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

Characteristics of cold-adapted carbonic anhydrase and efficient carbon dioxide capture based on cell surface display technology

Carbonic anhydrase (CA) is currently under investigation because of its potential to capture CO2. A novel N-domain of ice nucleoproteins (INPN)-mediated surface display technique was developed to produce CA with low-temperature capture CO2 based on the mining and characterization of Colwellia sp. CA (CsCA) with cold-adapted enzyme structural features and catalytic properties. CsCA and INPN were effectively integrated into the outer membrane of the cell as fusion proteins. Throughout the display process, the integrity of the membrane of engineered bacteria BL21/INPN-CsCA was maintained. Notably, the study affirmed positive applicability, wherein 94 % activity persisted after 5 d at 15 °C, and 73 % of the activity was regained after 5 cycles of CO2 capture. BL21/INPN-CsCA displayed a high CO2 capture capacity of 52 mg of CaCO3/mg of whole-cell biocatalysts during CO2 mineralization at 25 °C. Therefore, the CsCA functional cell surface display technology could contribute significantly to environmentally friendly CO2 capture.

Affinity-assisted covalent self-assembly of PduQ-SpyTag and Nox-SpyCatcher to construct multi-enzyme complexes on the surface of magnetic microsphere modified with chelated Ni2

It is of great significance to study the effect of multi-enzyme aggregation behavior at the interface on the formation of multi-enzyme complexes and their co-catalytic characteristics, which is helpful for us to design and construct immobilized multi-enzyme complex systems for in vitro synthetic biology. Here, a magnetic microsphere with chelated Ni2+, was prepared to explore the self-assembly characteristics of PduQ-SpyTag (P-T) and Nox-SpyCatcher (NC) on its surface, based on the mixed interaction mode consisting the affinity of His-tag/Ni2+ and covalent binding of SpyTag/SpyCatcher. After studying the effect of saturated or unsaturated adsorption of P-T on the covalent binding between P-T and NC at the interface, a possible multienzyme interaction mechanism for the affinity-assisted covalent self-assembly on the Ni2+ chelating surface was proposed. The time evolution of NADH showed that the immobilized P-T/N-C complex formed by this method and the free P-T/N-C complex exhibited similar synergistic catalytic properties, and presented higher catalytic efficiency than the simple mixing of P-T and NC. The optimal catalytic conditions, stability and reusability of the immobilized multi-enzyme complexes prepared in this study were also discussed by comparing them with free enzymes. In this study, we demonstrate a simple and effective strategy for self-assembling SpyTag/SpyCatcher fusion proteins on the surface of magnetic beads, which is inspiring for the construction of more cascade enzyme systems at the interface. It provides a new method for facilitating the rapid construction of immobilized multi-enzyme complexes in vitro from the crude cell lysis.

Metabolic Engineering of Escherichia coli for Production of 2,5-Dimethylpyrazine

2,5-Dimethylpyrazine (2,5-DMP) is a high-value-added alkylpyrazine compound with important applications in both the food and pharmaceutical fields. In response to the increasing consumer preference for natural products over chemically synthesized ones, efforts have been made to develop efficient microbial cell factories for the production of 2,5-DMP. However, the previously reported recombinant strains have exhibited low yields and relied on expensive antibiotics and inducers. In this study, we employed metabolic engineering strategies to develop an Escherichia coli strain capable of producing 2,5-DMP at high levels without the need for inducers or antibiotics. Initially, the biosynthesis pathway of 2,5-DMP was constructed that realized 2,5-DMP production from glucose. Subsequently, efforts focused on enhancing 2,5-DMP production by improving the availability of the cofactor NAD+ and precursor l-threonine. Additionally, the supply and conversion of l-threonine were balanced by optimizing the copy number of the key gene tdh on the chromosome and by modifying the l-threonine transport system. The final engineering strain D19 produced 3.1 g/L of 2,5-DMP, which is the highest titer for fermentative production of 2,5-DMP using glucose as the carbon source up to date. The strategies used in this study lay a good foundation for the production of 2,5-DMP on a large scale.

Cloning, heterologous expression, and molecular characterization of a highly active and stable non-specific endonuclease from Pseudomonas fluorescens

Non-specific endonucleases can be used for the digestion of nucleic acids because they hydrolyze DNA/RNA into 3-5 base pairs (bp) length oligonucleotide fragments without strict selectivity. In this work, a novel non-specific endonuclease from Pseudomonas fluorescens (PfNuc) with high activities for both DNA and RNA was successfully cloned and expressed in Escherichia coli. The production of PfNuc in flask scale could be achieved to 1.73 × 106 U/L and 4.82 × 106 U/L for DNA and RNA by investigation of the culture and induction conditions. The characterization of PfNuc indicated that it was Mg2+-dependent and the catalytic activity was enhanced by 3.74 folds for DNA and 1.06 folds for RNA in the presence of 5 mM Mg2+. The specific activity of PfNuc for DNA was 1.44 × 105 U/mg at pH 8.0 and 40 °C, and 3.93 × 105 U/mg for RNA at pH 8.5 and 45 °C. The Km of the enzyme for both DNA and RNA was close to 43 µM. The Vmax was 6.40 × 105 U/mg and 1.11 × 106 U/mg for DNA and RNA, respectively. There was no observed activity loss when PfNuc was stored at 4 °C and - 20 °C after 28 days or 10 repeated freeze-thaw cycles at - 80 °C. Molecular docking revealed that PfNuc formed 17 and 19 hydrogen bonds with single-stranded RNA and double-stranded DNA, respectively. These results could explain the high activity and stability of PfNuc, suggesting its great potential applications in the industry and clinic.

Heparinase III with High Activity and Stability: Heterologous Expression, Biochemical Characterization, and Application in Depolymerization of Heparin

A novel heparinase III from Pedobacter schmidteae (PsHep-III) with high activity and good stability was successfully cloned, expressed, and characterized. PsHep-III displayed the highest specific activity ever reported of 192.8 U mg-1 using heparin as the substrate. It was stable at 25 °C with a half-life of 323 h in an aqueous solution. PsHep-III was employed for the depolymerization of heparin, and the enzymatic hydrolyzed products were analyzed with gel permeation chromatography and high-performance liquid chromatography. PsHep-III can break glycosidic bonds in heparin like →4]GlcNAc/GlcNAc6S/GlcNS/GlcNS6S/GlcN/GlcN6S(1 → 4)ΔUA/ΔUA2S[1 → and efficiently digest heparin into seven disaccharides including N-acetylated, N-sulfated, and N-unsubstituted modification, with molecular masses of 503, 605, 563, 563, 665, 360, and 563 Da, respectively. These results indicated that PsHep-III with broad substrate specificity could be combined with heparinase I to overcome the low selectivity at the N-acetylated modification binding sites of heparinase I. This work will contribute to the application of PsHep-III for characterizing heparin and producing low-molecular-weight heparin effectively.

Pharmacoinformatics-Based Approach for Uncovering the Quorum-Quenching Activity of Phytocompounds against the Oral Pathogen, Streptococcus mutans

Streptococcus mutans, a gram-positive oral pathogen, is the primary causative agent of dental caries. Biofilm formation, a critical characteristic of S. mutans, is regulated by quorum sensing (QS). This study aimed to utilize pharmacoinformatics techniques to screen and identify effective phytochemicals that can target specific proteins involved in the quorum sensing pathway of S. mutans. A computational approach involving homology modeling, model validation, molecular docking, and molecular dynamics (MD) simulation was employed. The 3D structures of the quorum sensing target proteins, namely SecA, SMU1784c, OppC, YidC2, CiaR, SpaR, and LepC, were modeled using SWISS-MODEL and validated using a Ramachandran plot. Metabolites from Azadirachta indica (Neem), Morinda citrifolia (Noni), and Salvadora persica (Miswak) were docked against these proteins using AutoDockTools. MD simulations were conducted to assess stable interactions between the highest-scoring ligands and the target proteins. Additionally, the ADMET properties of the ligands were evaluated using SwissADME and pkCSM tools. The results demonstrated that campesterol, meliantrol, stigmasterol, isofucosterol, and ursolic acid exhibited the strongest binding affinity for CiaR, LepC, OppC, SpaR, and Yidc2, respectively. Furthermore, citrostadienol showed the highest binding affinity for both SMU1784c and SecA. Notably, specific amino acid residues, including ASP86, ARG182, ILE179, GLU143, ASP237, PRO101, and VAL84 from CiaR, LepC, OppC, SecA, SMU1784c, SpaR, and YidC2, respectively, exhibited significant interactions with their respective ligands. While the docking study indicated favorable binding energies, the MD simulations and ADMET studies underscored the substantial binding affinity and stability of the ligands with the target proteins. However, further in vitro studies are necessary to validate the efficacy of these top hits against S. mutans.

Characterization of 17β-estradiol-degrading enzyme from Microbacterium sp. MZT7 and its function on E2 biodegradation in wastewater

BACKGROUND

17β-estradiol (E2) residues exhibit harmful effects both for human and animals and have got global attention of the scientific community. Microbial enzymes are considered as one of the effective strategies having great potential for removal E2 residues from the environment. However, limited literature is available on the removal of E2 from wastewater using short-chain dehydrogenase.

RESULTS

In this study, 17β-estradiol degrading enzyme (17β-HSD-0095) was expressed and purified from Microbacterium sp. MZT7. The optimal pH and temperature for reaction was 7 and 40 °C, respectively. Molecular docking studies have shown that the ARG215 residue form a hydrogen bond with oxygen atom of the substrate E2. Likewise, the point mutation results have revealed that the ARG215 residue play an important role in the E2 degradation by 17β-HSD-0095. In addition, 17β-HSD-0095 could remediate E2 contamination in synthetic livestock wastewater.

CONCLUSIONS

These findings offer some fresh perspectives on the molecular process of E2 degradation and the creation of enzyme preparations that can degrade E2.

Application of cofactors in the regulation of microbial metabolism: A state of the art review

Cofactors are crucial chemicals that maintain cellular redox balance and drive the cell to do synthetic and catabolic reactions. They are involved in practically all enzymatic activities that occur in live cells. It has been a hot research topic in recent years to manage their concentrations and forms in microbial cells by using appropriate techniques to obtain more high-quality target products. In this review, we first summarize the physiological functions of common cofactors, and give a brief overview of common cofactors acetyl coenzyme A, NAD(P)H/NAD(P)+, and ATP/ADP; then we provide a detailed introduction of intracellular cofactor regeneration pathways, review the regulation of cofactor forms and concentrations by molecular biological means, and review the existing regulatory strategies of microbial cellular cofactors and their application progress, to maximize and rapidly direct the metabolic flux to target metabolites. Finally, we speculate on the future of cofactor engineering applications in cell factories. Graphical Abstract.

N20D/N116E Combined Mutant Downward Shifted the pH Optimum of Bacillus subtilis NADH Oxidase

Cofactor regeneration is indispensable to avoid the addition of large quantities of cofactor NADH or NAD+ in oxidation-reduction reactions. Water-forming NADH oxidase (Nox) has attracted substantive attention as it can oxidize cytosolic NADH to NAD+ without concomitant accumulation of by-products. However, its applications have some limitations in some oxidation-reduction processes when its optimum pH is different from its coupled enzymes. In this study, to modify the optimum pH of BsNox, fifteen relevant candidates of site-directed mutations were selected based on surface charge rational design. As predicted, the substitution of this asparagine residue with an aspartic acid residue (N22D) or with a glutamic acid residue (N116E) shifts its pH optimum from 9.0 to 7.0. Subsequently, N20D/N116E combined mutant could not only downshift the pH optimum of BsNox but also significantly increase its specific activity, which was about 2.9-fold at pH 7.0, 2.2-fold at pH 8.0 and 1.2-fold at pH 9.0 that of the wild-type. The double mutant N20D/N116E displays a higher activity within a wide range of pH from 6 to 9, which is wider than the wide type. The usability of the BsNox and its variations for NAD+ regeneration in a neutral environment was demonstrated by coupling with a glutamate dehydrogenase for α-ketoglutaric acid (α-KG) production from L-glutamic acid (L-Glu) at pH 7.0. Employing the variation N20D/N116E as an NAD+ regeneration coenzyme could shorten the process duration; 90% of L-Glu were transformed into α-KG within 40 min vs. 70 min with the wild-type BsNox for NAD+ regeneration. The results obtained in this work suggest the promising properties of the BsNox variation N20D/N116E are competent in NAD+ regeneration applications under a neutral environment.

Doped Graphene To Mimic the Bacterial NADH Oxidase for One-Step NAD+ Supplementation in Mammals

Nicotinamide adenine dinucleotide (NAD) is a critical regulator of metabolic networks, and declining levels of its oxidized form, NAD⁺, are closely associated with numerous diseases. While supplementing cells with precursors needed for NAD⁺ synthesis has shown poor efficacy in combatting NAD⁺ decline, an alternative strategy is the development of synthetic materials that catalyze the oxidation of NADH into NAD⁺, thereby taking over the natural role of the NADH oxidase (NOX) present in bacteria. Herein, we discovered that metal-nitrogen-doped graphene (MNGR) materials can catalyze the oxidation of NADH into NAD⁺. Among MNGR materials with different transition metals, Fe-, Co-, and Cu-NGR displayed strong catalytic activity combined with >80% conversion of NADH into NAD⁺, similar specificity to NOX for abstracting hydrogen from the pyridine ring of nicotinamide, and higher selectivity than 51 other nanomaterials. The NOX-like activity of FeNGR functioned well in diverse cell lines. As a proof of concept of the in vivo application, we showed that FeNGR could specifically target the liver and remedy the metabolic flux anomaly in obesity mice with NAD⁺-deficient cells. Overall, our study provides a distinct insight for exploration of drug candidates by design of synthetic materials to mimic the functions of unique enzymes (e.g., NOX) in bacteria.

Characterization of a xylitol dehydrogenase from Aspergillus flavus and its application in l-xylulose production

An NAD⁺-dependent xylitol dehydrogenase from A. flavus (AfXDH) was cloned and successfully expressed in Escherichia coli. AfXDH gene sequence revealed an open reading frame of 1,110 bp, encoding a polypeptide of 369 amino acids with a calculated molecular mass of 38,893 Da. Among various polyols, sorbitol and xylitol were preferred substrates of AfXDH with K_m values of 16.2 and 16.9 mM, respectively. AfXDH showed the highest activity in Tris-glycine-NaOH buffer (pH 9.5) at 50°C; it required Zn²⁺ or Mn²⁺ for enzyme activity. The half-life at 40°C and half denaturation temperature (T_1/2) was 200 min and 45°C, respectively. Bioinformatic analyses along with biochemical properties confirmed that AfXDH belonged to the medium-chain dehydrogenase/reductase family. AfXDH exhibits higher thermostability and k_cat values than those of other XDHs. The feasibility of using AfXDH in l-xylulose production was demonstrated. AfXDH, when coupled with Streptococcus pyogenes NADH oxidase, efficiently converted xylitol to l-xylulose with 97% yield, suggesting its usefulness for the industrial l-xylulose production from xylitol.

Immobilization of Alcohol Dehydrogenase, Acetaldehyde Lyase, and NADH Oxidase for Cascade Enzymatic Conversion of Ethanol to Acetoin

Acetoin, a four-carbon hydroxyl-keto compound, is used in the food, pharmaceutical, and chemical industries. The cascade enzymatic production is considered a promising and efficient method to produce acetoin. However, the stability and compatibility of the enzymes under the same catalytic conditions are challenges that need to be resolved. In this work, alcohol dehydrogenase, acetaldehyde lyase, and NADH oxidase were selected to work at the same conditions to efficiently convert ethanol into acetoin. These three enzymes were immobilized on epoxy-modified magnetic nanomaterials to obtain highly stable biocatalysts. The stability and the immobilization conditions, including temperature, pH, enzyme–carrier ratio, and immobilization time, were optimized to obtain the immobilized enzymes with a high catalytic activity. The cascade reactions catalyzed by the immobilized enzymes yielded a high conversion of 90%, suggesting that the use of immobilized enzymes is a promising way to produce acetoin.

Novel Salt-Tolerant Leucine Dehydrogenase from Marine Pseudoalteromonas rubra DSM 6842

This study reported the cloning, expression, and characterization of a new salt-tolerant leucine dehydrogenase (PrLeuDH) from Pseudoalteromonas rubra DSM 6842. A codon-optimized 1038 bp gene encoding PrLeuDH was successfully expressed on pET-22b( +) in E. coli BL21(DE3). The purified recombinant PrLeuDH showed a single band of about 38.7 kDa on SDS-PAGE. It exhibited the maximum activity at 40 °C and pH 10.5, while kept high activities in the range of 25-45 °C and pH 9.5-12. The K_m value and turnover number k_cat for leucine of PrLeuDH were 2.23 ± 0.12 mM and 35.39 ± 0.05 s^-1, respectively, resulting in a catalytic efficiency k_cat/K_m of 15.87 s^-1/mM. Importantly, PrLeuDH remained 92.1 ± 2.67% active in the presence of 4.0 M NaCl. The study provides the first in-depth understanding of LeuDH from marine Pseudoalteromonas rubra, meanwhile the unique properties of high activity at low temperature and high salt tolerance make it a promising biocatalyst for the synthesis of non-protein amino acids and α-ketoacids under special conditions in pharmaceutical industry.

Preparation of combined cross-linked enzyme aggregates containing galactitol dehydrogenase and NADH oxidase for L-tagatose synthesis via in situ cofactor regeneration

The combined cross-linked enzyme aggregates (combi-CLEAs) containing galactitol dehydrogenase (Gdh) and NADH oxidase (Nox) were prepared for L-tagatose synthesis. To prevent the excess consumption of cofactor, Nox in the combi-CLEAs was used to in situ regenerate NAD⁺. In the immobilization process, ammonia sulfate and glutaraldehyde were used as the precipitant and cross-linking reagent, respectively. The preparation conditions were optimized as follows: 60% ammonium sulfate, 1:1 (molar ratio) of Gdh to Nox, 20:1 (molar ratio) of protein to glutaraldehyde, and 6 h of cross-linking time at 35 °C. Under these conditions, the activity of the combi-CLEAs was 210 U g^-1. The combi-CLEAs exhibited higher thermostability and preserved 51.5% of the original activity after eight cycles of reuses at 45 °C. The combi-CLEAs were utilized for the preparation of L-tagatose without by-products. Therefore, the combi-CLEAs have the industrial potential for the bioconversion of galactitol to L-tagatose.

Cloning, expression, and characterization of an arabitol dehydrogenase and coupled with NADH oxidase for effective production of L-xylulose

A novel arabitol dehydrogenase (ArDH) gene was cloned from a bacterium named Aspergillus nidulans and expressed heterologously in Escherichia coli. The purified ArDH exhibited the maximal activity in pH 9.5 Tris-HCl buffer at 40 °C, showed K_m and V_max of 1.2 mg/mL and 9.1 U/mg, respectively. The ArDH was used to produce the L-xylulose and coupled with the NADH oxidase (Nox) for the regeneration of NAD⁺. In further optimization, a high conversion of 84.6% in 8 hours was achieved under the optimal conditions: 20 mM of xylitol, 100 µM NAD⁺ in pH 9.0 Tris-HCl buffer at 30 °C. The results indicated the coupling system with cofactor regeneration provides a promising approach for L-xylulose production from xylitol.

Cloning, Expression, Characterization, and Antioxidant Protection of Glutaredoxin3 From Psychrophilic Bacterium Psychrobacter sp. ANT206

Glutaredoxins (Grxs) are proteins that catalyze the glutathione (GSH)-dependent reduction of protein disulfides. In this study, a Grx-related gene (264 bp), encoding a Ps-Grx3, was cloned from Psychrobacter sp. ANT206. Sequence analysis indicated the presence of the active site motif CPYC in this protein. Homology modeling showed that Ps-Grx3 had fewer hydrogen bonds and salt bridges, as well as a lower Arg/(Arg + Lys) ratio than its mesophilic homologs, indicative of an improved catalytic ability at low temperatures. Site-directed mutagenesis demonstrated that the Cys13, Pro14, and Cys16 sites were essential for the catalytic activity of Ps-Grx3, while circular dichroism (CD) spectroscopy confirmed that point mutations in these amino acid residues led to the loss or reduction of enzyme activity. Furthermore, analysis of the biochemical properties of Ps-Grx3 showed that the optimum temperature of this enzyme was 25 °C. Importantly, Ps-Grx3 was more sensitive to tBHP and CHP than to H₂O₂, and retained approximately 40% activity even when the H₂O₂ concentration was increased to 1 mm Regarding substrate specificity, Ps-Grx3 had a higher affinity for HED, L-cystine, and DHA than for S-sulfocysteine and BSA. We also investigated the DNA-protective ability of Ps-Grx3 using the pUC19 plasmid, and found that Ps-Grx3 could protect supercoiled DNA from oxidation-induced damage at 15°C for 1.5 h. This study provides new insights into the structure and catalytic activity of a cold-adapted Grx3.

Using galactitol dehydrogenase coupled with water-forming NADH oxidase for efficient enzymatic synthesis of L-tagatose

L-Tagatose, a promising building block in the production of many value-added chemicals, is generally produced by chemical routes with a low yield, which may not meet the increasing demands. Synthesis of l-tagatose by enzymatic oxidation of d-galactitol has not been applied on an industrial scale because of the high cofactor costs and the lack of efficient cofactor regeneration methods. In this work, an efficient and environmentally friendly enzymatic method containing a galactitol dehydrogenase for d-galactitol oxidation and a water-forming NADH oxidase for regeneration of NAD⁺ was first designed and used for l-tagatose production. Supplied with only 3 mM NAD⁺, subsequent reaction optimization facilitated the efficient transformation of 100 mM of d-galactitol into l-tagatose with a yield of 90.2 % after 12 h (obtained productivity: 7.61 mM.h^-1). Compared with the current chemical and biocatalytic methods, the strategy developed avoids by-product formation and achieves the highest yield of l-tagatose with low costs. It is expected to become a cleaner and more promising route for industrial biosynthesis of l-tagatose.

A highly active heparinase I from Bacteroides cellulosilyticus: Cloning, high level expression, and molecular characterization

As one of the most extensively studied glycosaminoglycan lyases, heparinase I has been used in producing low or ultra-low molecular weight heparin. Its' important applications are to neutralize the heparin in human blood and analyze heparin structure in the clinic. However, the low productivity and activity of the enzyme have greatly hindered its applications. In this study, a novel Hep-I from Bacteroides cellulosilyticus (BcHep-I) was successfully cloned and heterologously expressed in E. coli BL21 (DE3) as a soluble protein. The molecular mass and isoelectric point (pI) of the enzyme are 44.42 kDa and 9.02, respectively. And the characterization of BcHep-I after purified with Ni-NTA affinity chromatography suggested that it is a mesophilic enzyme. BcHep-I can be activated by 1 mM Ca2+, Mg2+, and Mn2+, while severely inhibited by Zn2+, Co2+, and EDTA. The specific activity of the enzyme was 738.3 U·mg-1 which is the highest activity ever reported. The Km and Vmax were calculated as 0.17 mg·mL-1 and 740.58 U·mg-1, respectively. Besides, the half-life of 300 min at 30°C showed BcHep-I has practical applications. Homology modeling and substrate docking revealed that Gln15, Lys74, Arg76, Lys104, Arg149, Gln208, Tyr336, Tyr342, and Lys338 were mainly involved in the substrate binding of Hep-I, and 11 hydrogen bonds were formed between heparin and the enzyme. These results indicated that BcHep-I with high activity has great potential applications in the industrial production of heparin, especially in the clinic to neutralize heparin.

Rhein inhibits the growth of Propionibacterium acnes by blocking NADH dehydrogenase-2 activity

Introduction. Rhein (4, 5-dihydroxyanthraquinone-2-carboxylic acid) has various properties, including anti-inflammatory, antioxidant and anticancer activities. However, the mechanism underlying the role of rhein in antimicrobial activity remains largely unknown.Aim. This study aims to identify potential natural compounds of rhein that are capable of inhibiting Cutibacterium acnes and elucidate the effects of rhein on NADH dehydrogenase-2 activity in C. acnes.Methodology. The anti-C. acnes activity of compounds was analysed using minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), the paper disc diffusion test and the checkerboard dilution test. To check whether rhein was inhibitory, putative type II NADH dehydrogenase (NDH-2) of C. acnes was analysed, cloned and expressed in Escherichia coli, and then NDH-2 purification was assessed with Ni-NTA before rhein inhibition of NADH dehydrogenase-2 activity was checked with ferricyanide [K₃Fe(CN)₆] as a substrate.Results. The results showed that the MIC of rhein against C. acnes was 6.25 µg ml^-1, while the MBC was 12.5 µg ml^-1, and there was a 38 mm inhibition zone in the paper disc diffusion test. Rhein showed an additive two- to fourfold reduction of the MIC value with four antibiotics on the checkerboard dilution test. The purified NADH dehydrogenase gene product showed a size of approximately 51 kDa and had a V _max of 23 µmol and a K _m of 280 µm. The inhibitory effect of rhein against NADH dehydrogenase-2 activity was non-competitive with ferricyanide [K₃Fe(CN)₆] with a K _i value of 3.5-4.5 µm.Conclusion. This study provided evidence of the inhibitory effects of rhein on the growth of C. acnes by blocking of NADH dehydrogenase-2 activity. This mechanism of inhibitory activity in the reduction of ROS formation and ATP productivity should be further tested in C. acnes and the question of whether rhein inhibits the natural growth of C. acnes should be investigated.

A New Cold-Adapted and Salt-Tolerant Glutathione Reductase from Antarctic Psychrophilic Bacterium Psychrobacter sp. and Its Resistance to Oxidation

A new glutathione reductase gene (psgr) coding for glutathione reductase (GR) from an Antarctic bacterium was cloned and overexpressed into Escherichia coli (E. coli). A sequence analysis revealed that PsGR is a protein consisting of 451 amino acids, and homology modeling demonstrated that PsGR has fewer hydrogen bonds and salt bridges, which might lead to improved conformational flexibility at low temperatures. PsGR possesses the flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate (NADPH) binding motifs. Recombinant PsGR (rPsGR) was purified using Ni-NTA affinity chromatography and was found to have a molecular mass of approximately 53.5 kDa. rPsGR was found to be optimally active at 25 °C and a pH of 7.5. It was found to be a cold-adapted enzyme, with approximately 42% of its optimal activity remaining at 0 °C. Moreover, rPsGR was most active in 1.0 M NaCl and 62.5% of its full activity remained in 3.0 M NaCl, demonstrating its high salt tolerance. Furthermore, rPsGR was found to have a higher substrate affinity for NADPH than for GSSG (oxidized glutathione). rPsGR provided protection against peroxide (H₂O₂)-induced oxidative stress in recombinant cells, and displayed potential application as an antioxidant protein. The results of the present study provide a sound basis for the study of the structural characteristics and catalytic characterization of cold-adapted GR.

Cloning, expression, and characterization of a novel heparinase I from Bacteroides eggerthii

Heparinase I (Hep I) specifically degrades heparin to oligosaccharide or unsaturated disaccharide and has been widely used in preparation of low molecular weight heparin (LMWH). In this work, a novel Hep I from Bacteroides eggerthii VPI T5-42B-1 was cloned and overexpressed in Escherichia coli BL21 (DE3). The enzyme has specific activity of 480 IU·mg^-1 at the optimal temperature and pH of 30 °C and pH 7.5, and the K_m and V_max were 3.6 mg·mL^-1 and 647.93 U·mg^-1, respectively. The Hep I has good stability with t_1/2 values of 350 and 60 min at 30 and 37 °C, respectively. And it showed a residual relative activity of 70.8% after 21 days incubation at 4 °C. Substrate docking study revealed that Lys99, Arg101, Gln241, Lys270, Asn275, and Lys292 were mainly involved in the substrate binding of Hep I. The shorter hydrogen bonds formed between heparin and these residues suggested the higher specific activity of BeHep I. And the minimum conformational entropy value of 756 J·K^-1 provides an evidence for the improved stability of this enzyme. This Hep I could be of interest in the industrial preparation of LMWH for its high specific activity and good stability.

Combined cross-linked enzyme aggregates of glycerol dehydrogenase and NADH oxidase for high efficiency in situ NAD+ regeneration

Cofactor regeneration is an important method to avoid the consumption of large quantities of oxidized cofactor NAD⁺ in enzyme-catalyzed reactions. Herein, glycerol dehydrogenase (GDH) and NADH oxidase preparations by aggregating enzymes with ammonium sulphate followed by cross-linking formed aggregates for effective regeneration of NAD⁺. After optimization, the activity of combi-CLEAs and separate CLEAs mixtures were 950 and 580 U/g, respectively. And the catalytic stability of combi-CLEAs against pH and temperature was superior to the free enzyme mixture. After ten cycles of reuse, the catalytic efficiency could still retain 63.3% of its initial activity, indicating that the constructed combi-CLEAs system had excellent reusability. Also, the conversion of glycerol to 1,3-dihydroxyacetone (DHA) was improved by the constructed NAD⁺ regeneration system, resulting in 4.6%, which was 2.5 times of the free enzyme system. Thus, wide applications of this co-immobilization method in the production of various chiral chemicals could be expected in the industry for its high efficiency at a low cost.

Switching the substrate specificity from NADH to NADPH by a single mutation of NADH oxidase from Lactobacillus rhamnosus

Enzymatic NADP⁺ regeneration is a promising approach to produce valuable chemicals under economic conditions. Among all the enzymatic routes, using water-forming NADH oxidase is an ideal one because there is no by-product. However, most NADH oxidases have a low specific activity to NADPH. In this work, a thermostable NADH oxidase from Lactobacillus rhamnosus (LrNox) was rationally engineered to switch its specificity from NADH to NADPH. The results show that mutants D177A, G178R, D177A/G178R, D177A/G178R/L179S improved the NADPH activity by a factor of 4-6. The highest NADPH catalytic efficiency (K_cat/K_m 223.71 S^-1 μm^-1, 47.6-fold higher than wild-type LrNox) and 51% of NADH activity retention were achieved by replacing the single amino acid Leu179 for serine (L179S) in LrNox. Modeling of L179S-NADPH complex reveals that the phosphate group of NADPH interacts with the hydroxyl of Ser179 with a strong hydrogen bond and several shorter hydrogen bonds with the amino group of Lys185 could stabilize the binding of NADPH in the L179S mutant. This work provides an efficient method for converting NAD(P)H specificity and shows that L179S mutant is a potential and efficient auxiliary enzyme for NADP⁺ regeneration.

Cloning, Expression and Characterization of a Highly Active Alcohol Dehydrogenase for Production of Ethyl (S)-4-Chloro-3-Hydroxybutyrate

A novel alcohol dehydrogenase from Bartonella apis (BaADH) was heterologous expressed in Escherichia coli. Its biochemical properties were investigated and used to catalyze the synthesis of ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE), which is a chiral intermediate of the cholesterol-lowering drug atorvastatin. The purified recombinant BaADH displayed 182.4 U/mg of the specific activity using ethyl 4-chloroacetoacetate as substrate under the conditions of 50 °C in pH 7.0 Tris-HCl buffer. It was stable in storage buffers of pH 7 to 9 and retains up to 96.7% of the initial activity after 24 h. The K _m and V _max values of BaADH were 0.11 mM and 190.4 μmol min^-1 mg^-1, respectively. Synthesis of (S)-CHBE catalyzed by BaADH was performed with a cofactor regeneration system using a glucose dehydrogenase, and a conversion of 94.9% can be achieved after 1 h reaction. Homology modeling and substrate docking revealed that a typical catalytic triad is in contact with local water molecules to form a catalytic system. The results indicated this ADH could contribute to the further enzymatic synthesis of (S)-CHBE.

A Novel Cold-Adapted Leucine Dehydrogenase from Antarctic Sea-Ice Bacterium Pseudoalteromonas sp. ANT178

l-tert-leucine and its derivatives are useful as pharmaceutical active ingredients, in which leucine dehydrogenase (LeuDH) is the key enzyme in their enzymatic conversions. In the present study, a novel cold-adapted LeuDH, psleudh, was cloned from psychrotrophic bacteria Pseudoalteromonas sp. ANT178, which was isolated from Antarctic sea-ice. Bioinformatics analysis of the gene psleudh showed that the gene was 1209 bp in length and coded for a 42.6 kDa protein containing 402 amino acids. PsLeuDH had conserved Phe binding site and NAD⁺ binding site, and belonged to a member of the Glu/Leu/Phe/Val dehydrogenase family. Homology modeling analysis results suggested that PsLeuDH exhibited more glycine residues, reduced proline residues, and arginine residues, which might be responsible for its catalytic efficiency at low temperature. The recombinant PsLeuDH (rPsLeuDH) was purified a major band with the high specific activity of 275.13 U/mg using a Ni-NTA affinity chromatography. The optimum temperature and pH for rPsLeuDH activity were 30 °C and pH 9.0, respectively. Importantly, rPsLeuDH retained at least 40% of its maximum activity even at 0 °C. Moreover, the activity of rPsLeuDH was the highest in the presence of 2.0 M NaCl. Substrate specificity and kinetic studies of rPsLeuDH demonstrated that l-leucine was the most suitable substrate, and the catalytic activity at low temperatures was ensured by maintaining a high k_cat value. The results of the current study would provide insight into Antarctic sea-ice bacterium LeuDH, and the unique properties of rPsLeuDH make it a promising candidate as a biocatalyst in medical and pharmaceutical industries.

Cloning, expression and enzymatic characteristics of a 2-Cys peroxiredoxin from Antarctic sea-ice bacterium Psychrobacter sp. ANT206

Peroxiredoxin (Prx, EC 1.11.1.15) is a family of the thiol-dependent antioxidant enzyme. In this study, a cold-adapted Prx gene from Antarctic psychrophilic bacterium Psychrobacter sp. ANT206 (PsPrx) consisted of an open reading frame (ORF) of 567 bp was cloned. Amino acid sequence analysis revealed that PsPrx contained one catalytic site (Thr45, Cys48 and Arg121) and could be categorized as a typical 2-Cys Prx. Compared with the mesophilic StPrx, PsPrx with a reduced amount of hydrogen bonds and salt bridges and other characteristics, may be responsible for its enzymatic stability and flexibility at low temperature. The recombinant PsPrx (rPsPrx) was purified to homogeneity by Ni-NTA and its enzymatic characterization was described. Interestingly, rPsPrx exhibited the maximum activity at 30 °C and remained 42.6% of its maximum activity at 0 °C. rPsPrx was a salt-tolerance enzyme that showed 42.2% of its maximum activity under 2.5 M NaCl. The kinetic parameters of different substrates revealed that it could efficiently catalyze the peroxides, especially H₂O₂ and t-BOOH (tert‑butyl hydroperoxide). Moreover, rPsPrx exhibited the ability to protect super-coiled DNA from oxidative damage. These results indicated that rPsPrx has special catalytic properties and may be a promising candidate for food and industrial applications.