Tagatose, a new antidiabetic and obesity control drug

Impact of artificial sweeteners and rare sugars on the gut microbiome

Alternative sugars are often used as sugar substitutes because of their low calories and glycemic index. Recently, consumption of these sweeteners in diet foods and beverages has increased dramatically, raising concerns about their health effects. This review examines the types and characteristics of artificial sweeteners and rare sugars and analyzes their impact on the gut microbiome. In the section on artificial sweeteners, we have described the chemical structures of different sweeteners, their digestion and absorption processes, and their effects on the gut microbiota. We have also discussed the biochemical properties and production methods of rare sugars and their positive and negative effects on gut microbial communities. Finally, we have described how artificial sweeteners and rare sugars alter the gut microbiome and how these changes affect the gut environment. Our observations aim to improve our understanding regarding the potential health implications of the consumption of artificial sweeteners and low-calorie sugars.

D-Tagatose: A Rare Sugar with Functional Properties and Antimicrobial Potential against Oral Species

Carbohydrates have a dietary role, but excessive consumption of high-calorie sugars can contribute to an increased incidence of metabolic diseases and dental caries. Recently, carbohydrates with sweetening properties and low caloric value, such as D-tagatose, have been investigated as alternative sugars. D-tagatose is a rare sugar that has nutritional and functional properties of great interest for health. This literature review presents an approach to the biological effects of D-tagatose, emphasizing its benefits for oral health. Studies report that D-tagatose has antioxidant and prebiotic effects, low digestibility, reduced glycemic and insulinemic responses, and the potential to improve the lipid profile, constituting an alternative for diabetes mellitus and obesity. It can also be observed that D-tagatose has an antioxidant action, favoring the elimination of free radicals and, consequently, causing a reduction in cellular oxidative stress. Furthermore, it also has antibacterial potential against oral species. Regarding oral health, studies have shown that D-tagatose efficiently reversed bacterial coaggregations, including periodontopathogenic species, and impaired the activity and growth of cariogenic bacteria, such as S. mutans. D-tagatose significantly inhibited biofilm formation, pH decrease and insoluble glucan synthesis in S. mutans cultures. Salivary S. mutans counts were also significantly reduced by the consumption of chewing gum containing D-tagatose and xylitol. In addition, there is evidence that tagatose is effective as an air-polishing powder for biofilm decontamination. The literature indicates that D-tagatose can contribute to the prevention of systemic diseases, also constituting a promising agent to improve oral health.

Efficient synthesis of rare sugars from galactose in hot compressed water using eggshells as an environmentally friendly catalyst

Aqueous galactose solutions containing eggshell was heated at 120 °C to produce calcium supplements containing rare sugars. Galactose was isomerized to rare sugars with improving rare sugar yields compared to those without eggshell. Organic acids were also formed as byproducts during the reaction. These acids were neutralized by dissolving eggshells with increasing the calcium ion concentration in the solution. When eggshell components (calcium carbonate, magnesium carbonate, or calcium phosphate) were used for the treatment, rare sugars were also formed. Especially, addition of magnesium carbonate improved rare sugar yield, but byproduct formation became more pronounced. Eggshells used in the treatment were used for repeated treatments. When eggshells were used three times, rare sugar yield changed only slightly but the selectivity of rare sugars improved significantly. By these processes, we obtained an aqueous solution of rare sugars containing calcium ion at 295 mg/L, which has potential as ingredients for dietary supplements.

Highly efficient production and simultaneous purification of d-tagatose through one-pot extraction-assisted isomerization of d-galactose

A one-pot extraction-assisted d-galactose-to-d-tagatose isomerization strategy was proposed based on the selective extraction of d-tagatose by phenylborate anions. 4-Vinylphenylboronic acid was selected with high extraction efficiency and selectivity towards d-tagatose. The extracted sugars could be desorbed through a two-staged stripping process with the purity of d-tagatose significantly increased. In-situ extraction-assisted d-galactose-to-d-tagatose isomerization was implemented for the first time ever reported, and the effect of boron-to-sugar ratio (boron: sugar) was investigated. The conversion yield of d-tagatose at 60 °C increased from ∼ 39 % (boron: sugar = 0.5) to ∼ 56 % (boron: sugar = 1) but then decreased to ∼ 44 % (boron: sugar = 1.5). With temperature increased to 70 °C, the conversion yield of d-tagatose was further improved to ∼ 61 % (boron: sugar = 1.5), with the minimized formation of byproducts. Moreover, high purity (∼83 %) and concentrated d-tagatose solution (∼40 g/L) was obtained after sequential desorption. The proposed extraction-assisted isomerization strategy achieved improving the yield and purity of d-tagatose, proving its feasibility in industrial applications.

Natural Deep Eutectic Solvents for Solubility and Selective Fractionation of Bioactive Low Molecular Weight Carbohydrates

Natural deep eutectic solvents (NADESs) have been shown to be selective and environmentally friendly solvents for the extraction of bioactive compounds. However, studies on the solubility of low-molecular-weight carbohydrates (LMWCs) in NADESs are scarce. In this work, new solubility data of LMWCs in NADESs are provided and a new approach based on the use of these solvents for the efficient fractionation of bioactive carbohydrates was explored for the first time. Several mono- and disaccharides and three NADESs based on choline chloride (ChCl) and different donors (2-ethylene glycol (EtG), glycerol (Gly) and ethanedioic acid dihydrate (Eth)) were considered. While the degradation of carbohydrates, mainly ketoses, was detected with ChCl:Eth due to its acidic nature, ChCl:EtG and ChCl:Gly were found to be useful alternatives for selectively separating bioactive ketoses and their corresponding aldoses (e.g., lactulose/lactose and tagatose/galactose) present in equimolar binary mixtures. In addition, the usefulness of ChCl:EtG for the selective enrichment of lactulose to be used as food ingredient or nutraceutical was proven (from a 25% in the reaction mixture to a 56% in the purified sample). NADESs could be used for the selective fractionation of value-added carbohydrates from interfering sugars for several applications, including food science, engineering or pharmaceuticals.

Low-no-calorie sweeteners exert marked compound-specific impact on the human gut microbiota ex vivo

Low-no-calorie sweeteners (LNCS) are used as sugar substitutes as part of strategies to reduce the risk of chronic diseases related to high sugar intake (e.g. type 2 diabetes (T2D)). This study investigated how a range of sweeteners [tagatose (TA)/maltitol (MA)/sorbitol (SO)/stevia (ST)/sucralose (SU)/acesulfame K (ACK)] impact the gut microbiota of T2D subjects and healthy human adults using the ex vivo SIFR® technology (n = 12). The cohort covered clinically relevant interpersonal and T2D-related differences. ACK/SU remained intact while not impacting microbial composition and metabolite production. In contrast, TA/SO and ST/MA were respectively readily and gradually fermented. ST and particularly TA/SO/MA increased bacterial density and SCFA production product-specifically: SO increased acetate (∼Bifidobacterium adolescentis), whilst MA/ST increased propionate (∼Parabacteroides distasonis). TA exerted low specificity as it increased butyrate for healthy subjects, yet propionate for T2D subjects. Overall, LNCS exerted highly compound-specific effects stressing that results obtained for one LNCS cannot be generalised to other LNCS.

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.

The Bioactivity and Phytochemicals of Pachyrhizus erosus (L.) Urb.: A Multifunctional Underutilized Crop Plant

Pachyrhizus erosus (L.) Urb. is an underutilized crop plant belonging to the Fabaceae family. In recent years, the plant received huge attention and was introduced in different countries owing to properties such as a high nutritional content, its nitrogen-fixing abilities, and different biological activities such as its antioxidant, immune modulation, anticancer, anti-diabetes, anti-osteoporosis, antiviral, and antiaging affects, among others. In this review, an attempt has been made to comprehensively compile the biological activities of the plant to provide a panoramic view of the current efforts and further directions, which may lead to the development of pharmacological applications. This information will be helpful in creating interest towards P. erosus and it may be useful in developing the plant for medical applications and/or as a functional food. More than 50 phytochemicals have been reported from the plant, which belong to different chemical classes such as triterpenoids, organic acid, flavonoids, and fatty acids. Numerous biological activities were reported from the plant through in vivo, in vitro, ex vivo, and human studies. However, well-defined clinical studies are still lacking for the establishment of any biological properties that could be further developed. Suggestions for the further development of P. erosus, according to current knowledge about the different biological properties, has also been provided.

The Pentose Phosphate Pathway in Cancer: Regulation and Therapeutic Opportunities

BACKGROUND

Tumorigenesis is associated with deregulation of nutritional requirements, intermediary metabolites production, and microenvironment interactions. Unlike their normal cell counterparts, tumor cells rely on aerobic glycolysis, through the Warburg effect.

SUMMARY

The pentose phosphate pathway (PPP) is a major glucose metabolic shunt that is upregulated in cancer cells. The PPP comprises an oxidative and a nonoxidative phase and is essential for nucleotide synthesis of rapidly dividing cells. The PPP also generates nicotinamide adenine dinucleotide phosphate, which is required for reductive metabolism and to counteract oxidative stress in tumor cells. This article reviews the regulation of the PPP and discusses inhibitors that target its main pathways. Key Message: Exploiting the metabolic vulnerability of the PPP offers potential novel therapeutic opportunities and improves patients' response to cancer therapy.

The Differential Growth Inhibition of Phytophthora spp. Caused by the Rare Sugar Tagatose Is Associated With Species-Specific Metabolic and Transcriptional Changes

Tagatose is a rare sugar with no negative impacts on human health and selective inhibitory effects on plant-associated microorganisms. Tagatose inhibited mycelial growth and negatively affected mitochondrial processes in Phytophthora infestans, but not in Phytophthora cinnamomi. The aim of this study was to elucidate metabolic changes and transcriptional reprogramming activated by P. infestans and P. cinnamomi in response to tagatose, in order to clarify the differential inhibitory mechanisms of tagatose and the species-specific reactions to this rare sugar. Phytophthora infestans and P. cinnamomi activated distinct metabolic and transcriptional changes in response to the rare sugar. Tagatose negatively affected mycelial growth, sugar content and amino acid content in P. infestans with a severe transcriptional reprogramming that included the downregulation of genes involved in transport, sugar metabolism, signal transduction, and growth-related process. Conversely, tagatose incubation upregulated genes related to transport, energy metabolism, sugar metabolism and oxidative stress in P. cinnamomi with no negative effects on mycelial growth, sugar content and amino acid content. Differential inhibitory effects of tagatose on Phytophthora spp. were associated with an attempted reaction of P. infestans, which was not sufficient to attenuate the negative impacts of the rare sugar and with an efficient response of P. cinnamomi with the reprogramming of multiple metabolic processes, such as genes related to glucose transport, pentose metabolism, tricarboxylic acid cycle, reactive oxygen species detoxification, mitochondrial and alternative respiration processes. Knowledge on the differential response of Phytophthora spp. to tagatose represent a step forward in the understanding functional roles of rare sugars.

Rare Sugars: Recent Advances and Their Potential Role in Sustainable Crop Protection

Rare sugars are monosaccharides with a limited availability in the nature and almost unknown biological functions. The use of industrial enzymatic and microbial processes greatly reduced their production costs, making research on these molecules more accessible. Since then, the number of studies on their medical/clinical applications grew and rare sugars emerged as potential candidates to replace conventional sugars in human nutrition thanks to their beneficial health effects. More recently, the potential use of rare sugars in agriculture was also highlighted. However, overviews and critical evaluations on this topic are missing. This review aims to provide the current knowledge about the effects of rare sugars on the organisms of the farming ecosystem, with an emphasis on their mode of action and practical use as an innovative tool for sustainable agriculture. Some rare sugars can impact the plant growth and immune responses by affecting metabolic homeostasis and the hormonal signaling pathways. These properties could be used for the development of new herbicides, plant growth regulators and resistance inducers. Other rare sugars also showed antinutritional properties on some phytopathogens and biocidal activity against some plant pests, highlighting their promising potential for the development of new sustainable pesticides. Their low risk for human health also makes them safe and ecofriendly alternatives to agrochemicals.

A head-to-head comparison review of biological and toxicological studies of isomaltulose, d-tagatose, and trehalose on glycemic control

Diabetes mellitus is the most common metabolic disorder contributing to significant morbidity and mortality in humans. Different preventive and therapeutic agents, as well as various pharmacological strategies or non-pharmacological tools, improve the glycemic profile of diabetic patients. Isomaltulose, d-tagatose, and trehalose are naturally occurring, low glycemic sugars that are not synthesized by humans but widely used in food industries. Various studies have shown that these carbohydrates can regulate glucose metabolism and provide support in maintaining glucose homeostasis in patients with diabetes, but also can improve insulin response, subsequently leading to better control of hyperglycemia. In this review, we discussed the anti-hyperglycemic effects of isomaltulose, D-tagatose, and trehalose, comparing their properties with other known sweeteners, and highlighting their importance for the development of the pharmaceutical and food industries.

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.

A Three-Step Process for the Bioconversion of Whey Permeate into a Glucose-Free D-Tagatose Syrup

We have developed a sustainable three-stage process for the revaluation of cheese whey permeate into D-tagatose, a rare sugar with functional properties used as sweetener. The experimental conditions (pH, temperature, cofactors, etc.) for each step were independently optimized. In the first step, concentrated whey containing 180–200 g/L of lactose was fully hydrolyzed by β-galactosidase from Bifidobacterium bifidum (Saphera®) in 3 h at 45 °C. Secondly, glucose was selectively removed by treatment with Pichia pastoris cells for 3 h at 30 °C. The best results were obtained with 350 mg of cells (previously grown for 16 h) per mL of solution. Finally, L-arabinose isomerase US100 from Bacillus stearothermophilus was employed to isomerize D-galactose into D-tagatose at pH 7.5 and 65 °C, in presence of 0.5 mM MnSO4. After 7 h, the concentration of D-tagatose was approximately 30 g/L (33.3% yield, referred to the initial D-galactose present in whey). The proposed integrated process takes place under mild conditions (neutral pH, moderate temperatures) in a short time (13 h), yielding a glucose-free syrup containing D-tagatose and galactose in a ratio 1:2 (w/w).

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.

Rare mono- and disaccharides as healthy alternative for traditional sugars and sweeteners?

Obesity and type 2 diabetes are major health problems affecting hundreds of millions of people. Caloric overfeeding with calorie-dense food ingredients like sugars may contribute to these chronic diseases. Sugar research has also identified mechanisms via which conventional sugars like sucrose and fructose can adversely influence metabolic health. To replace these sugars, numerous sugar replacers including artificial sweeteners and sugar alcohols have been developed. Rare sugars became new candidates to replace conventional sugars and their health effects are already reported in individual studies, but overviews and critical appraisals of their health effects are missing. This is the first paper to provide a detailed review of the metabolic health effects of rare sugars as a group. Especially allulose has a wide range of health effects. Tagatose and isomaltulose have several health effects as well, while other rare sugars mainly provide health benefits in mechanistic studies. Hardly any health claims have been approved for rare sugars due to a lack of evidence from human trials. Human trials with direct measures for disease risk factors are needed to allow a final appraisal of promising rare sugars. Mechanistic cell culture studies and animal models are required to enlarge our knowledge on understudied rare sugars.

From the Laboratory to the Kitchen: New Alternatives to Healthier Bakery Products

Due to the growing interest in improving the nutritional profile of bakery products, we have dealt with the most recent and relevant contributions regarding potential replacements for carbohydrates, proteins, and fats. Focusing on the influence of carbohydrates on metabolism, their excess implies obesity, diabetes and tooth decay. However, they are technologically important, since they are responsible for the structure of many bakery products. Regarding of the lipid profile, saturated fats have a great impact on the appearance of cardiovascular disease. Fortunately, nature and the food industry offer alternatives to traditional oils/butters with large amounts of omega 3 and other components that can mitigate these problems. Other relevant aspects are related to allergies concerning egg proteins, gluten or even requirements for vegan consumers. Several studies have been performed in this line, replacing eggs with milk serum, different mucilages obtained from legumes or some gums, etc. In conclusion, many papers have been published showing the possibility of successfully replacing (both at technological and sensory levels) less healthy ingredients with others that are nutritionally better. The challenge now is to combine these better components in a given product, as well as to evaluate possible interactions among them.

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.

Acute Effects of Nutritive and Non-Nutritive Sweeteners on Postprandial Blood Pressure

Postprandial hypotension (PPH) is under-recognised, but common, particularly in the elderly, and is of clear clinical importance due to both the independent association between PPH and an increase in mortality and lack of effective management for this condition. Following health concerns surrounding excessive consumption of sugar, there has been a trend in the use of low- or non-nutritive sweeteners as an alternative. Due to the lack of literature in this area, we conducted a systematic search to identify studies relevant to the effects of different types of sweeteners on postprandial blood pressure (BP). The BP response to ingestion of sweeteners is generally unaffected in healthy young subjects, however in elderly subjects, glucose induces the greatest decrease in postprandial BP, while the response to sucrose is less pronounced. The limited studies investigating other nutritive and non-nutritive sweeteners have demonstrated minimal or no effect on postprandial BP. Dietary modification by replacing high nutritive sweeteners (glucose, fructose, and sucrose) with low nutritive (d-xylose, xylitol, erythritol, maltose, maltodextrin, and tagatose) and non-nutritive sweeteners may be a simple and effective management strategy for PPH.

Overcoming the thermodynamic equilibrium of an isomerization reaction through oxidoreductive reactions for biotransformation

Isomerases perform biotransformations without cofactors but often cause an undesirable mixture of substrate and product due to unfavorable thermodynamic equilibria. We demonstrate the feasibility of using an engineered yeast strain harboring oxidoreductase reactions to overcome the thermodynamic limit of an isomerization reaction. Specifically, a yeast strain capable of consuming lactose intracellularly is engineered to produce tagatose from lactose through three layers of manipulations. First, GAL1 coding for galactose kinase is deleted to eliminate galactose utilization. Second, heterologous xylose reductase (XR) and galactitol dehydrogenase (GDH) are introduced into the ∆gal1 strain. Third, the expression levels of XR and GDH are adjusted to maximize tagatose production. The resulting engineered yeast produces 37.69 g/L of tagatose from lactose with a tagatose and galactose ratio of 9:1 in the reaction broth. These results suggest that in vivo oxidoreaductase reactions can be employed to replace isomerases in vitro for biotransformation.

A desired product cannot be obtained at higher concentration than its equilibrium concentration when isomerases are used for biotransformation. Here, the authors engineer in vivo oxidoreductive reactions in yeast to overcome the equilibrium limitation of in vitro isomerases-based tagatose production.

Overcoming NADPH product inhibition improves D-sorbitol conversion to L-sorbose

Gluconobacter oxydans sorbitol dehydrogenase (GoSLDH) exhibits a higher catalytic efficiency than other l-sorbose producing enzymes. During the reaction catalysed by GoSLDH, NADP⁺ is reduced to NADPH and d-sorbitol is oxidized to l-sorbose. However, GoSLDH activity is inhibited by the NADPH (K_i = 100 μM) formed during the enzymatic reaction. Therefore, Escherichia coli_{gosldh-lrenox} producing both GoSLDH for d-sorbitol oxidation and LreNOX (NAD(P)H oxidase from Lactobacillus reuteri) for NADP⁺ regeneration was generated and used for l-sorbose production. Whole cell biocatalysts with the LreNOX cofactor recycling system showed a high conversion rate (92%) of d-sorbitol to l-sorbose in the presence of low concentration of NADP⁺ (0.5 mM). By alleviating NADPH accumulation during the catalytic reactions, E. coli_{gosldh-lrenox} exhibited 23-fold higher conversion rate of d-sorbitol than E. coli_gosldh. l-Sorbose production by E. coli_{gosldh-lrenox} reached 4.1 g/L after 40 min, which was 20.5-fold higher than that of E. coli_gosldh. We also constructed G. oxydans_gosldh and G. oxydans_{gosldh-lrenox} strains, and they exhibited 1.2- and 2.9-fold higher conversion rates than the wild-type G. oxydans KCTC 1091. The results indicate that overcoming NADPH product inhibition using LreNOX improves chemical production in NADP⁺-dependent enzymatic reactions.

Elegant and Efficient Biotransformation for Dual Production of d-Tagatose and Bioethanol from Cheese Whey Powder

In this study, the dual production of valuable d-tagatose and bioethanol from lactose and cheese whey powder is presented. First, a one-pot biosynthesis involving lactose hydrolysis and d-galactose isomerization for d-tagatose production was established using crude enzymes of recombinant Escherichia coli with l-arabinose isomerase (L-AI) at 50 °C. Compared to the current enzymatic system, only L-AI was overexpressed, because of the unexpectedly thermotolerant β-galactosidase in E. coli BL21(DE3). Moreover, this high temperature rendered the d-glucose catabolism of E. coli inactive, while retaining all fermentable sugars for bioethanol fermentation. Thereafter, the mixed sugar syrup was fermented by Saccharomyces cerevisiae NL22. A total of 23.5 g/L d-tagatose and 26.9 g/L bioethanol was achieved from cheese whey powder containing 100 g/L lactose. This bioprocess not only provides an efficient method for the functionalization of byproduct whey, but also offsets the high production cost of d-tagatose and bioethanol.

The Ability of Different Ketohexoses to Alter Apo-A-I Structure and Function In Vitro and to Induce Hepatosteatosis, Oxidative Stress, and Impaired Plasma Lipid Profile in Hyperlipidemic Zebrafish

In the current study, we have tested the nonenzymatic glycation activities of ketohexoses, such as tagatose and psicose. Although tagatose-treated apoA-I (t-A-I) and psicose-treated apoA-I (p-A-I) exerted more inhibitory activity you cupric ion-mediated low-density lipoprotein (LDL) oxidation and oxidized LDL (oxLDL) phagocytosis into macrophage than fructose-treated apoA-I (f-A-I). In the lipid-free state, t-A-I and f-A-I showed more multimerized band without crosslinking. Since t-A-I lost its phospholipid binding ability, the rHDL formation was not as successful as f-A-I. However, injecting t-A-I showed more antioxidant activities in zebrafish embryo under the presence of oxLDL. Three weeks of consumption of fructose (50% of wt in Tetrabit/4% cholesterol) showed a 14% elevation of serum triacylglycerol (TG), while tagatose-administered group showed 30% reduction in serum TG compared to high cholesterol control. Fructose-fed group showed the biggest area of Oil Red O staining with the intensity as strong as the HCD control. However, tagatose-consumed group showed much lesser Oil Red O-stained area with the reduction of lipid accumulation. In conclusion, although tagatose treatment caused modification of apoA-I, the functional loss was not as much severe as the fructose treatment in macrophage cell model, zebrafish embryo, and hypercholesterolemic zebrafish model.

Biochemical properties of L-arabinose isomerase from Clostridium hylemonae to produce D-tagatose as a functional sweetener

d-Tagatose has gained substantial interest due to its potential functionalities as a sucrose substitute. In this study, the gene araA, encoding l-arabinose isomerase (l-AI) from Clostridium hylemonae (DSM 15053), was cloned and expressed in Escherichia coli BL21 (DE3). This gene consists of 1,506 nucleotides and encodes a protein of 501 amino acid residues with a calculated molecular mass of 56,554 Da. Since l-AI was expressed as an intracellular inclusion body, this enzyme was solubilized with guanidine hydrochloride, refolded, and activated with a descending concentration gradient of urea. The purified enzyme exhibited the greatest activity at 50°C, pH 7-7.5, and required 1 mM of Mg2+ as a cofactor. Notably, the catalytic efficiency (3.69 mM-1sec-1) of l-AI from C. hylemonae on galactose was significantly greater than that of other previously reported enzymes. The bioconversion yield of d-tagatose using the C. hylemonae l-arabinose isomerase at 60°C reached approximately 46% from 10 mM of d-galactose after 2 h. From these results, it is suggested that the l-arabinose isomerase from C. hylemonae could be utilized as a potential enzyme for d-tagatose production due to its high conversion yield at an industrially competitive temperature.

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.

Influence of Low Glycaemic Index Sweeteners on Antioxidant, Sensory, Mechanical, and Physicochemical Properties of a Watermelon Jelly

The replacement of sucrose by new noncariogenic and low glycaemic index sweeteners (isomaltulose and tagatose) and the addition of natural watermelon juice in jelly have been assessed in terms of composition, texture, colour, antioxidant activity, microbiology, and sensory properties. These analyses were performed initially and after 15 days of storage. Furthermore, the values were compared with those obtained in the analyses of a commercial watermelon jelly. The results showed that the antioxidant activity increased with the storage time in the control sample and in samples combining isomaltulose and tagatose. In addition, noncariogenic and low glycaemic index sweeteners did not affect the instrumental texture. However, the colour changed, especially in the sample containing tagatose only. Finally, the dessert containing tagatose and isomaltulose in equal proportion achieved a similar score in the sensory evaluation as the commercial one, showing the feasibility of using these sweeteners to reformulate watermelon jelly.

Biochemical analysis and the preliminary crystallographic characterization of D-tagatose 3-epimerase from Rhodobacter sphaeroides

Background

d-Tagatose 3-epimerase epimerizes d-fructose to yield d-psicose, which is a rare sugar that exists in small quantities in nature and is difficult to synthesize chemically. We aim to explore potential industrial biocatalysts for commercial-scale manufacture of this rare sugar. A d-tagatose 3-epimerase from Rhodobacter sphaeroides (RsDTE) has recently been identified as a d-tagatose 3-epimerase that can epimerize d-fructose to yield d-psicose with a high conversion rate.

Results

The purified RsDTE by Ni-affinity chromatography, ionic exchange chromatography and gel filtration forms a tetramer in solution. The maximal activity was in Tris–HCl buffer pH 8.5, and the optimal temperature was at 35 °C. The product, d-psicose, was confirmed using HPLC and NMR. Crystals of RsDTE were obtained using crystal kits and further refined under crystallization conditions such as 10% PEG 8000,0.1 M HEPES pH 7.5, and 8% ethylene glycol at 20 °C using the sitting-drop vapor diffusion method. The RsDTE homology model showed that it possessed the characteristic TIM-barrel fold. Four residues, Glu156, Asp189, Gln215 and Glu250, forms a hydrogen bond network with the active Mn(II) for the hydride transfer reaction. These residues may constitute the catalytic tetrad of RsDTE. The residues around O1, O2 and O3 of the substrates were conserved. However, the binding-site residues are different at O4, O5 and O6. Arg118 formed the unique hydrogen bond with O4 of d-fructose which indicates RsDTE’s preference of d-fructose more than any other family enzymes.

Conclusions

RsDTE possesses a different metal-binding site. Arg118, forming unique hydrogen bond with O4 of d-fructose, regulates the substrate recognition. The research on d-tagatose 3-epimerase or d-psicose 3-epimerase enzymes attracts enormous commercial interest and would be widely used for rare sugar production in the future.

Electronic supplementary material

The online version of this article (10.1186/s12934-017-0808-4) contains supplementary material, which is available to authorized users.

Biocatalytic Synthesis of the Rare Sugar Kojibiose: Process Scale-Up and Application Testing

Cost-efficient (bio)chemical production processes are essential to evaluate the commercial and industrial applications of promising carbohydrates and also are essential to ensure economically viable production processes. Here, the synthesis of the naturally occurring disaccharide kojibiose (2-O-α-d-glucopyranosyl-d-glucopyranoside) was evaluated using different Bifidobacterium adolescentis sucrose phosphorylase variants. Variant L341I_Q345S was found to efficiently synthesize kojibiose while remaining fully active after 1 week of incubation at 55 °C. Process optimization allowed kojibiose production at the kilogram scale, and simple but efficient downstream processing, using a yeast treatment and crystallization, resulted in more than 3 kg of highly pure crystalline kojibiose (99.8%). These amounts allowed a deeper characterization of its potential in food applications. It was found to have possible beneficial health effects, including delayed glucose release and potential to trigger SCFA production. Finally, we compared the bulk functionality of highly pure kojibiose to that of sucrose, hereby mapping its potential as a new sweetener in confectionery products.

TM0416, a Hyperthermophilic Promiscuous Nonphosphorylated Sugar Isomerase, Catalyzes Various C5 and C6 Epimerization Reactions

There is currently little information on nonphosphorylated sugar epimerases, which are of potential interest for producing rare sugars. We found a gene (the TM0416 gene) encoding a putative d-tagatose-3-epimerase-related protein from the hyperthermophilic bacterium Thermotoga maritima We overexpressed the TM0416 gene in Escherichia coli and purified the resulting recombinant protein for detailed characterization. Amino acid sequence alignment and a structural similarity search revealed that TM0416 is a putative nonphosphorylated sugar epimerase. The recombinant enzyme exhibited maximal C-3 epimerization of l-ribulose to l-xylulose at ∼80°C and pH 7 in the presence of 1 mM Mn²⁺ In addition, this enzyme showed unusually high activity for the epimerization of d-tagatose to d-sorbose, with a conversion yield of 20% after 6 h at 80°C. Remarkably, the enzyme catalyzed the isomerization of d-erythrose or d-threose to d-erythrulose significantly, with conversion yields of 71% and 54.5%, respectively, after 6 h at 80°C at pH 7. To further investigate the substrate specificity of TM0416, we determined its crystal structures in complex with divalent metal ions and l-erythrulose at resolutions of 1.5 and 1.6 Å. Detailed inspection of the structural features and biochemical data clearly demonstrated that this metalloenzyme, with a freely accessible substrate-binding site and neighboring hydrophobic residues, exhibits different and promiscuous substrate preferences, compared with its mesophilic counterparts. Therefore, this study suggests that TM0416 can be functionally classified as a novel type of l-ribulose 3-epimerase (R3E) with d-erythrose isomerase activity.IMPORTANCE Rare sugars, which occur naturally in small amounts, have attracted considerable attention in the food and drug industries. However, there is little information on nonphosphorylated sugar epimerases, which might potentially be applied for the production of rare sugars. This study describes the characterization and functional annotation of a putative nonphosphorylated sugar 3-epimerase from a hyperthermophilic bacterium. Furthermore, we determined its crystal structures in complex with divalent metal ions and l-erythrulose, highlighting its metal-dependent, bifunctional, sugar-isomerizing activity. This hyperthermophilic R3E exhibited d-erythrose/d-threose isomerase activity, with structural features near the substrate-binding site distinct from those of its mesophilic counterparts. Moreover, this metalloenzyme showed unusually high activity for the epimerization of d-tagatose to d-sorbose at 70°C. Therefore, TM0416 can be functionally classified as a novel type of promiscuous R3E with a potential for the production of rare sugars for the food and pharmaceutical industries.

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.

Structural characterization of the thermostable Bradyrhizobium japonicumD-sorbitol dehydrogenase

Bradyrhizobium japonicum sorbitol dehydrogenase is NADH-dependent and is active at elevated temperatures. The best substrate is D-glucitol (a synonym for D-sorbitol), although L-glucitol is also accepted, giving it particular potential in industrial applications. Crystallization led to a hexagonal crystal form, with crystals diffracting to 2.9 Å resolution. In attempts to phase the data, a molecular-replacement solution based upon PDB entry 4nbu (33% identical in sequence to the target) was found. The solution contained one molecule in the asymmetric unit, but a tetramer similar to that found in other short-chain dehydrogenases, including the search model, could be reconstructed by applying crystallographic symmetry operations. The active site contains electron density consistent with D-glucitol and phosphate, but there was not clear evidence for the binding of NADH. In a search for the features that determine the thermostability of the enzyme, the Tm for the orthologue from Rhodobacter sphaeroides, for which the structure was already known, was also determined, and this enzyme proved to be considerably less thermostable. A continuous β-sheet is formed between two monomers in the tetramer of the B. japonicum enzyme, a feature not generally shared by short-chain dehydrogenases, and which may contribute to thermostability, as may an increased Pro/Gly ratio.

Two-step enzymatic synthesis of 6-deoxy-L-psicose

Rare sugars offer a plethora of applications in the pharmaceutical, medicinal, and industries, as well as in synthetic chemistry. However, studies of rare sugars have been hampered by their relative scarcity. In this work, we describe a two-step strategy to efficiently and conveniently prepare 6-deoxy-L-psicose from L-rhamnose. In the first reaction step, the isomerization of L-rhamnose (6-deoxy-L-mannose) to L-rhamnulose (6-deoxy-L-fructose) catalyzed by L-rhamnose isomerase (RhaI), and the epimerization of L-rhamnulose to 6-deoxy-L-psicose catalyzed by D-tagatose 3-epimerase (DTE) were coupled with selective phosphorylation reaction by fructose kinase from human (HK), which selectively phosphorylate 6-deoxy-L-psicose at C-1 position. 6-deoxy-L-psicose 1-phosphate was purified by a silver nitrate precipitation method. In the second step, the phosphate group of the 6-deoxy-L-sorbose 1-phosphate was hydrolyzed with acid phosphatase (AphA) to produce 6-deoxy-L-psicose in 81% yield with respect to L-rhamnose. This method allows that the 6-deoxy-L-psicose to be obtained from readily available starting materials with high purity and without having to undergo isomer separation.

The secreted L-arabinose isomerase displays anti-hyperglycemic effects in mice

Background

The l-arabinose isomerase is an intracellular enzyme which converts l-arabinose into l-ribulose in living systems and d-galactose into d-tagatose in industrial processes and at industrial scales. d-tagatose is a natural ketohexose with potential uses in pharmaceutical and food industries. The d-galactose isomerization reaction is thermodynamically equilibrated, and leads to secondary subproducts at high pH. Therefore, an attractive l-arabinose isomerase should be thermoactive and acidotolerant with high catalytic efficiency. While many reports focused on the set out of a low cost process for the industrial production of d-tagatose, these procedures remain costly. When compared to intracellular enzymes, the production of extracellular ones constitutes an interesting strategy to increase the suitability of the biocatalysts.

Results

The l-arabinose isomerase (l-AI) from Lactobacillus sakei was expressed in Lactococcus lactis in fusion with the signal peptide of usp45 (SP_Usp45). The l-AI protein and activity were detected only in the supernatant of the induced cultures of the recombinant L. lactis demonstrating the secretion in the medium of the intracellular L. sakeil-AI in an active form. Moreover, we showed an improvement in the enzyme secretion using either (1) L. lactis strains deficient for their two major proteases, ClpP and HtrA, or (2) an enhancer of protein secretion in L. lactis fused to the recombinant l-AI with the SP_Usp45. Th l-AI enzyme secreted by the recombinant L. lactis strains or produced intracellularly in E. coli, showed the same functional properties than the native enzyme. Furthermore, when mice are fed with the L. lactis strain secreting the l-AI and galactose, tagatose was produced in vivo and reduced the glycemia index.

Conclusions

We report for the first time the secretion of the intracellular l-arabinose isomerase in the supernatant of food grade L. lactis cultures with hardly display other secreted proteins. The secreted l-AI originated from the food grade L. sakei 23 K was active and showed the same catalytic and structural properties as the intracellular enzyme. The L. lactis strains secreting the l-arabinose isomerase has the ability to produce d-tagatose in vivo and conferred an anti-hyperglycemic effect to mice.

The metabolome profiling and pathway analysis in metabolic healthy and abnormal obesity

OBJECTIVES

Mechanisms of the development of abnormal metabolic phenotypes among obese population are not yet clear. In this study, we aimed to screen metabolomes of both healthy and subjects with abnormal obesity to identify potential metabolic pathways that may regulate the different metabolic characteristics of obesity.

METHODS

We recruited subjects with body mass index (BMI) over 25 from the weight-loss clinic of a central hospital in Taiwan. Metabolic healthy obesity (MHO) is defined as without having any form of hyperglycemia, hypertension and dyslipidemia, while metabolic abnormal obesity (MAO) is defined as having one or more abnormal metabolic indexes. Serum-based metabolomic profiling using both liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry of 34 MHO and MAO individuals with matching age, sex and BMI was performed. Conditional logistic regression and partial least squares discriminant analysis were applied to identify significant metabolites between the two groups. Pathway enrichment and topology analyses were conducted to evaluate the regulated pathways.

RESULTS

A differential metabolite panel was identified to be significantly differed in MHO and MAO groups, including L-kynurenine, glycerophosphocholine (GPC), glycerol 1-phosphate, glycolic acid, tagatose, methyl palmitate and uric acid. Moreover, several metabolic pathways were relevant in distinguishing MHO from MAO groups, including fatty acid biosynthesis, phenylalanine metabolism, propanoate metabolism, and valine, leucine and isoleucine degradation.

CONCLUSION

Different metabolomic profiles and metabolic pathways are important for distinguishing between MHO and MAO groups. We have identified and discussed the key metabolites and pathways that may prove important in the regulation of metabolic traits among the obese, which could provide useful clues to study the underlying mechanisms of the development of abnormal metabolic phenotypes.

Sn-Beta zeolites with borate salts catalyse the epimerization of carbohydrates via an intramolecular carbon shift

Carbohydrate epimerization is an essential technology for the widespread production of rare sugars. In contrast to other enzymes, most epimerases are only active on sugars substituted with phosphate or nucleotide groups, thus drastically restricting their use. Here we show that Sn-Beta zeolite in the presence of sodium tetraborate catalyses the selective epimerization of aldoses in aqueous media. Specifically, a 5 wt% aldose (for example, glucose, xylose or arabinose) solution with a 4:1 aldose:sodium tetraborate molar ratio reacted with catalytic amounts of Sn-Beta yields near-equilibrium epimerization product distributions. The reaction proceeds by way of a 1,2 carbon shift wherein the bond between C-2 and C-3 is cleaved and a new bond between C-1 and C-3 is formed, with C-1 moving to the C-2 position with an inverted configuration. This work provides a general method of performing carbohydrate epimerizations that surmounts the main disadvantages of current enzymatic and inorganic processes.

Epimerization of carbohydrates to rare sugars yields products that have potential applications as anti-viral drugs or chiral building blocks. Here, Sn-Beta zeolite in the presence of sodium tetraborate is shown to catalyze the selective epimerization of aldoses in aqueous media.

The Effect of D-Tagatose on Fructose Absorption in a Rat Model

D-tagatose is in development as a medication for the treatment of type 2 diabetes. The effect of oral D-tagatose on the absorption of D-fructose was assessed when co-administered in this study. In the pilot study, male Sprague-Dawley rats were fed C14 labeled fructose and glucose concomitantly to establish dose levels for the treatment group of rats fed C14 labeled fructose together with D-tagatose. Rats were administered 0, 600, 2000, 6000, or 12000 mg/kg of D-tagatose along with 2000 mg/kg of fructose. Blood samples were taken over 60 minutes and were assessed using scintillation counting. 600, 2000, and 6000 mg/kg of D-tagatose decreased fructose absorption by 1%, 26%, and 30% respectively (12000 mg/kg group was stopped short of completion due to intolerance) as measured by AUC of scintillation counts. The 600 and 2000 mg/kg of D-tagatose groups showed no difference in plasma glucose concentrations compared to placebo while a rise in glucose was seen in the 6000 mg/kg of D-tagatose groups. The results indicate that D-tagatose may be useful in reducing fructose absorption, which could lead to a beneficial outcome.

A method for the production of D-tagatose using a recombinant Pichia pastoris strain secreting β-D-galactosidase from Arthrobacter chlorophenolicus and a recombinant L-arabinose isomerase from Arthrobacter sp. 22c

Background

D-Tagatose is a natural monosaccharide which can be used as a low-calorie sugar substitute in food, beverages and pharmaceutical products. It is also currently being tested as an anti-diabetic and obesity control drug. D-Tagatose is a rare sugar, but it can be manufactured by the chemical or enzymatic isomerization of D-galactose obtained by a β-D-galactosidase-catalyzed hydrolysis of milk sugar lactose and the separation of D-glucose and D-galactose. L-Arabinose isomerases catalyze in vitro the conversion of D-galactose to D-tagatose and are the most promising enzymes for the large-scale production of D-tagatose.

Results

In this study, the araA gene from psychrotolerant Antarctic bacterium Arthrobacter sp. 22c was isolated, cloned and expressed in Escherichia coli. The active form of recombinant Arthrobacter sp. 22c L-arabinose isomerase consists of six subunits with a combined molecular weight of approximately 335 kDa. The maximum activity of this enzyme towards D-galactose was determined as occurring at 52°C; however, it exhibited over 60% of maximum activity at 30°C. The recombinant Arthrobacter sp. 22c L-arabinose isomerase was optimally active at a broad pH range of 5 to 9. This enzyme is not dependent on divalent metal ions, since it was only marginally activated by Mg²⁺, Mn²⁺ or Ca²⁺ and slightly inhibited by Co²⁺ or Ni²⁺. The bioconversion yield of D-galactose to D-tagatose by the purified L-arabinose isomerase reached 30% after 36 h at 50°C. In this study, a recombinant Pichia pastoris yeast strain secreting β-D-galactosidase Arthrobacter chlorophenolicus was also constructed. During cultivation of this strain in a whey permeate, lactose was hydrolyzed and D-glucose was metabolized, whereas D-galactose was accumulated in the medium. Moreover, cultivation of the P. pastoris strain secreting β-D-galactosidase in a whey permeate supplemented with Arthrobacter sp. 22c L-arabinose isomerase resulted in a 90% yield of lactose hydrolysis, the complete utilization of D-glucose and a 30% conversion of D-galactose to D-tagatose.

Conclusions

The method developed for the simultaneous hydrolysis of lactose, utilization of D-glucose and isomerization of D-galactose using a P. pastoris strain secreting β-D-galactosidase and recombinant L-arabinose isomerase seems to offer an interesting alternative for the production of D-tagatose from lactose-containing feedstock.

The acid-tolerant L-arabinose isomerase from the mesophilic Shewanella sp. ANA-3 is highly active at low temperatures

Background

L-arabinose isomerases catalyse the isomerization of L-arabinose into L-ribulose at insight biological systems. At industrial scale of this enzyme is used for the bioconversion of D-galactose into D-tagatose which has many applications in pharmaceutical and agro-food industries. The isomerization reaction is thermodynamically equilibrated, and therefore the bioconversion rates is shifted towards tagatose when the temperature is increased. Moreover, to prevent secondary reactions it will be of interest to operate at low pH. The profitability of this D-tagatose production process is mainly related to the use of lactose as cheaper raw material. In many dairy products it will be interesting to produce D-tagatose during storage. This requires an efficient L-arabinose isomerase acting at low temperature and pH values.

Results

The gene encoding the L-arabinose isomerase from Shewanella sp. ANA-3 was cloned and overexpressed in Escherichia coli. The purified protein has a tetrameric arrangement composed by four identical 55 kDa subunits. The biochemical characterization of this enzyme showed that it was distinguishable by its maximal activity at low temperatures comprised between 15-35°C. Interestingly, this biocatalyst preserves more than 85% of its activity in a broad range of temperatures from 4.0 to 45°C. Shewanella sp. ANA-3 L-arabinose isomerase was also optimally active at pH 5.5-6.5 and maintained over 80% of its activity at large pH values from 4.0 to 8.5. Furthermore, this enzyme exhibited a weak requirement for metallic ions for its activity evaluated at 0.6 mM Mn²⁺. Stability studies showed that this protein is highly stable mainly at low temperature and pH values. Remarkably, T268K mutation clearly enhances the enzyme stability at low pH values. Use of this L-arabinose isomerase for D-tagatose production allows the achievement of attractive bioconversion rates of 16% at 4°C and 34% at 35°C.

Conclusions

Here we reported the purification and the biochemical characterization of the novel Shewanella sp. ANA-3 L-arabinose isomerase. Determination of the biochemical properties demonstrated that this enzyme was highly active at low temperatures. The generated T268K mutant displays an increase of the enzyme stability essentially at low pH. These features seem to be very attractive for the bioconversion of D-galactose into D-tagatose at low temperature which is very interesting from industrial point of view.

Identification and characterization of a novel L-arabinose isomerase from Anoxybacillus flavithermus useful in D-tagatose production

D-Tagatose is a highly functional rare ketohexose and many attempts have been made to convert D-galactose into the valuable D-tagatose using L-arabinose isomerase (L-AI). In this study, a thermophilic strain possessing L-AI gene was isolated from hot spring sludge and identified as Anoxybacillus flavithermus based on its physio-biochemical characterization and phylogenetic analysis of its 16s rRNA gene. Furthermore, the gene encoding L-AI from A. flavithermus (AFAI) was cloned and expressed at a high level in E. coli BL21(DE3). L-AI had a molecular weight of 55,876 Da, an optimum pH of 10.5 and temperature of 95°C. The results showed that the conversion equilibrium shifted to more D-tagatose from D-galactose by raising the reaction temperatures and adding borate. A 60% conversion of D-galactose to D-tagatose was observed at an isomerization temperature of 95°C with borate. The catalytic efficiency (k (cat) /K (m)) for D-galactose with borate was 9.47 mM(-1) min(-1), twice as much as that without borate. Our results indicate that AFAI is a novel hyperthermophilic and alkaliphilic isomerase with a higher catalytic efficiency for D-galactose, suggesting its great potential for producing D-tagatose.

Effect of diets containing sucrose vs. D-tagatose in hypercholesterolemic mice

Effects of functional sweeteners on the development of the metabolic syndrome and atherosclerosis are unknown. The objective was to compare the effect of dietary carbohydrate in the form of sucrose (SUCR) to D-tagatose (TAG; an isomer of fructose currently used as a low-calorie sweetener) on body weight, blood cholesterol concentrations, hyperglycemia, and atherosclerosis in low-density lipoprotein receptor deficient (LDLr(-/-)) mice. LDLr(-/-) male and female mice were fed either standard murine diet or a diet enriched with TAG or SUCR as carbohydrate sources for 16 weeks. TAG and SUCR diets contained equivalent amounts (g/kg) of protein, fat, and carbohydrate. We measured food intake, body weight, adipocyte diameter, serum cholesterol and lipoprotein concentrations, and aortic atherosclerosis. Macrophage immunostaining and collagen content were examined in aortic root lesions. CONTROL and TAG-fed mice exhibited similar energy intake, body weights and blood glucose and insulin concentrations, but SUCR-fed mice exhibited increased energy intake and became obese and hyperglycemic. Adipocyte diameter increased in female SUCR-fed mice compared to TAG and CONTROL. Male and female SUCR-fed mice had increased serum cholesterol and triglyceride concentrations compared to TAG and CONTROL. Atherosclerosis was increased in SUCR-fed mice of both genders compared to TAG and CONTROL. Lesions from SUCR-fed mice exhibited pronounced macrophage immunostaining and reductions in collagen content compared to TAG and CONTROL mice. These results demonstrate that in comparison to sucrose, equivalent substitution of TAG as dietary carbohydrate does not result in the same extent of obesity, hyperglycemia, hyperlipidemia, and atherosclerosis.