Transforming monosaccharides: Recent advances in rare sugar production and future exploration

Rare sugars are defined as monosaccharides and their derivatives that do not exist in nature at all or that exist in extremely limited amounts despite being theoretically possible. At present, no comprehensive dogma has been presented regarding how and why these rare sugars have deviated from the naturally selected monosaccharides. In this minireview, we adopt a hypothesis on the origin and evolution of elementary hexoses, previously presented by one of the authors (Hirabayashi, Q Rev Biol, 1996, 71:365-380). In this scenario, monosaccharides, which constitute various kinds of glycans in nature, are assumed to have been generated by formose reactions on the prebiotic Earth (chemical evolution era). Among them, the most stable hexoses, i.e., fructose, glucose, and mannose remained accumulated. After the birth of life, the "chemical origin" saccharides thus survived were transformed into a variety of "bricolage products", which include galactose and other recognition saccharides like fucose and sialic acid through the invention of diverse metabolic pathways (biological evolution era). The remaining monosaccharides that have deviated from this scenario are considered rare sugars. If we can produce rare sugars as we wish, it is expected that various more useful biomaterials will be created by using them as raw materials. Thanks to the pioneering research of the Izumori group in the 1990's, and to a few other investigations by other groups, almost all monosaccharides including l-sugars can now be produced by combining both chemical and enzymatic approaches. After briefly giving an overview of the origin of elementary hexoses and the current state of the rare sugar production, we will look ahead to the next generation of monosaccharide research which also targets glycosides including disaccharides.

Artificial Cascade Transformation Biosystem for One-Pot Biomanufacturing of Odd-Numbered Neoagarooligosaccharides and d-Tagatose through Wiser Agarose Utilization

The application of agarose oligosaccharides has garnered great attention, with their biological activities varying among different structures. However, it still meets a great bottleneck for the targeted production of odd-numbered neoagarooligosaccharides (NAOSs), such as neoagarotriose (NA3), due to the lack of one-step hydrolases. In this work, the α-agarase AgaA33 and β-galactosidase BgaD were synergistically used to prepare NA3 with agarose as a substrate. Additionally, an l-arabinose isomerase CaLAI from Clostridium acetobutylicum was characterized to valorize low-value byproducts (d-galactose) by forming d-tagatose, which exhibited good thermal stability without the need for additional metal ions. Under the optimal reaction conditions, the production of NA3 and d-galactose catalyzed by these three enzymes was 0.40 and 0.15 g/L, respectively. The artificial three-enzyme-based cascade transformation system not only achieved the highest production of NA3 until now but also allowed for the valorization of d-galactose, providing a wiser application route for agarose utilization.

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.

Interplay between dental caries pathogens, periodontal pathogens, and sugar molecules: approaches for prevention and treatment

Globally, oral diseases affect nearly 3.5 billion people, accounting for 4.6% of the healthcare expenditure. Common oral diseases include dental caries and periodontal disease, associated with biofilms formed by cariogenic pathogens. Epidemiological studies associate carbohydrates with these diseases due to  the sugars metabolized by cariogenic pathogens. This review focuses on dental caries and periodontal pathogens, quorum sensing, lectin-carbohydrate interactions, and various sugar molecules. Cariogenic sugars significantly influence biofilms by enhancing pathogen adhesion, viability, and gene expressions associated with biofilm formation. Moreover, lectin-carbohydrate interactions contribute to biofilm stability. Disrupting these interactions is a potential strategy for oral disease prevention. The use of nanoparticles, such as quantum dots, provides novel insights into lectin-sugar interactions and the development of inhibitors. Additionally, nanomaterials like calcium phosphate nanoparticles neutralize acids and inhibit microbial growth. This overview emphasizes understanding the relationships between oral diseases, microbial communities, and sugars to devise preventive and therapeutic strategies against oral diseases.

Rational design of Shewanella sp. l-arabinose isomerase for d-galactose isomerase activity under mesophilic conditions

d-Tagatose, a potential low calorific substitute for sucrose, can be produced by bioconversion of d-galactose catalysed by l-arabinose isomerase. l-Arabinose isomerase from Shewanella sp. ANA-3 is unique for its ability to catalyse bioconversion reactions under mesophilic conditions. However, d-galactose not being a natural substrate for l-arabinose isomerase is catalysed at a slower rate. We attempted to increase the biocatalytic efficiency of Shewanella sp. l-arabinose isomerase by rational design to enhance galactose isomerisation activity. In silico molecular docking, analysis has revealed that F279 is sterically hindering the binding of d-galactose at the C6 position. Substitution of bulky Phe residue with smaller hydrophilic residues such as Asn and Thr increased the galactose isomerase activity by 86 % and 12 % respectively. At mesophilic conditions, F279N mutant catalysed the bioconversion of d-galactose more efficiently than l-arabinose, indicating a shift in substrate preference.

Optimization of fermentation conditions for production of l-arabinose isomerase of Lactobacillus plantarum WU14

As a substitute sweetener for sucrose, d-tagatose is widely used in products, such as health drinks, yogurt, fruit juices, baked goods, confectionery, and pharmaceutical preparations. In the fermentation process of l-AI produced by Lactobacillus plantarum, d-tagatose is produced through biotransformation and this study was based on the fermentation process of Lactobacillus plantarum WU14 producing l-AI to further research the biotransformation and separation process of d-tagatose. The kinetics of cell growth, substrate consumption, and l-arabinose isomerase formation were established by nonlinear fitting, and the fitting degrees were 0.996, 0.994, and 0.991, respectively, which could better reflect the change rule of d-tagatose biotransformation in the fermentation process of L. plantarum WU14. The separation process of d-tagatose was identified by decolorization, protein removal, desalination, and freeze drying, initially. Finally, the volume ratio of whole cell catalysts, d-galactose, and borate was 5:1:2 at 60°C, pH 7.17 through borate complexation; then, after 24 hr of conversion, the yield of d-tagatose was 58 g/L.

Engineering of Alicyclobacillus hesperidum L-arabinose isomerase for improved catalytic activity and reduced pH optimum using random and site-directed mutagenesis

A mutation, D478N, was obtained by an error-prone polymerase chain reaction using the L-arabinose isomerase (L-AI) gene from Alicyclobacillus hesperidum URH17-3-68 as the template. The mutated isomerase showed higher activity for D-galactose isomerization. The mutation site obtained from random mutagenesis was then introduced as a single-site mutation using site-directed mutagenesis. Single-site variants, D478N, D478Q, D478A, D478K, and D478R, were constructed. The optimum temperatures were all higher than 60 °C. D478A, D478N, and D478Q retained more than 80 % of the maximum relative activity of the wild-type L-AI at 75 °C. With the exception of the D478A variant, all variants showed decreased optimum pH values in the acidic range (6.0-6.5). All of the variant L-AIs could be significantly activated by the addition of Co(2+) and Mn(2+). D478N and D478Q showed higher catalytic efficiencies (k cat/K m) toward D-galactose than that of wild-type L-AI. In addition, the D478N and D478Q variants exhibited a much higher conversion ratio of D-galactose to D-tagatose at 6.0 than the wild-type L-AI. According to the molecular model, residue D478 was located on the surface of the enzyme and distant from the active site. It was supposed that the charged state of residue 478 may influence the optimum pH for substrate binding or isomerization.