Enzymatic conversion of D-galactose to D-tagatose: cloning, overexpression and characterization of L-arabinose isomerase from Pediococcus pentosaceus PC-5

L-arabinose isomerase from Lactobacillus fermentum C6: Enzymatic characteristics and its recombinant Bacillus subtilis whole cells achieving a significantly increased production of D-tagatose

L-arabinose isomerase (L-AI) is a functional enzyme for the isomerizing of D-galactose to produce D-tagatose. In this study, L-AI-C6-encoding gene from the probiotic Lactobacillus fermentum C6 was cloned and expressed in Bacillus subtilis WB600 for investigating enzymatic characteristics and bioconverting D-tagatose by means of whole-cell catalysis. Results showed that the engineered B. subtilis WB600-pMA5-LAI achieved a maximum specific activity of L-AI-C6 (232.65 ± 15.54 U/mg protein) under cultivation in LB medium at 28 °C for 40 h. The recombinant L-AI-C6 was purified, and enzymatic characteristics test showed its optimum reaction temperature and pH at 60 °C and 8.0, respectively. In addition, L-AI-C6 exhibited good stability within the pH range of 5.5-9.0. By using B. subtilis WB600-pMA5-LAI cells as whole-cell catalyst, the highest D-tagatose yield reached 42.91 ± 0.28 % with D-galactose as substrate, which was 2.41 times that of L. fermentum C6 (17.79 ± 0.11 %). This suggested that the cloning and heterologous expression of L-AI-C6 was an effective strategy for improving D-tagatose conversion by whole-cell catalysis. In brief, the present study demonstrated that the reaction temperature, pH, and stability of L-AI-C6 from L. fermentum C6 meet the demands of industrial application, and the constructed B. subtilis WB600-pMA5-LAI shows promising potential for the whole-cell biotransformation of D-tagatose.

Exploring an l-arabinose isomerase from cryophile bacteria Arthrobacter psychrolactophilus B7 for d-tagatose production

A novel l-arabinose isomerase (L-AI) from Arthrobacter psychrolactophilus (Ap L-AI) was successfully cloned and characterized. The enzyme catalyzes the isomerization of d-galactose into a rare sugar d-tagatose. The recombinant Ap L-AI had an approximate molecular weight of about 258 kDa, suggesting it was an aggregate of five 58 kDa monomers and became the first record as a homo-pentamer L-AI. The catalytic efficiency (kcat/Km) and Km for d-galactose were 0.32 mM-1 min-1 and 51.43 mM, respectively, while for l-arabinose, were 0.64 mM-1 min-1 and 23.41 mM, respectively. It had the highest activity at pH 7.0-7.5 and 60 °C in the presence of 0.250 mM Mn2+. Ap L-AI was discovered to be an outstanding thermostable enzyme that only lost its half-life value at 60 °C for >1000 min. These findings suggest that l-arabinose isomerase from Arthrobacter psychrolactophilus is a promising candidate for d-tagatose mass-production due to its industrially competitive temperature.

Robust nano-enzyme conjugates for the sustainable synthesis of a rare sugar D-tagatose

Aim of present study was to develop biological catalysts of L-arabinose isomerase (L-AI) by immobilizing on four different supports such as multiwalled carbon nanotube (MWCNT), graphene oxide (GOx), Santa Barbara Amorphous (SBA-15) and mobile composite matter (MCM-41). Also, comparative analysis of the developed catalysts was performed to evolve the best in terms of transformation efficiency for D-tagatose production. The developed nano-enzyme conjugates (NECs) were characterized using the high resolution transmission electron microscopy (HR-TEM) and elemental analysis was performed by energy dispersive X-ray spectroscopy (EDS). The functional groups were investigated by Fourier transform infra red spectroscopy. Also, the thermo gravimetric analysis (TGA) was employed to plot a thermal degradation weight loss profile of NECs. The conjugated L-AI with MWCNT and GOx were found to be more promising immobilized catalysts due to their ability to provide more surface area. Conversion of D-Galactose to D-Tagatose at moderate temperature and pH was observed to attain the equilibrium level of transformation (~50%). On the contrary, NECs prepared using SBA-15 and MCM-41 as support matrix were unable to reach the equilibrium level of conversion. Additionally, the developed NECs were suitable for reuse in multiple batch cycles. Thus, promising nanotechnology coupled with biocatalysis made the transformation of D-Galactose into D-tagatose more economically sustainable.

Isomerization of Galactose to Tagatose: Recent Advances in Non-enzymatic Isomerization

The valorization of galactose derived from acid whey to low-calorie tagatose has gained increasing attention. Enzymatic isomerization is of great interest but faces several challenges, such as poor thermal stability of enzymes and a long processing time. In this work, non-enzymatic (supercritical fluids, triethylamine, arginine, boronate affinity, hydrotalcite, Sn-β zeolite, and calcium hydroxide) pathways for galactose to tagatose isomerization were critically discussed. Unfortunately, most of these chemicals showed poor tagatose yields (<30%), except for calcium hydroxide (>70%). The latter is able to form a tagatose-calcium hydroxide-water complex, which stimulates the equilibrium toward tagatose and prevents sugar degradation. Nevertheless, the excessive use of calcium hydroxide may pose challenges in terms of economic and environmental feasibility. Moreover, the proposed mechanisms for the base (enediol intermediate) and Lewis acid (hydride shift between C-2 and C-1) catalysis of galactose were elucidated. Overall, it is crucial to explore novel and effective catalysts as well as integrated systems for isomerizing of galactose to tagatose.

Synthesis of a Healthy Sweetener d-Tagatose from Starch Catalyzed by Semiartificial Cell Factories

d-Tagatose is one of the several healthy sweeteners that can be a substitute for sucrose and fructose in our daily life. Whole cell-catalyzed phosphorylation and dephosphorylation previously reported by our group afford a thermodynamic-driven strategy to achieve tagatose production directly from starch with high product yields. Nonetheless, the poor structural stability of cells and difficulty in biocatalyst recycling restrict its practical application. Herein, an efficient and stable semiartificial cell factory (SACF) was developed by constructing an organosilica network (OSN) artificial shell on the cells bearing five thermophilic enzymes to produce tagatose. The OSN artificial shell, the thickness of which can be regulated by changing the tetraethyl silicate concentration, exhibited tunable permeability and superior mechanical strength. In contrast with cells, SACFs showed a relative activity of 99.5% and an extended half-life from 33.3 to 57.8 h. Over 50% of initial activity was retained after 20 reuses. The SACFs can catalyze seven consecutive reactions with tagatose yields of over 40.7% in field applications.

Low-calorie bulk sweeteners: Recent advances in physical benefits, applications, and bioproduction

At present, with the continuous improvement of living standards, people are paying increasing attention to dietary nutrition and health. Low sugar and low energy consumption have become important dietary trends. In terms of sugar control, more and more countries have implemented sugar taxes in recent years. Hence, as the substitute for sugar, low-calorie sweeteners have been widely used in beverage, bakery, and confectionary industries. In general, low-calorie sweeteners consist of high-intensity and low-calorie bulk sweeteners (some rare sugars and sugar alcohols). In this review, recent advances and challenges in low-calorie bulk sweeteners are explored. Bioproduction of low-calorie bulk sweeteners has become the focus of many researches, because it has the potential to replace the current industrial scale production through chemical synthesis. A comprehensive summary of the physicochemical properties, physiological functions, applications, bioproduction, and regulation of typical low-calorie bulk sweeteners, such as D-allulose, D-tagatose, D-mannitol, sorbitol, and erythritol, is provided.

Characterization of l-Arabinose Isomerase from Klebsiella pneumoniae and Its Application in the Production of d-Tagatose from d-Galactose

d-Tagatose, a functional sweetener, is converted from d-galactose by l-arabinose isomerase, which catalyzes the conversion of l-arabinose to l-ribulose. In this study, the araA gene encoding l-arabinose isomerase from Klebsiella pneumoniae was cloned and expressed in Escherichia coli, and the expressed enzyme was purified and characterized. The purified l-arabinose isomerase, a soluble protein with 11.6-fold purification and a 22% final yield, displayed a specific activity of 1.8 U/mg for d-galactose and existed as a homohexamer of 336 kDa. The enzyme exhibited maximum activity at pH 8.0 and 40 °C in the presence of Mn2+ and relative activity for pentoses and hexoses in the order l-arabinose > d-galactose > l-ribulose > d-xylulose > d-xylose > d-tagatose > d-glucose. The thermal stability of recombinant E. coli cells expressing l-arabinose isomerase from K. pneumoniae was higher than that of the enzyme. Thus, the reaction conditions of the recombinant cells were optimized to pH 8.0, 50 °C, and 4 g/L cell concentration using 100 g/L d-galactose with 1 mM Mn2+. Under these conditions, 33.5 g/L d-tagatose was produced from d-galactose with 33.5% molar yield and 67 g/L/h productivity. Our findings will help produce d-tagatose using whole-cell reactions, extending its industrial application.

Reagentless D-Tagatose Biosensors Based on the Oriented Immobilization of Fructose Dehydrogenase onto Coated Gold Nanoparticles- or Reduced Graphene Oxide-Modified Surfaces: Application in a Prototype Bioreactor

As electrode nanomaterials, thermally reduced graphene oxide (TRGO) and modified gold nanoparticles (AuNPs) were used to design bioelectrocatalytic systems for reliable D-tagatose monitoring in a long-acting bioreactor where the valuable sweetener D-tagatose was enzymatically produced from a dairy by-product D-galactose. For this goal D-fructose dehydrogenase (FDH) from Gluconobacter industrius immobilized on these electrode nanomaterials by forming three amperometric biosensors: AuNPs coated with 4-mercaptobenzoic acid (AuNP/4-MBA/FDH) or AuNPs coated with 4-aminothiophenol (AuNP/PATP/FDH) monolayer, and a layer of TRGO on graphite (TRGO/FDH) were created. The immobilized FDH due to changes in conformation and spatial orientation onto proposed electrode surfaces catalyzes a direct D-tagatose oxidation reaction. The highest sensitivity for D-tagatose of 0.03 ± 0.002 μA mM⁻¹cm⁻² was achieved using TRGO/FDH. The TRGO/FDH was applied in a prototype bioreactor for the quantitative evaluation of bioconversion of D-galactose into D-tagatose by L-arabinose isomerase. The correlation coefficient between two independent analyses of the bioconversion mixture: spectrophotometric and by the biosensor was 0.9974. The investigation of selectivity showed that the biosensor was not active towards D-galactose as a substrate. Operational stability of the biosensor indicated that detection of D-tagatose could be performed during six hours without loss of sensitivity.

An overview of D-galactose utilization through microbial fermentation and enzyme-catalyzed conversion

D-Galactose is an abundant carbohydrate monomer in nature and widely exists in macroalgae, plants, and dairy wastes. D-Galactose is useful as a raw material for biomass fuel production or low-calorie sweetener production, attracting increased attention. This article summarizes the studies on biotechnological processes for galactose utilization. Two main research directions of microbial fermentation and enzyme-catalyzed conversion from galactose-rich biomass are extensively reviewed. The review provides the recent discoveries for biofuel production from macroalgae, including the innovative methods in the pretreatment process and technological development in the fermentation process. As modern people pay more attention to health, enzyme technologies for low-calorie sweetener production are more urgently needed. D-Tagatose is a promising low-calorie alternative to sugar. We discuss the recent studies on characterization and genetic modification of L-arabinose isomerase to improve the bioconversion of D-galactose to D-tagatose. In addition, the trends and critical challenges in both research directions are outlined at the end. KEY POINTS: • The value and significance of galactose utilization are highlighted. • Biofuel production from galactose-rich biomass is accomplished by fermentation. • L-arabinose isomerase is a tool for bioconversion of D-galactose to D-tagatose.

Alternative Heterologous Expression of L-Arabinose Isomerase from Enterococcus faecium DBFIQ E36 By Residual Whey Lactose Induction

This study reports an alternative strategy for the expression of a recombinant L-AI from Enterococcus faecium DBFIQ E36 by auto-induction using glucose and glycerol as carbon sources and residual whey lactose as inducer agent. Commercial lactose and isopropyl β-D-1-thiogalactopyranoside (IPTG) were also evaluated as inducers for comparison of enzyme expression levels. The enzymatic extracts were purified by affinity chromatography, characterized, and applied in the bioconversion of D-galactose into D-tagatose. L-AI presented a catalytic activity of 1.67 ± 0.14, 1.52 ± 0.01, and 0.7 ± 0.04 U/mL, when expressed using commercial lactose, lactose from whey, and IPTG, respectively. Higher activities could be obtained by changing the protocol of enzyme extraction and, for instance, the enzymatic extract produced with whey presented a catalytic activity of 3.8 U/mL. The specific activity of the enzyme extracts produced using lactose (commercial or residual whey) after enzyme purification was also higher when compared to the enzyme expressed with IPTG. Best results were achieved when enzyme expression was conducted using 4 g/L of residual whey lactose for 11 h. These results proved the efficacy of an alternative and economic protocol for the effective expression of a recombinant L-AI aiming its high-scale production.

Exploring a Highly D-Galactose Specific L-Arabinose Isomerase From Bifidobacterium adolescentis for D-Tagatose Production

D-Galactose-specific L-arabinose isomerase (L-AI) would have much potential for the enzymatic conversion of D-Galactose into D-tagatose, while most of the reported L-AIs are L-arabinose specific. This study explored a highly D-Galactose-specific L-AI from Bifidobacterium adolescentis (BAAI) for the production of D-tagatose. In the comparative protein-substrate docking for D-Galactose and L-arabinose, BAAI showed higher numbers of hydrogen bonds in D-Galactose-BAAI bonding site than those found in L-arabinose-BAAI bonding site. The activity of BAAI was 24.47 U/mg, and it showed good stability at temperatures up to 65°C and a pH range 6.0–7.5. The K_m, V_max, and K_cat/K_m of BAAI were found to be 22.4 mM, 489 U/mg and 9.3 mM^–1 min^–1, respectively for D-Galactose, while the respective values for L-arabinose were 40.2 mM, 275.1 U/mg, and 8.6 mM^–1 min^–1. Enzymatic conversion of D-Galactose into D-tagatose by BAAI showed 56.7% conversion efficiency at 55°C and pH 6.5 after 10 h.

Biochemical Characterization of Heat-Tolerant Recombinant L-Arabinose Isomerase from Enterococcus faecium DBFIQ E36 Strain with Feasible Applications in D-Tagatose Production

D-Tagatose is a ketohexose, which presents unique properties as a low-calorie functional sweetener possessing a sweet flavor profile similar to D-sucrose and having no aftertaste. Considered a generally recognized as safe (GRAS) substance by FAO/WHO, D-tagatose can be used as an intermediate for the synthesis of other optically active compounds as well as an additive in detergent, cosmetic, and pharmaceutical formulations. This study reports important features for L-arabinose isomerase (EC 5.3.1.4) (L-AI) use in industry. We describe arabinose (araA) gene virulence analysis, gene isolation, sequencing, cloning, and heterologous overexpression of L-AI from the food-grade GRAS bacterium Enterococcus faecium DBFIQ E36 in Escherichia coli and assess biochemical properties of this recombinant enzyme. Recombinant L-AI (rL-AI) was one-step purified to homogeneity by Ni²⁺-agarose resin affinity chromatography and biochemical characterization revealed low identity with both thermophilic and mesophilic L-AIs but high degree of conservation in residues involved in substrate recognition. Optimal conditions for rL-AI activity were 50 °C, pH 5.5, and 0.3 mM Mn²⁺, exhibiting a low cofactor concentration requirement and an acidic optimum pH. Half-life at 45 °C and 50 °C were 1427 h and 11 h, respectively, and 21.5 h and 39.5 h at pH 4.5 and 5.6, respectively, showing the high stability of the enzyme in the presence of a metallic cofactor. Bioconversion yield for D-tagatose biosynthesis was 45% at 50 °C after 48 h. These properties highlight the technological potential of E. faecium rL-AI as biocatalyst for D-tagatose production.

Tagatose as a Potential Nutraceutical: Production, Properties, Biological Roles, and Applications

Nutraceuticals are gaining importance owing to their potential applications in numerous sectors including food and feed industries. Among the emerging nutraceuticals, d-tagatose occupies a significant niche because of its low calorific value, antidiabetic property and growth promoting effects on beneficial gut bacteria. As d-tagatose is present in minute quantities in naturally occurring food substances, it is produced mainly by chemical or biological means. Recently, attempts were made for bio-production of d-tagatose using l-arabinose isomerase enzyme to overcome the challenges of chemical process of production. Applications of d-tagatose for maintaining health and wellbeing are increasing due to growing consumer awareness and apprehension against modern therapeutic agents. This review outlines the current status on d-tagatose, particularly its production, properties, biological role, applications, and the future perspectives.

Towards efficient enzymatic conversion of D-galactose to D-tagatose: purification and characterization of L-arabinose isomerase from Lactobacillus brevis

L-arabinose isomerase (L-AI) (EC 5. 3. 1. 4. L-AI) that mediates the isomerization of D-galactose to D-tagatose was isolated from Lactobacillus brevis (MF 465792), and was further purified and characterized. Pure enzyme with molecular weight of 60.1 kDa was successfully obtained after the purification using Native-PAGE gel extraction method, which was a monomer in solution. The L-AI was found to be stable at 45-75 °C, and at pH 7.0-9.0. Its optimum temperature and pH was determined as 65 °C and 7.0, respectively. Besides, we found that Ca²⁺, Cu²⁺, and Ba²⁺ ions inhibited the enzyme activity, whereas the enzyme activity was significantly enhanced in the presence of Mg²⁺, Mn²⁺, or Co²⁺ ions. The optimum concentration of Mn²⁺ and Co²⁺ was determined to be 1 mM. Furthermore, we characterized the kinetic parameters for L-AI and determined the K_m (129 mM) and the V_max (0.045 mM min^- 1) values. Notably, L. brevisL-AI exhibited a high bioconversion yield of 43% from D-galactose to D-tagatose under the optimal condition, and appeared to be a more efficient catalyst compared with other L-AIs from various organisms.

Reduced Susceptibility to Sugar-Induced Metabolic Derangements and Impairments of Myocardial Redox Signaling in Mice Chronically Fed with D-Tagatose when Compared to Fructose

Background

D-tagatose is an isomer of fructose and is ~90% as sweet as sucrose with less caloric value. Nowadays, D-tagatose is used as a nutritive or low-calorie sweetener. Despite clinical findings suggesting that D-tagatose could be beneficial in subjects with type 2 diabetes, there are no experimental data comparing D-tagatose with fructose, in terms of metabolic derangements and related molecular mechanisms evoked by chronic exposure to these two monosaccharides.

Materials and methods

C57Bl/6j mice were fed with a control diet plus water (CD), a control diet plus 30% fructose syrup (L-Fr), a 30% fructose solid diet plus water (S-Fr), a control diet plus 30% D-tagatose syrup (L-Tg), or a 30% D-tagatose solid diet plus water (S-Tg), during 24 weeks.

Results

Both solid and liquid fructose feeding led to increased body weight, abnormal systemic glucose homeostasis, and an altered lipid profile. These effects were associated with vigorous increase in oxidative markers. None of these metabolic abnormalities were detected when mice were fed with both the solid and liquid D-tagatose diets, either at the systemic or at the local level. Interestingly, both fructose formulations led to significant Advanced Glycation End Products (AGEs) accumulation in mouse hearts, as well as a robust increase in both myocardial AGE receptor (RAGE) expression and NF-κB activation. In contrast, no toxicological effects were shown in hearts of mice chronically exposed to liquid or solid D-tagatose.

Conclusion

Our results clearly suggest that chronic overconsumption of D-tagatose in both formulations, liquid or solid, does not exert the same deleterious metabolic derangements evoked by fructose administration, due to differences in carbohydrate interference with selective proinflammatory and oxidative stress cascades.

D-Tagatose Is a Promising Sweetener to Control Glycaemia: A New Functional Food

The objective of the current research was to review and update evidence on the dietary effect of the consumption of tagatose in type 2 diabetes, as well as to elucidate the current approach that exists on its production and biotechnological utility in functional food for diabetics. Articles published before July 1, 2017, were included in the databases PubMed, EBSCO, Google Scholar, and Scielo, including the terms "Tagatose", "Sweeteners", "Diabetes Mellitus type 2", "Sweeteners", "D-Tag". D-Tagatose (D-tag) is an isomer of fructose which is approximately 90% sweeter than sucrose. Preliminary studies in animals and preclinical studies showed that D-tag decreased glucose levels, which generated great interest in the scientific community. Recent studies indicate that tagatose has low glycemic index, a potent hypoglycemic effect, and eventually could be associated with important benefits for the treatment of obesity. The authors concluded that D-tag is promising as a sweetener without major adverse effects observed in these clinical studies.

Metabolic engineering pathways for rare sugars biosynthesis, physiological functionalities, and applications-a review

Biomolecules like rare sugars and their derivatives are referred to as monosaccharides particularly uncommon in nature. Remarkably, many of them have various known physiological functions and biotechnological applications in cosmetics, nutrition, and pharmaceutical industries. Also, they can be exploited as starting materials for synthesizing fascinating natural bioproducts with significant biological activities. Regrettably, most of the rare sugars are quite expensive, and their synthetic chemical routes are both limited and economically unfeasible due to expensive raw materials. On the other hand, their production by enzymatic means often suffers from low space-time yields and high catalyst costs due to hasty enzyme denaturation/degradation. In this context, biosynthesis of rare sugars with industrial importance is receiving renowned scientific attention, across the globe. Moreover, the utilization of renewable resources as energy sources via microbial fermentation or microbial metabolic engineering has appeared a new tool. This article presents a comprehensive review of physiological functions and biotechnological applications of rare ketohexoses and aldohexoses, including D-psicose, D-tagatose, L-tagatose, D-sorbose, L-fructose, D-allose, L-glucose, D-gulose, L-talose, L-galactose, and L-fucose. Novel in-vivo recombination pathways based on aldolase and phosphatase for the biosynthesis of rare sugars, particularly D-psicose and D-sorbose using robust microbial strains are also deliberated.

Enhanced d-tagatose production by spore surface-displayed l-arabinose isomerase from isolated Lactobacillus brevis PC16 and biotransformation

In the present study, a new strain of Lactobacillus brevis producing d-tagatose was isolated and identified. Then, the l-arabinose isomerase (L-AI) of this strain was displayed on the spore surface of Bacillus subtilis DB403 by using an anchoring protein CotG and a peptide linker (Gly-Gly-Gly-Gly-Ser). This displayed L-AI with high specific activity and stability was used as a novel immobilized biocatalyst for producing d-tagatose through batch and semi-continuous biotransformation. The conversion rate of d-tagatose from 125 g/L d-galactose was achieved 79.7% at 28 h, and the volumetric productivity reached 4.3 g/L/h at 20 h. Furthermore, the displayed L-AI showed a good performance on the reusability and remained 87% of the specific activity and 40.7% of the conversion rate after five recycles. A high efficient immobilized method for producing food-grade d-tagatose was established using spore surface-displayed L-AI.

Engineering the l-Arabinose Isomerase from Enterococcus Faecium for d-Tagatose Synthesis

l-Arabinose isomerase (EC 5.3.1.4) (l-AI) from Enterococcus faecium DBFIQ E36 was overproduced in Escherichia coli by designing a codon-optimized synthetic araA gene. Using this optimized gene, two N- and C-terminal His-tagged-l-AI proteins were produced. The cloning of the two chimeric genes into regulated expression vectors resulted in the production of high amounts of recombinant N-His-l-AI and C-His-l-AI in soluble and active forms. Both His-tagged enzymes were purified in a single step through metal-affinity chromatography and showed different kinetic and structural characteristics. Analytical ultracentrifugation revealed that C-His-l-AI was preferentially hexameric in solution, whereas N-His-l-AI was mainly monomeric. The specific activity of the N-His-l-AI at acidic pH was higher than that of C-His-l-AI and showed a maximum bioconversion yield of 26% at 50 °C for d-tagatose biosynthesis, with Km and Vmax parameters of 252 mM and 0.092 U mg^-1, respectively. However, C-His-l-AI was more active and stable at alkaline pH than N-His-l-AI. N-His-l-AI follows a Michaelis-Menten kinetic, whereas C-His-l-AI fitted to a sigmoidal saturation curve.

d-Tagatose production by permeabilized and immobilized Lactobacillus plantarum using whey permeate

The aim of the work is to produce d-Tagatose by direct addition of alginate immobilized Lactobacillus plantarum cells to lactose hydrolysed whey permeate. The cells were untreated and immobilized (UIC), permeabilized and immobilized (PIC) and the relative activities were compared with purified l-arabinose isomerase (l-AI) for d-galactose isomerization. Successive lactose hydrolysis by β-galactosidase from Escherichia coli and d-galactose isomerization using l-AI from Lactobacillus plantarum was performed to investigate the in vivo production of d-tagatose in whey permeate. In whey permeate, maximum conversion of 38% and 33% (w/w) d-galactose isomerization by PIC and UIC has been obtained. 162mg/g and 141mg/g of d-tagatose production was recorded in a 48h reaction time at 50°C, pH 7.0 with 5mM Mn²⁺ ion concentration in the initial substrate mixture.

Rational Design of Bacillus coagulans NL01 l-Arabinose Isomerase and Use of Its F279I Variant in d-Tagatose Production

d-Tagatose is a prospective functional sweetener that can be produced by l-arabinose isomerase (AI) from d-galactose. To improve the activity of AI toward d-galactose, the AI of Bacillus coagulans was rationally designed on the basis of molecular modeling and docking. After alanine scanning and site-saturation mutagenesis, variant F279I that exhibited improved activity toward d-galactose was obtained. The optimal temperature and pH of F279I were determined to be 50 °C and 8.0, respectively. This variant possessed 1.4-fold catalytic efficiency compared with the wild-type (WT) enzyme. The recombinant Escherichia coli overexpressing F279I also showed obvious advantages over the WT in biotransformation. Under optimal conditions, 67.5 and 88.4 g L^-1 d-tagatose could be produced from 150 and 250 g L^-1 d-galactose, respectively, in 15 h. The biocatalyst constructed in this study presents a promising alternative for large-scale d-tagatose production.

Cloning, Expression, and Characterization of a Novel L-Arabinose Isomerase from the Psychrotolerant Bacterium Pseudoalteromonas haloplanktis

L-Arabinose isomerase (L-AI, EC 5.3.1.4) catalyzes the isomerization between L-arabinose and L-ribulose, and most of the reported ones can also catalyze D-galactose to D-tagatose, except Bacillus subtilis L-AI. In this article, the L-AI from the psychrotolerant bacterium Pseudoalteromonas haloplanktis ATCC 14393 was characterized. The enzyme showed no substrate specificity toward D-galactose, which was similar to B. subtilis L-AI but distinguished from other reported L-AIs. The araA gene encoding the P. haloplanktis L-AI was cloned and overexpressed in E. coli BL21 (DE3). The recombinant enzyme was purified by one-step nickel affinity chromatography . The enzyme displayed the maximal activity at 40 °C and pH 8.0, and showed more than 75 % of maximal activity from pH 7.5-9.0. Metal ion Mn²⁺ was required as optimum metal cofactor for activity simulation, but it did not play a significant role in thermostability improvement as reported previously. The Michaelis-Menten constant (K _m), turnover number (k _cat), and catalytic efficiency (k _cat/K _m) for substrate L-arabinose were measured to be 111.68 mM, 773.30/min, and 6.92/mM/min, respectively. The molecular docking results showed that the active site residues of P. haloplanktis L-AI could only immobilize L-arabinose and recognized it as substrate for isomerization.

Pathway Construction in Corynebacterium glutamicum and Strain Engineering To Produce Rare Sugars from Glycerol

Rare sugars are valuable natural products widely used in pharmaceutical and food industries. In this study, we expected to synthesize rare ketoses from abundant glycerol using dihydroxyacetone phosphate (DHAP)-dependent aldolases. First, a new glycerol assimilation pathway was constructed to synthesize DHAP. The enzymes which convert glycerol to 3-hydroxypropionaldehyde and l-glyceraldehyde were selected, and their corresponding aldehyde synthesis pathways were constructed in vivo. Four aldol pathways based on different aldolases and phosphorylase were gathered. Next, three pathways were assembled and the resulting strains synthesized 5-deoxypsicose, 5-deoxysorbose, and 5-deoxyfructose from glucose and glycerol and produce l-fructose, l-tagatose, l-sorbose, and l-psicose with glycerol as the only carbon source. To achieve higher product titer and yield, the recombinant strains were further engineered and fermentation conditions were optimized. Fed-batch culture of engineered strains obtained 38.1 g/L 5-deoxypsicose with a yield of 0.91 ± 0.04 mol product per mol of glycerol and synthesized 20.8 g/L l-fructose, 10.3 g/L l-tagatose, 1.2 g/L l-sorbose, and 0.95 g/L l-psicose.

Increased Production of Food-Grade d-Tagatose from d-Galactose by Permeabilized and Immobilized Cells of Corynebacterium glutamicum, a GRAS Host, Expressing d-Galactose Isomerase from Geobacillus thermodenitrificans

The generally recognized as safe microorganism Corynebacterium glutamicum expressing Geobacillus thermodenitrificans d-galactose isomerase (d-GaI) was an efficient host for the production of d-tagatose, a functional sweetener. The d-tagatose production at 500 g/L d-galactose by the host was 1.4-fold higher than that by Escherichia coli expressing d-GaI. The d-tagatose-producing activity of permeabilized C. glutamicum (PCG) cells treated with 1% (w/v) Triton X-100 was 2.1-fold higher than that of untreated cells. Permeabilized and immobilized C. glutamicum (PICG) cells in 3% (w/v) alginate showed a 3.1-fold longer half-life at 50 °C and 3.1-fold higher total d-tagatose concentration in repeated batch reactions than PCG cells. PICG cells, which produced 165 g/L d-tagatose after 3 h, with a conversion of 55% (w/w) and a productivity of 55 g/L/h, showed significantly higher d-tagatose productivity than that reported for other cells. Thus, d-tagatose production by PICG cells may be an economical process to produce food-grade d-tagatose.

Characterization of an L-arabinose isomerase from Bacillus coagulans NL01 and its application for D-tagatose production

Background

L-arabinose isomerase (AI) is a crucial catalyst for the biotransformation of D-galactose to D-tagatose. In previous reports, AIs from thermophilic bacterial strains had been wildly researched, but the browning reaction and by-products formed at high temperatures restricted their applications. By contrast, AIs from mesophilic Bacillus strains have some different features including lower optimal temperatures and lower requirements of metallic cofactors. These characters will be beneficial to the development of a more energy-efficient and safer production process. However, the relevant data about the kinetics and reaction properties of Bacillus AIs in D-tagatose production are still insufficient. Thus, in order to support further applications of these AIs, a comprehensive characterization of a Bacillus AI is needed.

Results

The coding gene (1422 bp) of Bacillus coagulans NL01 AI (BCAI) was cloned and overexpressed in the Escherichia coli BL21 (DE3) strain. The enzymatic property test showed that the optimal temperature and pH of BCAI were 60 °C and 7.5 respectively. The raw purified BCAI originally showed high activity in absence of outsourcing metallic ions and its thermostability did not change in a low concentration (0.5 mM) of Mn²⁺ at temperatures from 70 °C to 90 °C. Besides these, the catalytic efficiencies (k_cat/K_m) for L-arabinose and D-galactose were 8.7 mM^-1 min^-1 and 1.0 mM^-1 min^-1 respectively. Under optimal conditions, the recombinant E. coli cell containing BCAI could convert 150 g L^-1 and 250 g L^-1 D-galactose to D-tagatose with attractive conversion rates of 32 % (32 h) and 27 % (48 h).

Conclusions

In this study, a novel AI from B. coagulans NL01was cloned, purified and characterized. Compared with other reported AIs, this AI could retain high proportions of activity at a broader range of temperatures and was less dependent on metallic cofactors such as Mn²⁺. Its substrate specificity was understood deeply by carrying out molecular modelling and docking studies. When the recombinant E. coli expressing the AI was used as a biocatalyst, D-tagatose could be produced efficiently in a simple one-pot biotransformation system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0286-5) contains supplementary material, which is available to authorized users.

L-Arabinose isomerase and its use for biotechnological production of rare sugars

L-Arabinose isomerase (AI), a key enzyme in the microbial pentose phosphate pathway, has been regarded as an important biological catalyst in rare sugar production. This enzyme could isomerize L-arabinose into L-ribulose, as well as D-galactose into D-tagatose. Both the two monosaccharides show excellent commercial values in food and pharmaceutical industries. With the identification of novel AI family members, some of them have exhibited remarkable potential in industrial applications. The biological production processes for D-tagatose and L-ribose (or L-ribulose) using AI have been developed and improved in recent years. Meanwhile, protein engineering techniques involving rational design has effectively enhanced the catalytic properties of various AIs. Moreover, the crystal structure of AI has been disclosed, which sheds light on the understanding of AI structure and catalytic mechanism at molecular levels. This article reports recent developments in (i) novel AI screening, (ii) AI-mediated rare sugar production processes, (iii) molecular modification of AI, and (iv) structural biology study of AI. Based on previous reports, an analysis of the future development has also been initiated.

Biosynthesis of rare ketoses through constructing a recombination pathway in an engineered Corynebacterium glutamicum

Rare sugars have various known biological functions and potential for applications in pharmaceutical, cosmetics, and food industries. Here we designed and constructed a recombination pathway in Corynebacterium glutamicum, in which dihydroxyacetone phosphate (DHAP), an intermediate of the glycolytic pathway, and a variety of aldehydes were condensed to synthesize rare ketoses sequentially by rhamnulose-1-phosphate aldolase (RhaD) and fructose-1-phosphatase (YqaB) obtained from Escherichia coli. A wild-type strain harboring this artificial pathway had the ability to produce D-sorbose and D-psicose using D-glyceraldehyde and glucose as the substrates. The tpi gene, encoding triose phosphate isomerase was further deleted, and the concentration of DHAP increased to nearly 20-fold relative to that of the wild-type. After additional optimization of expression levels from rhaD and yqaB genes and of the fermentation conditions, the engineered strain SY6(pVRTY) exhibited preferable performance for rare ketoses production. Its yield increased to 0.59 mol/mol D-glyceraldehyde from 0.33 mol/mol D-glyceraldehyde and productivity to 2.35 g/L h from 0.58 g/L h. Moreover, this strain accumulated 19.5 g/L of D-sorbose and 13.4 g/L of D-psicose using a fed-batch culture mode under the optimal conditions. In addition, it was verified that the strain SY6(pVRTY) meanwhile had the ability to synthesize C4, C5, C6, and C7 rare ketoses when a range of representative achiral and homochiral aldehydes were applied as the substrates. Therefore, the platform strain exhibited the potential for microbial production of rare ketoses and deoxysugars.

L-Arabinose isomerase and D-xylose isomerase from Lactobacillus reuteri: characterization, coexpression in the food grade host Lactobacillus plantarum, and application in the conversion of D-galactose and D-glucose

The L-arabinose isomerase (L-AI) and the D-xylose isomerase (D-XI) encoding genes from Lactobacillus reuteri (DSMZ 17509) were cloned and overexpressed in Escherichia coli BL21 (DE3). The proteins were purified to homogeneity by one-step affinity chromatography and characterized biochemically. L-AI displayed maximum activity at 65 °C and pH 6.0, whereas D-XI showed maximum activity at 65 °C and pH 5.0. Both enzymes require divalent metal ions. The genes were also ligated into the inducible lactobacillal expression vectors pSIP409 and pSIP609, the latter containing a food grade auxotrophy marker instead of an antibiotic resistance marker, and the L-AI- and D-XI-encoding sequences/genes were coexpressed in the food grade host Lactobacillus plantarum . The recombinant enzymes were tested for applications in carbohydrate conversion reactions of industrial relevance. The purified L-AI converted D-galactose to D-tagatose with a maximum conversion rate of 35%, and the D-XI isomerized D-glucose to D-fructose with a maximum conversion rate of 48% at 60 °C.