-self-sufficient. Thus, (S)-phenylglycinol was isolated in an 81% yield and >99.4% ee starting from ca. 100 mg of the diol intermediate. The second route consisted of a one-pot concurrent two-step cascade in which the oxidative and reductive steps were not interconnected. In this case, the diol intermediate was oxidized to either (S)- or (R)-2-hydroxy-2-phenylacetaldehyde by an alcohol oxidase and then aminated by an amine dehydrogenase to give the enantiomerically pure phenylethanolamine. The addition of a formate dehydrogenase and sodium formate was required to provide the reducing equivalents for the reductive amination step. Thus, (R)-phenylethanolamine was isolated in a 92% yield and >99.9% ee starting from ca. 100 mg of the diol intermediate. In summary, l-phenylalanine was converted into enantiomerically pure 2-phenylglycinol and phenylethanolamine in overall yields of 61% and 69%, respectively. This work exemplifies how linear and divergent enzyme cascades can enable the synthesis of high-value chiral molecules such as amino alcohols from a renewable material such as l-phenylalanine with high atom economy and improved sustainability.

Generation of Oxidoreductases with Dual Alcohol Dehydrogenase and Amine Dehydrogenase Activity

The l-lysine-ϵ-dehydrogenase (LysEDH) from Geobacillus stearothermophilus naturally catalyzes the oxidative deamination of the ϵ-amino group of l-lysine. We previously engineered this enzyme to create amine dehydrogenase (AmDH) variants that possess a new hydrophobic cavity in their active site such that aromatic ketones can bind and be converted into α-chiral amines with excellent enantioselectivity. We also recently observed that LysEDH was capable of reducing aromatic aldehydes into primary alcohols. Herein, we harnessed the promiscuous alcohol dehydrogenase (ADH) activity of LysEDH to create new variants that exhibited enhanced catalytic activity for the reduction of substituted benzaldehydes and arylaliphatic aldehydes to primary alcohols. Notably, these novel engineered dehydrogenases also catalyzed the reductive amination of a variety of aldehydes and ketones with excellent enantioselectivity, thus exhibiting a dual AmDH/ADH activity. We envisioned that the catalytic bi-functionality of these enzymes could be applied for the direct conversion of alcohols into amines. As a proof-of-principle, we performed an unprecedented one-pot "hydrogen-borrowing" cascade to convert benzyl alcohol to benzylamine using a single enzyme. Conducting the same biocatalytic cascade in the presence of cofactor recycling enzymes (i.e., NADH-oxidase and formate dehydrogenase) increased the reaction yields. In summary, this work provides the first examples of enzymes showing "alcohol aminase" activity.

Kinetic Resolution of Racemic Primary Amines Using Geobacillus stearothermophilus Amine Dehydrogenase Variant

A NADH-dependent engineered amine dehydrogenase from Geobacillus stearothermophilus (LE-AmDH-v1) was applied together with a NADH-oxidase from Streptococcus mutans (NOx) for the kinetic resolution of pharmaceutically relevant racemic α-chiral primary amines. The reaction conditions (e. g., pH, temperature, type of buffer) were optimised to yield S-configured amines with up to >99 % ee.

Regio- and stereoselective multi-enzymatic aminohydroxylation of β-methylstyrene using dioxygen, ammonia and formate

We report an enzymatic route for the formal regio- and stereoselective aminohydroxylation of β-methylstyrene that consumes only dioxygen, ammonia and formate; carbonate is the by-product. The biocascade entails highly selective epoxidation, hydrolysis and hydrogen-borrowing alcohol amination. Thus, β-methylstyrene was converted into 1R,2R and 1S,2R-phenylpropanolamine in 59-63% isolated yields, and up to >99.5: <0.5 dr and er.

A biocatalytic method for the chemoselective aerobic oxidation of aldehydes to carboxylic acids

Herein, we present a study on the oxidation of aldehydes to carboxylic acids using three recombinant aldehyde dehydrogenases (ALDHs). The ALDHs were used in purified form with a nicotinamide oxidase (NOx), which recycles the catalytic NAD+ at the expense of dioxygen (air at atmospheric pressure). The reaction was studied also with lyophilised whole cell as well as resting cell biocatalysts for more convenient practical application. The optimised biocatalytic oxidation runs in phosphate buffer at pH 8.5 and at 40 °C. From a set of sixty-one aliphatic, aryl-aliphatic, benzylic, hetero-aromatic and bicyclic aldehydes, fifty were converted with elevated yield (up to >99%). The exceptions were a few ortho-substituted benzaldehydes, bicyclic heteroaromatic aldehydes and 2-phenylpropanal. In all cases, the expected carboxylic acid was shown to be the only product (>99% chemoselectivity). Other oxidisable functionalities within the same molecule (e.g. hydroxyl, alkene, and heteroaromatic nitrogen or sulphur atoms) remained untouched. The reaction was scaled for the oxidation of 5-(hydroxymethyl)furfural (2 g), a bio-based starting material, to afford 5-(hydroxymethyl)furoic acid in 61% isolated yield. The new biocatalytic method avoids the use of toxic or unsafe oxidants, strong acids or bases, or undesired solvents. It shows applicability across a wide range of substrates, and retains perfect chemoselectivity. Alternative oxidisable groups were not converted, and other classical side-reactions (e.g. halogenation of unsaturated functionalities, Dakin-type oxidation) did not occur. In comparison to other established enzymatic methods such as the use of oxidases (where the concomitant oxidation of alcohols and aldehydes is common), ALDHs offer greatly improved selectivity.

Cloning, expression, characterization and homology modeling of a novel water-forming NADH oxidase from Streptococcus mutans ATCC 25175

A novel nicotinamide adenine dinucleotide (NADH) oxidase from Streptococcus mutans ATCC 25175 (SmNox) was cloned and overexpressed in Escherichia coli BL21 (DE3). Sequence analysis revealed an open reading frame of 1374bp, capable of encoding a polypeptide of 457 amino acid residues. The molecular mass of the purified SmNox was estimated to be ∼49.9kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified SmNox had the highest specific activity of 281.2U·mg^-1 at optimal pH and temperature of 7.0 and 35°C, with a K_m of 57.7μM and a V_max of 154.3U·mg^-1. The good stability at room temperature was observed. Homology modeling and substrate docking were performed to evaluate the catalytic characteristics. The results indicated that Nicotinamide ring of NADH extends vertically toward to re-face of coenzyme (FAD), and the specific conformation of NADH suggested that the charges transfer in SmNox complex could be easier than in its homologous enzyme (LbNox) under alkaline environment. The characterization of the SmNox indicated it has potential in industrial regeneration of coenzyme NAD⁺ for coupling with dehydrogenases.

Characteristics of a water-forming NADH oxidase from Methanobrevibacter smithii, an archaeon in the human gut

NOX-ms catalysed the oxidization of NADH and converted O₂ to H₂O by using cysteine-mediated electron transfer. Its transcription was increased by oxidative stress and glucose.

NADH oxidases (NOXs) catalysing the oxidation of NADH to yield NAD⁺ and H₂O, H₂O₂, or both play an important role in protecting organisms from oxidative stress and maintaining the balance of NAD⁺/NADH. A gene encoding NOX was identified from Methanobrevibacter smithii (NOX-ms), the predominant archaeon in the human gut ecosystem. Subsequent analyses showed that it is an FAD-containing protein with a subunit molecular mass of 48 kDa. NOX-ms was purified to homogeneity after expression in Escherichia coli. NOX-ms catalysed the oxidization of NADH and converted O₂ to H₂O with an optimal pH of 7.5 and a temperature optimum of approximately 37°C. The V_max and K_m values were 42.6–44.1 unit/mg and 47.8–54.6 μM for NADH. The apparent V_max and K_m for oxygen were 189.5–196.1 unit/mg and 14.6–16.8 μM. The mutation analysis suggests that Cys⁴² in NOX-ms plays a key role in the four-electron reduction of O₂ to H₂O. Quantitative reverse transcription-PCR (RT-qPCR) revealed that transcription of NOX-ms was also up-regulated after exposing the cells to oxidative stress and glucose. Finally, the potential of NOX-ms as a target to control colonization of M. smithii and its possible applications are discussed.

Codon-Optimized NADH Oxidase Gene Expression and Gene Fusion with Glycerol Dehydrogenase for Bienzyme System with Cofactor Regeneration

NADH oxidases (NOXs) play an important role in maintaining balance of NAD⁺/NADH by catalyzing cofactors regeneration. The expression of nox gene from Lactobacillus brevis in Escherichia coli BL21 (BL21 (DE3)) was studied. Two strategies, the high AT-content in the region adjacent to the initiation codon and codon usage of the whole gene sequence consistent with the host, obtained the NOX activity of 59.9 U/mg and 73.3 U/mg (crude enzyme), with enhanced expression level of 2.0 and 2.5-folds, respectively. Purified NOX activity was 213.8 U/mg. Gene fusion of glycerol dehydrogenase (GDH) and NOX formed bifuctional multi-enzymes for bioconversion of glycerol coupled with coenzyme regeneration. Kinetic parameters of the GDH-NOX for each substrate, glycerol and NADH, were calculated as V_{max(Glycerol)} 20 μM/min, K_m(Glycerol) 19.4 mM, V_{max (NADH)} 12.5 μM/min and K_{m (NADH)} 51.3 μM, respectively, which indicated the potential application of GDH-NOX for quick glycerol analysis and dioxyacetone biosynthesis.

Cofactor Specificity Engineering of Streptococcus mutans NADH Oxidase 2 for NAD(P)(+) Regeneration in Biocatalytic Oxidations

Soluble water-forming NAD(P)H oxidases constitute a promising NAD(P)(+) regeneration method as they only need oxygen as cosubstrate and produce water as sole byproduct. Moreover, the thermodynamic equilibrium of O2 reduction is a valuable driving force for mostly energetically unfavorable biocatalytic oxidations. Here, we present the generation of an NAD(P)H oxidase with high activity for both cofactors, NADH and NADPH. Starting from the strictly NADH specific water-forming Streptococcus mutans NADH oxidase 2 several rationally designed cofactor binding site mutants were created and kinetic values for NADH and NADPH conversion were determined. Double mutant 193R194H showed comparable high rates and low K m values for NADPH (k cat 20 s(-1), K m 6 µM) and NADH (k cat 25 s(-1), K m 9 µM) with retention of 70% of wild type activity towards NADH. Moreover, by screening of a SeSaM library S. mutans NADH oxidase 2 variants showing predominantly NADPH activity were found, giving further insight into cofactor binding site architecture. Applicability for cofactor regeneration is shown for coupling with alcohol dehydrogenase from Sphyngobium yanoikuyae for 2-heptanone production.

Cloning and characterization of a thermostable H2O-forming NADH oxidase from Lactobacillus rhamnosus

NADH oxidase (Nox) catalyzes the conversion of NADH to NAD(+). A previously uncharacterized Nox gene (LrNox) was cloned from Lactobacillus rhamnosus and overexpressed in Escherichia coli BL21(DE3). Sequence analysis revealed an open reading frame of 1359 bp, capable of encoding a polypeptide of 453 amino acid residues. The molecular mass of the purified LrNox enzyme was estimated to be ~50 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and 100 kDa by gel filtration chromatography, suggesting that the enzyme is a homodimer. The enzyme had optimal activity at pH 5.6 and temperature 65 °C, and k(cat)/K(m) of 3.77×10(7) s(-1) M(-1), the highest ever reported. Heat inactivation studies revealed that LrNox had high thermostability, with a half-life of 120 min at 80 °C. Molecular dynamics simulation studies shed light on the factors contributing to the high activity of LrNox. Although the properties of Nox from several microorganisms have been reported, this is the first report on the characterization of a recombinant H(2)O-forming Nox with high activity and thermostability. The characteristics of the LrNox enzyme could prove to be of interest in industrial applications such as NAD(+) regeneration.

Regeneration of nicotinamide coenzymes: principles and applications for the synthesis of chiral compounds

Dehydrogenases which depend on nicotinamide coenzymes are of increasing interest for the preparation of chiral compounds, either by reduction of a prochiral precursor or by oxidative resolution of their racemate. The regeneration of oxidized and reduced nicotinamide cofactors is a very crucial step because the use of these cofactors in stoichiometric amounts is too expensive for application. There are several possibilities to regenerate nicotinamide cofactors: established methods such as formate/formate dehydrogenase (FDH) for the regeneration of NADH, recently developed electrochemical methods based on new mediator structures, or the application of gene cloning methods for the construction of "designed" cells by heterologous expression of appropriate genes.A very promising approach is enzymatic cofactor regeneration. Only a few enzymes are suitable for the regeneration of oxidized nicotinamide cofactors. Glutamate dehydrogenase can be used for the oxidation of NADH as well as NADPH while L: -lactate dehydrogenase is able to oxidize NADH only. The reduction of NAD(+) is carried out by formate and FDH. Glucose-6-phosphate dehydrogenase and glucose dehydrogenase are able to reduce both NAD(+) and NADP(+). Alcohol dehydrogenases (ADHs) are either NAD(+)- or NADP(+)-specific. ADH from horse liver, for example, reduces NAD(+) while ADHs from Lactobacillus strains catalyze the reduction of NADP(+). These enzymes can be applied by their inclusion in whole cell biotransformations with an NAD(P)(+)-dependent primary reaction to achieve in situ the regeneration of the consumed cofactor.Another efficient method for the regeneration of nicotinamide cofactors is the electrochemical approach. Cofactors can be regenerated directly, for example at a carbon anode, or indirectly involving mediators such as redox catalysts based on transition-metal complexes.An increasing number of examples in technical scale applications are known where nicotinamide dependent enzymes were used together with cofactor regenerating enzymes.

The two-component system ScnRK of Streptococcus mutans affects hydrogen peroxide resistance and murine macrophage killing

To survive macrophage killing is critical in the pathogenesis of viridians streptococci-induced infective endocarditis (IE). Streptococcus mutans, an opportunistic IE pathogen, generally does not survive well phagocytic killing in murine macrophage RAW 264.7 cells. A putative two-component system (TCS), ScnR/ScnK from S. mutans, was investigated to elucidate the mechanisms underlying bacteria-cellular interaction in this study. Both the wild-type and mutant strains were phagocytosed by RAW 264.7 cells at a comparable rate and an increased intracellular susceptibility during a 5 h incubation period was observed with the scnRK-null mutants. The amount of reactive oxygen species (ROS) in activated macrophages was reduced significantly after ingesting wild-type, but not scnRK-null mutant strains, suggesting that increased macrophage killing of these mutants is due to the impaired ability of S. mutans to counteract ROS. Additionally, both scnR- or scnRK-null mutants were more susceptible to hydrogen peroxide. Interestingly, scnRK expression was unaffected by hydrogen peroxide. These experimental results indicate that scnRK is important in counteracting oxidative stress in S. mutans, and decreased susceptibility to phagocytic killing is at least partly attributable to inhibition of intracellular ROS formation.

Structure of coenzyme F420H2 oxidase (FprA), a di-iron flavoprotein from methanogenic Archaea catalyzing the reduction of O2 to H2O

The di-iron flavoprotein F(420)H(2) oxidase found in methanogenic Archaea catalyzes the four-electron reduction of O(2) to 2H(2)O with 2 mol of reduced coenzyme F(420)(7,8-dimethyl-8-hydroxy-5-deazariboflavin). We report here on crystal structures of the homotetrameric F(420)H(2) oxidase from Methanothermobacter marburgensis at resolutions of 2.25 A, 2.25 A and 1.7 A, respectively, from which an active reduced state, an inactive oxidized state and an active oxidized state could be extracted. As found in structurally related A-type flavoproteins, the active site is formed at the dimer interface, where the di-iron center of one monomer is juxtaposed to FMN of the other. In the active reduced state [Fe(II)Fe(II)FMNH(2)], the two irons are surrounded by four histidines, one aspartate, one glutamate and one bridging aspartate. The so-called switch loop is in a closed conformation, thus preventing F(420) binding. In the inactive oxidized state [Fe(III)FMN], the iron nearest to FMN has moved to two remote binding sites, and the switch loop is changed to an open conformation. In the active oxidized state [Fe(III)Fe(III)FMN], both irons are positioned as in the reduced state but the switch loop is found in the open conformation as in the inactive oxidized state. It is proposed that the redox-dependent conformational change of the switch loop ensures alternate complete four-electron O(2) reduction and redox center re-reduction. On the basis of the known Si-Si stereospecific hydride transfer, F(420)H(2) was modeled into the solvent-accessible pocket in front of FMN. The inactive oxidized state might provide the molecular basis for enzyme inactivation by long-term O(2) exposure observed in some members of the FprA family.

Structure of coenzyme A-disulfide reductase from Staphylococcus aureus at 1.54 A resolution

Coenzyme A (CoASH) replaces glutathione as the major low molecular weight thiol in Staphylococcus aureus; it is maintained in the reduced state by coenzyme A-disulfide reductase (CoADR), a homodimeric enzyme similar to NADH peroxidase but containing a novel Cys43-SSCoA redox center. The crystal structure of S. aureus CoADR has been solved using multiwavelength anomalous dispersion data and refined at a resolution of 1.54 A. The resulting electron density maps define the Cys43-SSCoA disulfide conformation, with Cys43-S(gamma) located at the flavin si face, 3.2 A from FAD-C4aF, and the CoAS- moiety lying in an extended conformation within a cleft at the dimer interface. A well-ordered chloride ion is positioned adjacent to the Cys43-SSCoA disulfide and receives a hydrogen bond from Tyr361'-OH of the complementary subunit, suggesting a role for Tyr361' as an acid-base catalyst during the reduction of CoAS-disulfide. Tyr419'-OH is located 3.2 A from Tyr361'-OH as well and, based on its conservation in known functional CoADRs, also appears to be important for activity. Identification of residues involved in recognition of the CoAS-disulfide substrate and in formation and stabilization of the Cys43-SSCoA redox center has allowed development of a CoAS-binding motif. Bioinformatics analyses indicate that CoADR enzymes are broadly distributed in both bacterial and archaeal kingdoms, suggesting an even broader significance for the CoASH/CoAS-disulfide redox system in prokaryotic thiol/disulfide homeostasis.

Studies on NADH oxidase and alkyl hydroperoxide reductase produced by Porphyromonas gingivalis

Enzymes that detoxify oxygen or oxygen radicals are important to anaerobic microorganisms that inhabit oxygenated environments. In previous studies we have determined that Porphyromonas gingivalis W50 cell extracts possess NADH oxidase-like activity, which increases slightly under oxygenated conditions. The aim of this study was to characterize the protein responsible for this activity in order to establish whether it protects the microorganism from oxidative stress. Protein purification based on NADH oxidase activity did not isolate a conventional NADH oxidase. Instead, the NADH oxidase activity was found to be associated with a FAD-dependent enzyme identified as 4-hydroxybutyryl-CoA dehydratase (AbfD). The biological significance of this activity with respect to protection against oxidative stress is not clear; hydrogen peroxide (H2O2) was present after completion of the NADH oxidase assay with the purified protein. Northern blot analysis, examining the expression of other proteins likely to function as NADH oxidases/peroxidases in P. gingivalis, revealed the transcription of a protein similar to alkyl-hydroperoxide reductase (AhpF-C), which could serve as an NADH oxidase and H2O2-detoxification system. AhpF is transcribed in a polycystronic way with its neighboring gene, which encodes for the coupling protein AhpC. No transcript could be detected for the closest match to an NADH oxidase identified in the P. gingivalis genome sequence. In conclusion, P. gingivalis seems to lack a protective NADH oxidase but AhpF-C could contribute to its moderate tolerance to reactive oxygen species by metabolizing H2O2.

Molecular characterization of H2O2-forming NADH oxidases from Archaeoglobus fulgidus

Three NADH oxidase encoding genes noxA-1, noxB-1 and noxC were cloned from the genome of Archaeoglobus fulgidus, expressed in Escherichia coli, and the gene products were purified and characterized. Expression of noxA-1 and noxB-1 resulted in active gene products of the expected size. The noxC gene was expressed as well but the protein produced showed no activity in the standard Nox assay. NoxA-1 and NoxB-1 are both FAD-containing enzymes with subunit molecular masses of 48 and 69 kDa, respectively. NoxA-1 exists predominantly as homodimer, NoxB-1 as monomer. NoxA-1 and NoxB-1 showed pH optimum of 8.0 and 6.5, with specific NADH oxidase activities of 5.8 U.mg-1 and 4.1 U.mg-1, respectively. Both enzymes were specific for NADH as electron donor, but with different apparent Km values (NoxA-1, 0.13 mm; NoxB-1, 0.011 mm). The apparent Km values for oxygen differed significantly (NoxA-1, 0.06 mm; NoxB-1, 2.9 mm). In contrast with all mesophilic homologues, both enzymes were found to produce predominantly H2O2 instead of H2O. Despite apparent similarities, NoxB-1 is essentially different from NoxA-1. Whereas NoxA-1 resembles typical H2O-producing Nox enzymes that are expected to have a role in oxidative stress defence, NoxB-1 belongs to a small group of enzymes that is involved in catalysing the reduction of unsaturated acids and aldehydes, suggesting a role in fatty acid oxidation. Moreover, NoxB-1 contains a ferredoxin-like motif, which is absent in NoxA-1.

Increased production of hydrogen peroxide by Lactobacillus delbrueckii subsp. bulgaricus upon aeration: involvement of an NADH oxidase in oxidative stress

The growth of Lactobacillus delbrueckii subsp. bulgaricus (L. delbrueckii subsp. bulgaricus) on lactose was altered upon aerating the cultures by agitation. Aeration caused the bacteria to enter early into stationary phase, thus reducing markedly the biomass production but without modifying the maximum growth rate. The early entry into stationary phase of aerated cultures was probably related to the accumulation of hydrogen peroxide in the medium. Indeed, the concentration of hydrogen peroxide in aerated cultures was two to three times higher than in unaerated ones. Also, a similar shift from exponential to stationary phase could be induced in unaerated cultures by adding increasing concentrations of hydrogen peroxide. A significant fraction of the hydrogen peroxide produced by L. delbrueckii subsp. bulgaricus originated from the reduction of molecular oxygen by NADH catalyzed by an NADH:H(2)O(2) oxidase. The specific activity of this NADH oxidase was the same in aerated and unaerated cultures, suggesting that the amount of this enzyme was not directly regulated by oxygen. Aeration did not change the homolactic character of lactose fermentation by L. delbrueckii subsp. bulgaricus and most of the NADH was reoxidized by lactate dehydrogenase with pyruvate. This indicated that NADH oxidase had no (or a very small) energetic role and could be involved in eliminating oxygen.

Streptococcus mutans H2O2-forming NADH oxidase is an alkyl hydroperoxide reductase protein

Nox-1 from Streptococcus mutans, the bacteria which cause dental caries, was previously identified as an H2O2-forming reduced nicotinamide adenine dinucleotide (NADH) oxidase. Nox-1 is homologous with the flavoprotein component, AhpF, of Salmonella typhimurium alkyl hydroperoxide reductase. A partial open reading frame upstream of nox1, homologous with the other (peroxidase) component, ahpC, from the S. typhimurium system, was also identified. We report here the complete sequence of S. mutans ahpC. Analyses of purified AhpC together with Nox-1 have verified that these proteins act as a cysteine-based peroxidase system in S. mutans, catalyzing the NADH-dependent reduction of organic hydroperoxides or H2O2 to their respective alcohols and/or H2O. These proteins also catalyze the four-electron reduction of O2 to H2O2, clarifying the role of Nox-1 as a protective protein against oxygen toxicity. Major differences between Nox-1 and AhpF include: (i) the absolute specificity of Nox-1 for NADH; (ii) lower amounts of flavin semiquinone and a more prominent FADH2 to NAD+ charge transfer absorbance band stabilized by Nox-1; and (iii) even higher redox potentials of disulfide centers relative to flavin for Nox-1. Although Nox-1 and AhpC from S. mutans were shown to play a protective role against oxidative stress in vitro and in vivo in Escherichia coli, the lack of a significant effect on deletion of these genes from S. mutans suggests the presence of additional antioxidant proteins in these bacteria.

Pyruvate flux distribution in NADH-oxidase-overproducing Lactococcus lactis strain as a function of culture conditions

The influence of growth conditions on product formation from glucose by Lactococcus lactis strain NZ9800 engineered for NADH-oxidase overproduction was examined. In aerobic batch cultures, a large production of acetoin and diacetyl was found at acidic pH under pH-unregulated conditions. However, pyruvate flux was mainly driven towards lactate production when these cells were grown under strictly pH-controlled conditions. A decreased NADH-oxidase overproduction accompanied the homolactic fermentation, suggesting that the cellular energy was used with preference to maintain cellular homeostasis rather than for NADH-oxidase overproduction. The end product formation and NADH-oxidase activity were also studied in cells grown in aerobic continuous cultures under acidic conditions. A homoacetic type of fermentation as well as a low NADH-oxidase overproduction were observed at low dilution rates. NADH-oxidase was efficiently overproduced as the dilution rate was increased and consequently metabolic flux through lactate dehydrogenase drastically decreased. Under these conditions the flux limitation via pyruvate dehydrogenase was relieved and this enzymatic complex accommodated most of the pyruvate flux. Pyruvate was also significantly converted to acetoin and diacetyl via alpha-acetolactate synthase. At higher dilution rates, acetate production declined and the cultures turned to mixed-acid fermentation. These results suggest that the need to maintain the cellular homeostasis influenced NADH-oxidase overproduction and consequently the end product formation from glucose in these engineered strains.

Functions of two types of NADH oxidases in energy metabolism and oxidative stress of Streptococcus mutans

We have previously identified two distinct NADH oxidases corresponding to H(2)O(2)-forming oxidase (Nox-1) and H(2)O-forming oxidase (Nox-2) induced in Streptococcus mutans. Sequence analyses indicated a strong similarity between Nox-1 and AhpF, the flavoprotein component of Salmonella typhimurium alkyl hydroperoxide reductase; an open reading frame upstream of nox-1 also showed homology to AhpC, the direct peroxide-reducing component of S. typhimurium alkyl hydroperoxide reductase. To determine their physiological functions in S. mutans, we constructed knockout mutants of Nox-1, Nox-2, and/or the AhpC homologue; we verified that Nox-2 plays an important role in energy metabolism through the regeneration of NAD(+) but Nox-1 contributes negligibly. The Nox-2 mutant exhibited greatly reduced aerobic growth on mannitol, whereas there was no significant effect of aerobiosis on the growth on mannitol of the other strains or growth on glucose of any of the strains. Although the Nox-2 mutants grew well on glucose aerobically, the end products of glucose fermentation by the Nox-2 mutant were substantially shifted to higher ratios of lactic acid to acetic acid compared with wild-type cells. The resistance to cumene hydroperoxide of Escherichia coli TA4315 (ahpCF-defective mutant) transformed with pAN119 containing both nox-1 and ahpC genes was not only restored but enhanced relative to that of E. coli K-12 (parent strain), indicating a clear function for Nox-1 as part of an alkyl hydroperoxide reductase system in vivo in combination with AhpC. Surprisingly, the Nox-1 and/or AhpC deficiency had no effect on the sensitivity of S. mutans to cumene hydroperoxide and H(2)O(2), implying that the existence of some other antioxidant system(s) independent of Nox-1 in S. mutans compensates for the deficiency.

Cofactor engineering: a novel approach to metabolic engineering in Lactococcus lactis by controlled expression of NADH oxidase

NADH oxidase-overproducing Lactococcus lactis strains were constructed by cloning the Streptococcus mutans nox-2 gene, which encodes the H2O-forming NADH oxidase, on the plasmid vector pNZ8020 under the control of the L. lactis nisA promoter. This engineered system allowed a nisin-controlled 150-fold overproduction of NADH oxidase at pH 7.0, resulting in decreased NADH/NAD ratios under aerobic conditions. Deliberate variations on NADH oxidase activity provoked a shift from homolactic to mixed-acid fermentation during aerobic glucose catabolism. The magnitude of this shift was directly dependent on the level of NADH oxidase overproduced. At an initial growth pH of 6.0, smaller amounts of nisin were required to optimize NADH oxidase overproduction, but maximum NADH oxidase activity was twofold lower than that found at pH 7.0. Nonetheless at the highest induction levels, levels of pyruvate flux redistribution were almost identical at both initial pH values. Pyruvate was mostly converted to acetoin or diacetyl via alpha-acetolactate synthase instead of lactate and was not converted to acetate due to flux limitation through pyruvate dehydrogenase. The activity of the overproduced NADH oxidase could be increased with exogenously added flavin adenine dinucleotide. Under these conditions, lactate production was completely absent. Lactate dehydrogenase remained active under all conditions, indicating that the observed metabolic effects were only due to removal of the reduced cofactor. These results indicate that the observed shift from homolactic to mixed-acid fermentation under aerobic conditions is mainly modulated by the level of NADH oxidation resulting in low NADH/NAD+ ratios in the cells.

Metabolic engineering of sugar catabolism in lactic acid bacteria

Lactic acid bacteria are characterized by a relatively simple sugar fermentation pathway that, by definition, results in the formation of lactic acid. The extensive knowledge of traditional pathways and the accumulating genetic information on these and novel ones, allows for the rerouting of metabolic processes in lactic acid bacteria by physiological approaches, genetic methods, or a combination of these two. This review will discuss past and present examples and future possibilities of metabolic engineering of lactic acid bacteria for the production of important compounds, including lactic and other acids, flavor compounds, and exopolysaccharides.