Sucrose Phosphorylase and Related Enzymes in Glycoside Hydrolase Family 13: Discovery, Application and Engineering

High-yield synthesis of 2-O-α-D-glucosyl-D-glycerate by a bifunctional glycoside phosphorylase

Osmolytes are produced by various microorganisms as a defense mechanism to protect cells and macromolecules from damage caused by external stresses in harsh environments. Due to their useful stabilizing properties, these molecules are applied as active ingredients in a wide range of cosmetics and healthcare products. The metabolic pathways and biocatalytic syntheses of glycosidic osmolytes such as 2-O-α-D-glucosyl-D-glycerate often involve the action of a glycoside phosphorylase. Here, we report the discovery of a glucosylglycerate phosphorylase from carbohydrate-active enzyme family GH13 that is also active on sucrose, which contrasts the strict specificity of known glucosylglycerate phosphorylases that can only use α-D-glucose 1-phosphate as glycosyl donor in transglycosylation reactions. The novel enzyme can be distinguished from other phosphorylases from the same family by the presence of an atypical conserved sequence motif at specificity-determining positions in the active site. The promiscuity of the sucrose-active glucosylglycerate phosphorylase can be exploited for the high-yielding and rapid synthesis of 2-O-α-D-glucosyl-D-glycerate from sucrose and D-glycerate. KEY POINTS: • A Xylanimonas protaetiae glycoside phosphorylase can use both d-glycerate and fructose as glucosyl acceptor with high catalytic efficiency • Biocatalytic synthesis of the osmolyte 2-O-α-d-glucosyl-d-glycerate • Positions in the active site of GH13 phosphorylases act as convenient specificity fingerprints.

Sucrose phosphorylase from Alteromonas mediterranea: Structural insight into the regioselective α-glucosylation of (+)-catechin

Sucrose phosphorylases, through transglycosylation reactions, are interesting enzymes that can transfer regioselectively glucose from sucrose, the donor substrate, onto acceptors like flavonoids to form glycoconjugates and hence modulate their solubility and bioactivity. Here, we report for the first time the structure of sucrose phosphorylase from the marine bacteria Alteromonas mediterranea (AmSP) and its enzymatic properties. Kinetics of sucrose hydrolysis and transglucosylation capacities on (+)-catechin were investigated. Wild-type enzyme (AmSP-WT) displayed high hydrolytic activity on sucrose and was devoid of transglucosylation activity on (+)-catechin. Two variants, AmSP-Q353F and AmSP-P140D catalysed the regiospecific transglucosylation of (+)-catechin: 89 % of a novel compound (+)-catechin-4'-O-α-d-glucopyranoside (CAT-4') for AmSP-P140D and 92 % of (+)-catechin-3'-O-α-d-glucopyranoside (CAT-3') for AmSP-Q353F. The compound CAT-4' was fully characterized by NMR and mass spectrometry. An explanation for this difference in regiospecificity was provided at atomic level by molecular docking simulations: AmSP-P140D was found to preferentially bind (+)-catechin in a mode that favours glucosylation on its hydroxyl group in position 4' while the binding mode in AmSP-Q353F favoured glucosylation on its hydroxyl group in position 3'.

Converting Bulk Sugars into Functional Fibers: Discovery and Application of a Thermostable β-1,3-Oligoglucan Phosphorylase

Despite their broad application potential, the widespread use of β-1,3-glucans has been hampered by the high cost and heterogeneity associated with current production methods. To address this challenge, scalable and economically viable processes are needed for the production of β-1,3-glucans with tailorable molecular mass distributions. Glycoside phosphorylases have shown to be promising catalysts for the bottom-up synthesis of β-1,3-(oligo)glucans since they combine strict regioselectivity with a cheap donor substrate (i.e., α-glucose 1-phosphate). However, the need for an expensive priming substrate (e.g., laminaribiose) and the tendency to produce shorter oligosaccharides still form major bottlenecks. Here, we report the discovery and application of a thermostable β-1,3-oligoglucan phosphorylase originating from Anaerolinea thermophila (AtβOGP). This enzyme combines a superior catalytic efficiency toward glucose as a priming substrate, high thermostability, and the ability to synthesize high molecular mass β-1,3-glucans up to DP 75. Coupling of AtβOGP with a thermostable variant of Bifidobacterium adolescentis sucrose phosphorylase enabled the efficient production of tailorable β-1,3-(oligo)glucans from sucrose, with a near-complete conversion of >99 mol %. This cost-efficient process for the conversion of renewable bulk sugar into β-1,3-(oligo)glucans should facilitate the widespread application of these versatile functional fibers across various industries.

Strategies for the synthesis of the osmolyte glucosylglycerate and its precursor glycerate

Glycosidic osmolytes are widespread natural compounds that protect microorganisms and their macromolecules from the deleterious effects of various environmental stresses. Their protective properties have attracted considerable interest for industrial applications, especially as active ingredients in cosmetics and healthcare products. In that regard, the osmolyte glucosylglycerate is somewhat overlooked. Glucosylglycerate is typically accumulated by certain organisms when they are exposed to high salinity and nitrogen starvation, and its potent stabilizing effects have been demonstrated in vitro. However, the applications of this osmolyte have not been thoroughly explored due to the lack of a cost-efficient production process. Here, we present an overview of the progress that has been made in developing promising strategies for the synthesis of glucosylglycerate and its precursor glycerate, and discuss the remaining challenges. KEY POINTS: • Bacterial milking could be explored for fermentative production of glucosylglycerate • Glycoside phosphorylases of GH13_18 represent attractive alternatives for biocatalytic production • Conversion of glycerol with alditol oxidase is a promising strategy for generating the precursor glycerate.

Research overview on the genetic mechanism underlying the biosynthesis of polysaccharide in tuber plants

Tuber plants are of great significance in the world as human food crops. Polysaccharides, important metabolites in tuber plants, also serve as a source of innovative drugs with significant pharmacological effects. These drugs are particularly known for their immunomodulation and antitumor properties. To fully exploit the potential value of tuber plant polysaccharides and establish a synthetic system for their targeted synthesis, it is crucial to dissect their metabolic processes and genetic regulatory mechanisms. In this article, we provide a comprehensive summary of the basic pathways involved in the synthesis of various types of tuber plant polysaccharides. We also outline the key research progress that has been made in this area in recent years. We classify the main types and functions of tuber plant polysaccharides and analyze the biosynthetic processes and genetic regulation mechanisms of key enzymes involved in the metabolic pathways of starch, cellulose, pectin, and fructan in tuber plants. We have identified hexokinase and glycosyltransferase as the key enzymes involved in the polysaccharide synthesis process. By elucidating the synthesis pathway of polysaccharides in tuber plants and understanding the underlying mechanism of action of key enzymes in the metabolic pathway, we can provide a theoretical framework for enhancing the yield of polysaccharides and other metabolites in plant culture cells. This will ultimately lead to increased production efficiency.

Evolution of Phosphorylase Activity in an Ancestral Glycosyltransferase

The reconstruction of ancestral sequences can offer a glimpse into the fascinating process of molecular evolution by exposing the adaptive pathways that shape the proteins found in nature today. Here, we track the evolution of the carbohydrate-active enzymes responsible for the synthesis and turnover of mannogen, a critical carbohydrate reserve in Leishmania parasites. Biochemical characterization of resurrected enzymes demonstrated that mannoside phosphorylase activity emerged in an ancestral bacterial mannosyltransferase, and later disappeared in the process of horizontal gene transfer and gene duplication in Leishmania. By shuffling through plausible historical sequence space in an ancestral mannosyltransferase, we found that mannoside phosphorylase activity could be toggled on through various combinations of mutations at positions outside of the active site. Molecular dynamics simulations showed that such mutations can affect loop rigidity and shield the active site from water molecules that disrupt key interactions, allowing α-mannose 1-phosphate to adopt a catalytically productive conformation. These findings highlight the importance of subtle distal mutations in protein evolution and suggest that the vast collection of natural glycosyltransferases may be a promising source of engineering templates for the design of tailored phosphorylases.

Strategy for the Enzymatic Acylation of the Apple Flavonoid Phloretin Based on Prior α-Glucosylation

The acylation of flavonoids serves as a means to alter their physicochemical properties, enhance their stability, and improve their bioactivity. Compared with natural flavonoid glycosides, the acylation of nonglycosylated flavonoids presents greater challenges since they contain fewer reactive sites. In this work, we propose an efficient strategy to solve this problem based on a first α-glucosylation step catalyzed by a sucrose phosphorylase, followed by acylation using a lipase. The method was applied to phloretin, a bioactive dihydrochalcone mainly present in apples. Phloretin underwent initial glucosylation at the 4'-OH position, followed by subsequent (and quantitative) acylation with C8, C12, and C16 acyl chains employing an immobilized lipase from Thermomyces lanuginosus. Electrospray ionization-mass spectrometry (ESI-MS) and two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR) confirmed that the acylation took place at 6-OH of glucose. The water solubility of C8 acyl glucoside closely resembled that of aglycone, but for C12 and C16 derivatives, it was approximately 3 times lower. Compared with phloretin, the radical scavenging capacity of the new derivatives slightly decreased with 2,2-diphenyl-1-picrylhydrazyl (DPPH) and was similar to 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS•+). Interestingly, C12 acyl-α-glucoside displayed an enhanced (3-fold) transdermal absorption (using pig skin biopsies) compared to phloretin and its α-glucoside.

Turning waste into treasure: Biosynthesis of value-added 2-O-α-glucosyl glycerol and d-allulose from waste cane molasses through an in vitro synthetic biology platform

The efficient and economical conversion of agricultural waste into glycosides and rare sugars is challenging. Herein, an in vitro synthetic bienzyme system consisting of sucrose phosphorylase and d-allulose 3-epimerase was constructed to produce 2-O-α-glucosyl glycerol and d-allulose from cane molasses. Lactic acid in the cane molasses significantly induced sucrose phosphorylase to hydrolyze sucrose instead of glycosylation. Notably, lactic acid significantly inhibited the catalytic performance of d-allulose 3-epimerase only in the presence of Na+ and K+, with an inhibition rate of 75%. After removing lactic acid and metal ions, 116 g/L 2-O-α-glucosyl glycerol and 51 g/L d-allulose were synthesized from 500 mM sucrose in the treated cane molasses with a sucrose consumption rate of 97%. Our findings offer an economically efficient and environmentally friendly pathway for the industrial production of glycosides and rare sugars from food industry waste.

Characterization of a novel sucrose phosphorylase from Paenibacillus elgii and its use in biosynthesis of α-arbutin

α-Arbutin, a naturally occurring glycosylated derivative of hydroquinone (HQ), effectively inhibits melanin biosynthesis in epidermal cells. It is widely recognized as a fourth-generation whitening agent within the cosmetic industry. Currently, enzymatic catalysis is universally deemed the safest and most efficient method for α-arbutin synthesis. Sucrose phosphorylase (SPase), one of the most frequently employed glycosyltransferases, has been extensively reported for α-arbutin synthesis. In this study, a previously reported SPase known for its effectiveness in synthesizing α-arbutin, was used as a probe sequence to identify a novel SPase from Paenibacillus elgii (PeSP) in the protein database. The sequence similarity between PeSP and the probe was 39.71%, indicating a degree of novelty. Subsequently, the gene encoding PeSP was coexpressed with the molecular chaperone pG-Tf2 in Escherichia coli, significantly improving PeSP's solubility. Following this, PeSP was characterized and employed for α-arbutin biosynthesis. The specific activity of co-expressed PeSP reached 169.72 U/mg, exhibited optimal activity at 35℃ and pH 7.0, with a half-life of 3.6 h under the condition of 35℃. PeSP demonstrated excellent stability at pH 6.5-8.5 and sensitivity to high concentrations of metal ions. The kinetic parameters Km and kcat/Km were determined to be 14.50 mM and 9.79 min- 1·mM- 1, respectively.The reaction conditions for α-arbutin biosynthesis using recombinant PeSP were optimized, resulting in a maximum α-arbutin concentration of 52.60 g/L and a HQ conversion rate of 60.9%. The optimal conditions were achieved at 30℃ and pH 7.0 with 200 U/mL of PeSP, and by combining sucrose and hydroquinone at a molar ratio of 5:1 for a duration of 25 h.

Analyzing Current Trends and Possible Strategies to Improve Sucrose Isomerases' Thermostability

Due to their ability to produce isomaltulose, sucrose isomerases are enzymes that have caught the attention of researchers and entrepreneurs since the 1950s. However, their low activity and stability at temperatures above 40 °C have been a bottleneck for their industrial application. Specifically, the instability of these enzymes has been a challenge when it comes to their use for the synthesis and manufacturing of chemicals on a practical scale. This is because industrial processes often require biocatalysts that can withstand harsh reaction conditions, like high temperatures. Since the 1980s, there have been significant advancements in the thermal stabilization engineering of enzymes. Based on the literature from the past few decades and the latest achievements in protein engineering, this article systematically describes the strategies used to enhance the thermal stability of sucrose isomerases. Additionally, from a theoretical perspective, we discuss other potential mechanisms that could be used for this purpose.

Research progress and application of enzymatic synthesis of glycosyl compounds

Glucoside compounds are widely found in nature and have garnered significant attention in the medical, cosmetics, and food industries due to their diverse pharmaceutical properties, biological activities, and stable application characteristics. Glycosides are mainly obtained by direct extraction from plants, chemical synthesis, and enzymatic synthesis. Given the challenges associated with plant extraction, such as low conversion rates and the potential for environmental pollution with chemical synthesis, our review focuses on enzymatic synthesis. Here, we reviewed the enzymatic synthesis methods of 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G), 2-O-α-D-glucosyl glycerol (α-GG), arbutin and α-glucosyl hesperidin (Hsp-G), and other glucoside compounds. The types of enzymes selected in the synthesis process are comprehensively analyzed and summarized, as well as a series of enzyme transformation strategies adopted to improve the synthetic yield. KEY POINTS: • Glycosyl compounds have applications in the biomedical and food industries. • Enzymatic synthesis converts substrates into products using enzymes as catalysts. • Substrate bias and specificity are key to improving substrate conversion.

Combinatorial metabolic engineering enables high yield production of α-arbutin from sucrose by biocatalysis

α-Arbutin has been widely used as a skin-whitening ingredient. Previously, we successfully produced α-arbutin via whole-cell biocatalysis and found that the conversion rate of sucrose to α-arbutin was low (~45%). To overcome this issue, herein, we knocked out the genes of enzymes related to the sucrose hydrolysis, including sacB, sacC, levB, and sacA. The sucrose consumption was reduced by 17.4% in 24 h, and the sucrose conversion rate was increased to 51.5%. Furthermore, we developed an inducible protein degradation system with Lon protease isolated from Mesoplasma florum (MfLon) and proteolytic tag to control the PfkA activity, so that more fructose-6-phosphate (F6P) can be converted into glucose-1-phosphate (Glc1P) for α-arbutin synthesis, which can reduce the addition of sucrose and increase the sucrose conversion efficiency. Finally, the pathway of F6P to Glc1P was enhanced by integrating another copy of glucose 6-phosphate isomerase (Pgi) and phosphoglucomutase (PgcA); a high α-arbutin titer (~120 g/L) was obtained. The sucrose conversion rate was increased to 60.4% (mol/mol). In this study, the substrate utilization rate was boosted due to the attenuation of its hydrolysis and the assistance of the intracellular enzymes that converted the side product back into the substrate for α-arbutin synthesis. This strategy provides a new idea for the whole-cell biocatalytic synthesis of other products using sucrose as substrate, especially valuable glycosides.Key points The genes of sucrose metabolic pathway were knocked out to reduce the sucrose consumption. The by-product fructose was reused to synthesize α-arbutin. The optimized whole-cell system improved sucrose conversion by 15.3%.

Biocatalytic Approaches to Building Blocks for Enzymatic and Chemical Glycan Synthesis

While the field of biocatalysis has bloomed over the past 20-30 years, advances in the understanding and improvement of carbohydrate-active enzymes, in particular, the sugar nucleotides involved in glycan building block biosynthesis, have progressed relatively more slowly. This perspective highlights the need for further insight into substrate promiscuity and the use of biocatalysis fundamentals (rational design, directed evolution, immobilization) to expand substrate scopes toward such carbohydrate building block syntheses and/or to improve enzyme stability, kinetics, or turnover. Further, it explores the growing premise of using biocatalysis to provide simple, cost-effective access to stereochemically defined carbohydrate materials, which can undergo late-stage chemical functionalization or automated glycan synthesis/polymerization.

Molecular Level Sucrose Quantification: A Critical Review

Sucrose is a primary metabolite in plants, a source of energy, a source of carbon atoms for growth and development, and a regulator of biochemical processes. Most of the traditional analytical chemistry methods for sucrose quantification in plants require sample treatment (with consequent tissue destruction) and complex facilities, that do not allow real-time sucrose quantification at ultra-low concentrations (nM to pM range) under in vivo conditions, limiting our understanding of sucrose roles in plant physiology across different plant tissues and cellular compartments. Some of the above-mentioned problems may be circumvented with the use of bio-compatible ligands for molecular recognition of sucrose. Nevertheless, problems such as the signal-noise ratio, stability, and selectivity are some of the main challenges limiting the use of molecular recognition methods for the in vivo quantification of sucrose. In this review, we provide a critical analysis of the existing analytical chemistry tools, biosensors, and synthetic ligands, for sucrose quantification and discuss the most promising paths to improve upon its limits of detection. Our goal is to highlight the criteria design need for real-time, in vivo, highly sensitive and selective sucrose sensing capabilities to enable further our understanding of living organisms, the development of new plant breeding strategies for increased crop productivity and sustainability, and ultimately to contribute to the overarching need for food security.

Building mutational bridges between carbohydrate-active enzymes

The commercial value of specialty carbohydrates and glycosylated compounds has sparked considerable interest in the synthetic potential of carbohydrate-active enzymes (CAZymes). Protein engineering methods have proven to be highly successful in expanding the range of glycosylation reactions that these enzymes can perform efficiently and cost-effectively. The past few years have witnessed meaningful progress in this area, largely due to a sharper focus on the understanding of structure-function relationships and mechanistic intricacies. Here, we summarize recent studies that demonstrate how protein engineers have become much better at traversing the fitness landscape of CAZymes through mutational bridges that connect the different activity types.

Chitosan-regulated biomimetic hybrid nanoflower for efficiently immobilizing enzymes to enhance stability and by-product tolerance

Organic-inorganic hybrid nano-materials have been considered to be promising immobilization matrixes for enzymes due to their significantly enhanced reusability and stability of enzymes. Herein, we constructed a novel organic-inorganic hybrid nanoflower via biomacromolecule-regulated biomimetic mineralization to immobilize sucrose phosphorylase (SPase). It was found that chitosan (CS) effectively regulated the biomimetic mineralization of calcium phosphate (CaP), leading to the formation of flower-like hybrid materials for the entrapment of SPase via self-assembly to establish a nano-biocatalyst (CS-CaP@SPase). Upon immobilization, the obtained CS-CaP@SPase exhibited excellent pH, by-product and organic solvents tolerance, and storage stability. Specifically, at acidic condition (pH 4), CS-CaP@SPase performed over 80 % of initial activity, which was 2.42-folds higher than that of free SPase. The catalytic activity of free SPase was severely inhibited about 30 % in the presence of fructose (1.2 M), but CS-CaP@SPase only lost 5 % relative activity. The CS-CaP@SPase retained over 80 % of its relative activity, while the free SPase maintained <20 % of its relative activity in acetonitrile. The relative activity of CS-CaP@SPase was still retained about 80 % after 10 cycles and maintained 75 % after 15 days. Based on Raman spectra analysis, it was also found that the increased β-folding component of SPase in the secondary structure after immobilization was the main factor for its enhanced stability. It is reasonable to believe that biomacromolecule-regulated biomimetic mineralization could be potentially used as a promising method to immobilize enzymes with excellent stability and recyclability, thereby facilitating the preparation of highly efficient catalysts for industrial biocatalysts, biosensing, and biomedicine.

Enhancing regioselectivity of sucrose phosphorylase by loop engineering for glycosylation of L-ascorbic acid

Sucrose phosphorylase (SPase) has a remarkable capacity to synthesize numerous glucosides from abundantly available sucrose under mild conditions but suffers from specificity and regioselectivity issues. In this study, a loop engineering strategy was introduced to enhance the regioselectivity and substrate specificity of SPase for the efficient synthesis of 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G) via L-ascorbic acid (L-AA). P134, L341, and L343 were identified as "hotspots" for modulating the flexibility of loops, which significantly influenced the H-bonding network of L-AA in the active site, as well as the entrance of the substrate channel, thereby altering the regioselectivity and substrate specificity. Finally, the mutant L341V/L343F, with near-perfect control of the selectivity synthesis of the 2-OH group of L-AA (> 99%), was obtained. The AA-2G production by the mutant reached 244 g L^-1 in a whole-cell biotransformation system, and the conversion rate of L-AA reached 64%, which is the highest level reported to date. Our work also provides a successful loop engineering case for modulating the regioselectivity and specificity of sucrose phosphorylase. KEY POINTS: • "Hotspots" were identified in the flexible loops of sucrose phosphorylase. • Mutants exhibited improved regioselectivity and specificity against L-ascorbic acid. • Synthesized AA-2G with high yield and regioselectivity by whole-cell of mutant.

Efficient Production of 2-O-α-D-Glucosyl Glycerol Catalyzed by an Engineered Sucrose Phosphorylase from Bifidobacterium longum

2-O-α-D-Glucosyl glycerol (2-αGG) can be used as a multipurpose anti-aging, cell-stimulating, and skin moisturizing agent in the cosmetic industry. Sucrose phosphorylase (SPase) has been widely used in the production of 2-αGG. In this paper, the gene encoding sucrose phosphorylase from Bifidobacterium longum (BlSP) was inserted into pRSF-Duet-1 to construct the recombinant plasmid pRSF-BlSP and was functionally expressed in E. coli BL21(DE3) to be used as a biocatalyst for the synthesis of 2-αGG firstly. The mutations of BlSP were carried out based on alanine scanning, and a positive mutant G293A with a 50% increase in activity for 2-αGG production was identified. Mutant G293A has less K_m and bigger k_cat/K_m towards glycerol than the parental BlSP. Subsequently, the production of 177.6 g/L 2-αGG was attained from 1 M sucrose and 1.2 M glycerol catalyzed by 17 mg/mL G293A mutant. This study indicated that BlSP has good potential in the production of 2-αGG.

Comprehensive utilization of sucrose resources via chemical and biotechnological processes: A review

Sucrose, one of the most widespread disaccharides in nature, has been available in daily human life for many centuries. As an abundant and cheap sweetener, sucrose plays an essential role in our diet and the food industry. However, it has been determined that many diseases, such as obesity, diabetes, hyperlipidemia, etc., directly relate to the overconsumption of sucrose. It arouses many explorations for the conversion of sucrose to high-value chemicals. Production of valuable substances from sucrose by chemical methods has been studied since a half-century ago. Compared to chemical processes, biotechnological conversion approaches of sucrose are more environmentally friendly. Many enzymes can use sucrose as the substrate to generate functional sugars, especially those from GH68, GH70, GH13, and GH32 families. In this review, enzymatic catalysis and whole-cell fermentation of sucrose for the production of valuable chemicals were reviewed. The multienzyme cascade catalysis and metabolic engineering strategies were addressed.

Identification of Process-Related Impurities and Corresponding Control Strategy in Biocatalytic Production of 2-O-α-d-Glucopyranosyl-l-ascorbic Acid Using Sucrose Phosphorylase

2-O-α-d-Glucopyranosyl-l-ascorbic acid (AA-2G) is an ideal substitute for l-ascorbic acid because of its remarkable stability and improved biological activity, which can be easily applied in cosmetic, food, and medicine fields. However, impurity identification and control are significant procedures during the manufacturing of AA-2G. This study assessed a manufacturing routine of AA-2G synthesized by sucrose phosphorylase (SPase). First, three unknown process-related impurities were observed, which were further identified as 3-O-α-d-glucopyranosyl- l-ascorbic acid (impurity I), 2-O-α-d-glucopyranosyl-l-dehydroascorbic acid (impurity II), and 13-O-α-d-glucopyranosyl-2-O-α-d-glucopyranosyl-l-ascorbic acid (impurity III), respectively. Second, a comprehensive formation pathway of impurities was elucidated, and specific strategies corresponding to controlling each impurity were also proposed. Specifically, the content of impurity I can be reduced by 50% by fine tuning reaction conditions. The impurity II-free purification process was also achieved by applying a low concentration of alkali. Finally, a semi-rational design was introduced, and a single mutant L343F was obtained by site-directed mutagenesis, which reduced impurities I and III by 63.9 and 100%, respectively, without affecting the transglycosylation activity. It is expected that the reported impurity identification and control strategies during the AA-2G production will facilitate its industrial production.

Sweet Biotechnology: Enzymatic Production and Digestibility Screening of Novel Kojibiose and Nigerose Analogues

In view of the global pandemic of obesity and related metabolic diseases, there is an increased interest in alternative carbohydrates with promising physiochemical and health-related properties as a potential replacement for traditional sugars. However, our current knowledge is limited to only a small selection of carbohydrates, whereas the majority of alternative rare carbohydrates and especially their properties remain to be investigated. Unraveling their potential properties, like digestibility and glycemic content, could unlock their use in industrial applications. Here, we describe the enzymatic production and in vitro digestibility of three novel glycosides, namely, two kojibiose analogues (i.e., d-Glc_p-α-1,2-d-Gal and d-Glc_p-α-1,2-d-Rib) and one nigerose analogue (i.e., d-Glc_p-α-1,3-l-Ara). These novel sugars were discovered after an intensive acceptor screening with a sucrose phosphorylase originating from Bifidobacterium adolescentis (BaSP). Optimization and upscaling of this process led to roughly 100 g of these disaccharides. Digestibility, absorption, and caloric potential were assessed using brush border enzymes of rat origin and human intestinal Caco-2 cells. The rare disaccharides showed a reduced digestibility and a limited impact on energy metabolism, which was structure-dependent and even more pronounced for the three novel disaccharides in comparison to their respective glucobioses, translating to a low-caloric potential for these novel rare disaccharides.

Discovery and Biotechnological Exploitation of Glycoside-Phosphorylases

Among carbohydrate active enzymes, glycoside phosphorylases (GPs) are valuable catalysts for white biotechnologies, due to their exquisite capacity to efficiently re-modulate oligo- and poly-saccharides, without the need for costly activated sugars as substrates. The reversibility of the phosphorolysis reaction, indeed, makes them attractive tools for glycodiversification. However, discovery of new GP functions is hindered by the difficulty in identifying them in sequence databases, and, rather, relies on extensive and tedious biochemical characterization studies. Nevertheless, recent advances in automated tools have led to major improvements in GP mining, activity predictions, and functional screening. Implementation of GPs into innovative in vitro and in cellulo bioproduction strategies has also made substantial advances. Herein, we propose to discuss the latest developments in the strategies employed to efficiently discover GPs and make the best use of their exceptional catalytic properties for glycoside bioproduction.

Probing Genome-Scale Model Reveals Metabolic Capability and Essential Nutrients for Growth of Probiotic Limosilactobacillus reuteri KUB-AC5

Limosilactobacillus reuteri KUB-AC5 displays the hallmark features of probiotic properties for food and feed industries. Optimization of cultivation condition for the industrial production is important to reach cell concentration and cost reduction. Considering the strain-specific growth physiology, metabolic capability, and essential nutrients of L. reuteri KUB-AC5, the genome-scale metabolic model (GSMM) of L. reuteri KUB-AC5 was developed. Hereby, the GSMM of iTN656 was successfully constructed which contained 656 genes, 831 metabolites, and 953 metabolic reactions. The iTN656 model could show a metabolic capability under various carbon sources and guide potentially 14 essential single nutrients (e.g., vitamin B complex and amino acids) and 2 essential double nutrients (pairwise glutamine-glutamate and asparagine-aspartate) for L. reuteri KUB-AC5 growth through single and double omission analysis. Promisingly, the iTN656 model was further integrated with transcriptome data suggesting that putative metabolic routes as preferable paths e.g., sucrose uptake, nucleotide biosynthesis, urea cycle, and glutamine transporter for L. reuteri KUB-AC5 growth. The developed GSMM offers a powerful tool for multi-level omics analysis, enabling probiotic strain optimization for biomass overproduction on an industrial scale.

Metabolism and Health Effects of Rare Sugars in a CACO-2/HepG2 Coculture Model

Non-alcoholic fatty liver disease (NAFLD) has become the most prevalent liver disease worldwide and is impacted by an unhealthy diet with excessive calories, although the role of sugars in NAFLD etiology remains largely unexplored. Rare sugars are natural sugars with alternative monomers and glycosidic bonds, which have attracted attention as sugar replacers due to developments in enzyme engineering and hence an increased availability. We studied the impact of (rare) sugars on energy production, liver cell physiology and gene expression in human intestinal colorectal adenocarcinoma (Caco-2) cells, hepatoma G2 (HepG2) liver cells and a coculture model with these cells. Fat accumulation was investigated in the presence of an oleic/palmitic acid mixture. Glucose, fructose and galactose, but not mannose, l-arabinose, xylose and ribose enhanced hepatic fat accumulation in a HepG2 monoculture. In the coculture model, there was a non-significant trend (p = 0.08) towards higher (20–55% increased) median fat accumulation with maltose, kojibiose and nigerose. In this coculture model, cellular energy production was increased by glucose, maltose, kojibiose and nigerose, but not by trehalose. Furthermore, glucose, fructose and l-arabinose affected gene expression in a sugar-specific way in coculture HepG2 cells. These findings indicate that sugars provide structure-specific effects on cellular energy production, hepatic fat accumulation and gene expression, suggesting a health potential for trehalose and l-arabinose, as well as a differential impact of sugars beyond the distinction of conventional and rare sugars.

OUP accepted manuscript

Cellobiose, a natural disaccharide, attracts extensive attention as a potential functional food/feed additive. In this study, we present an inorganic phosphate (Pi) self-sufficient biotransformation system to produce cellobiose by co-expressing sucrose phosphorylase (SP) and cellobiose phosphorylase (CBP). The Bifidobacterium adolescentis SP (BASP) and Cellvibrio gilvus CBP (CGCBP) were co-expressed in Escherichia coli. Escherichia coli cells containing BASP and CGCBP were used as whole-cell catalysts to convert sucrose and glucose to cellobiose. The effects of reaction pH, temperature, Pi concentration, and substrate concentration were investigated. In the optimum biotransformation conditions, 800 mM cellobiose was produced from 1.0 M sucrose, 1.0 M glucose, and 50 mM Pi, within 12 hr. The by-product fructose and residual substrate (sucrose and glucose) were efficiently removed by treatment with yeast, to help purify the product cellobiose. The wider applicability of this Pi self-sufficiency strategy was demonstrated in the production of laminaribiose by co-expressing SP and laminaribiose phosphorylase. This study suggests that the Pi self-sufficiency strategy through co-expressing two phosphorylases has the advantage of great flexibility for enhanced production of cellobiose (or laminaribiose).

Enzymatic glucosylation of polyphenols using glucansucrases and branching sucrases of glycoside hydrolase family 70

Polyphenols exhibit various beneficial biological activities and represent very promising candidates as active compounds for food industry. However, the low solubility, poor stability and low bioavailability of polyphenols have severely limited their industrial applications. Enzymatic glycosylation is an effective way to improve the physicochemical properties of polyphenols. As efficient transglucosidases, glycoside hydrolase family 70 (GH70) glucansucrases naturally catalyze the synthesis of polysaccharides and oligosaccharides from sucrose. Notably, GH70 glucansucrases show broad acceptor substrate promiscuity and catalyze the glucosylation of a wide range of non-carbohydrate hydroxyl group-containing molecules, including benzenediol, phenolic acids, flavonoids and steviol glycosides. Branching sucrase enzymes, a newly established subfamily of GH70, are shown to possess a broader acceptor substrate binding pocket that acts efficiently for glucosylation of larger size polyphenols such as flavonoids. Here we present a comprehensive review of glucosylation of polyphenols using GH70 glucansucrase and branching sucrases. Their catalytic efficiency, the regioselectivity of glucosylation and the structure of generated products are described for these reactions. Moreover, enzyme engineering is effective for improving their catalytic efficiency and product specificity. The combined information provides novel insights on the glucosylation of polyphenols by GH70 glucansucrases and branching sucrases, and may promote their applications.

Biosynthesis and applications of curdlan

Curdlan is widely applied in the food and pharmaceutical industries. This review focuses on the biosynthetic pathways, regulatory mechanisms and metabolic engineering strategies for curdlan production. Firstly, curdlan biosynthesis is discussed. Furthermore, various strategies to increase curdlan production are summarized from four aspects, including the overexpression of genes for curdlan biosynthesis, weakening/knockdown of genes from competing pathways, increasing the supply of curdlan precursors, and optimization of fermentation conditions. Moreover, the emerging and advanced applications of curdlan are introduced. Finally, the challenges that are frequently encountered during curdlan biosynthesis are noted with a discussion of directions for curdlan production.

Effective bioconversion of 1,3-propanediol from biodiesel-derived crude glycerol using organic acid resistance-enhanced Lactobacillus reuteri JH83

Organic acids produced during the fermentation of lactic acid bacteria inhibit cellular growth and the production of 1,3-propanediol (1,3-PDO). Lactobacillus reuteri JH83, which has an increase of 18.6% in organic acid resistance, was obtained through electron beam irradiation mutagenesis irrelevant to the problem of genetically modified organisms. The maximum bioconversion of 1,3-PDO in fed-batch fermentation using pure glycerol by L. reuteri JH83 was 93.2 g/L at 72 h, and the productivity was 1.29 g/L·h, which achieved an increase by 34.6%, compared to that of the wild-type strain. In addition, the result of fed-batch fermentation for the production of 1,3-PDO using crude glycerol was not significantly different from that of pure glycerol. Additionally, transcriptome analysis confirmed changes in the expression levels of sucrose phosphorylase, which is a major facilitator superfamily transporter, and muramyl ligase family proteins, which protect lactic acid bacteria from various stressors, such as organic acids.

Recent advances in enzymatic synthesis of β-glucan and cellulose

Bottom-up synthesis of β-glucans such as callose, fungal β-(1,3)(1,6)-glucan and cellulose, can create the defined compounds that are needed to perform fundamental studies on glucan properties and develop applications. With the importance of β-glucans and cellulose in high-profile fields such as nutrition, renewables-based biotechnology and materials science, the enzymatic synthesis of such relevant carbohydrates and their derivatives has attracted much attention. Here we review recent developments in enzymatic synthesis of β-glucans and cellulose, with a focus on progress made over the last five years. We cover the different types of biocatalysts employed, their incorporation in cascades, the exploitation of enzyme promiscuity and their engineering, and reaction conditions affecting the production as well as in situ self-assembly of (non)functionalised glucans. The recent achievements in the application of glycosyl transferases and β-1,4- and β-1,3-glucan phosphorylases demonstrate the high potential and versatility of these biocatalysts in glucan synthesis in both industrial and academic contexts.

Development of thermostable sucrose phosphorylase by semi-rational design for efficient biosynthesis of alpha-D-glucosylglycerol

Abstract

Sucrose phosphorylase (SPase) can specifically catalyze transglycosylation reactions and can be used to enzymatically synthesize α-D-glycosides. However, the low thermostability of SPase has been a bottleneck for its industrial application. In this study, a SPase gene from Leuconostoc mesenteroides ATCC 12,291 (LmSPase) was synthesized with optimized codons and overexpressed successfully in Escherichia coli. A semi-rational design strategy that combined the FireProt (a web server designing thermostable proteins), structure–function analysis, and molecular dynamic simulations was used to improve the thermostability of LmSPase. Finally, one single-point mutation T219L and a combination mutation I31F/T219L/T263L/S360A (Mut4) with improved thermostability were obtained. The half-lives at 50 °C of T219L and Mut4 both increased approximately two-fold compared to that of wild-type LmSPase (WT). Furthermore, the two variants T219L and Mut4 were used to produce α-D-glucosylglycerol (αGG) from sucrose and glycerol by incubating with 40 U/mL crude extracts at 37 °C for 60 h and achieved the product concentration of 193.2 ± 12.9 g/L and 195.8 ± 13.1 g/L, respectively, which were approximately 1.3-fold higher than that of WT (150.4 ± 10.0 g/L). This study provides an effective strategy for improving the thermostability of an industrial enzyme.

Key points

• Predicted potential hotspot residues directing the thermostability of LmSPase by semi-rational design

• Screened two positive variants with higher thermostability and higher activity

• Synthesized α-D-glucosylglycerol to a high level by two screened positive variants

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11551-0.

Three-level hybrid modeling for systematic optimization of biocatalytic synthesis: α-glucosyl glycerol production by enzymatic trans-glycosylation from sucrose

Mechanism-based kinetic models are rigorous tools to analyze enzymatic reactions, but their extension to actual conditions of the biocatalytic synthesis can be difficult. Here, we demonstrate (mechanistic-empirical) hybrid modeling for systematic optimization of the sucrose phosphorylase-catalyzed glycosylation of glycerol from sucrose, to synthesize the cosmetic ingredient α-glucosyl glycerol (GG). The empirical model part was developed to capture nonspecific effects of high sucrose concentrations (up to 1.5 M) on microscopic steps of the enzymatic trans-glycosylation mechanism. Based on verified predictions of the enzyme performance under initial rate conditions (Level 1), the hybrid model was expanded by microscopic terms of the reverse reaction to account for the full-time course of GG synthesis (Level 2). Lastly (Level 3), the application of the hybrid model for comprehensive window-of-operation analysis and constrained optimization of the GG production (~250 g/L) was demonstrated. Using two candidate sucrose phosphorylases (from Leuconostoc mesenteroides and Bifidobacterium adolescentis), we reveal the hybrid model as a powerful tool of "process decision making" to guide rational selection of the best-suited enzyme catalyst. Our study exemplifies a closing of the gap between enzyme kinetic models considered for mechanistic research and applicable in technologically relevant reaction conditions; and it highlights the important benefit thus realizable for biocatalytic process development.

Continuous process technology for glucoside production from sucrose using a whole cell-derived solid catalyst of sucrose phosphorylase

Advanced biotransformation processes typically involve the upstream processing part performed continuously and interlinked tightly with the product isolation. Key in their development is a catalyst that is highly active, operationally robust, conveniently produced, and recyclable. A promising strategy to obtain such catalyst is to encapsulate enzymes as permeabilized whole cells in porous polymer materials. Here, we show immobilization of the sucrose phosphorylase from Bifidobacterium adolescentis (P134Q-variant) by encapsulating the corresponding E. coli cells into polyacrylamide. Applying the solid catalyst, we demonstrate continuous production of the commercial extremolyte 2-α-D-glucosyl-glycerol (2-GG) from sucrose and glycerol. The solid catalyst exhibited similar activity (≥70%) as the cell-free extract (~800 U g-1 cell wet weight) and showed excellent in-operando stability (40 °C) over 6 weeks in a packed-bed reactor. Systematic study of immobilization parameters related to catalyst activity led to the identification of cell loading and catalyst particle size as important factors of process optimization. Using glycerol in excess (1.8 M), we analyzed sucrose conversion dependent on space velocity (0.075-0.750 h-1) and revealed conditions for full conversion of up to 900 mM sucrose. The maximum 2-GG space-time yield reached was 45 g L-1 h-1 for a product concentration of 120 g L-1. Collectively, our study establishes a step-economic route towards a practical whole cell-derived solid catalyst of sucrose phosphorylase, enabling continuous production of glucosides from sucrose. This strengthens the current biomanufacturing of 2-GG, but also has significant replication potential for other sucrose-derived glucosides, promoting their industrial scale production using sucrose phosphorylase. KEY POINTS: • Cells of sucrose phosphorylase fixed in polyacrylamide were highly active and stable. • Solid catalyst was integrated with continuous flow to reach high process efficiency. • Generic process technology to efficiently produce glucosides from sucrose is shown.

Selective β-Mono-Glycosylation of a C15-Hydroxylated Metabolite of the Agricultural Herbicide Cinmethylin Using Leloir Glycosyltransferases

Cinmethylin is a well-known benzyl-ether derivative of the natural terpene 1,4-cineole that is used industrially as a pre-emergence herbicide in grass weed control for crop protection. Cinmethylin detoxification in plants has not been reported, but in animals, it prominently involves hydroxylation at the benzylic C15 methyl group. Here, we show enzymatic β-glycosylation of synthetic 15-hydroxy-cinmethylin to prepare a putative phase II detoxification metabolite of the cinmethylin in plants. We examined eight Leloir glycosyltransferases for reactivity with 15-hydroxy cinmethylin and revealed the selective formation of 15-hydroxy cinmethylin β-d-glucoside from uridine 5'-diphosphate (UDP)-glucose by the UGT71E5 from safflower (Carthamus tinctorius). The UGT71E5 showed a specific activity of 431 mU/mg, about 300-fold higher than that of apple (Malus domestica) UGT71A15 that also performed the desired 15-hydroxy cinmethylin mono-glycosylation. Bacterial glycosyltransferases (OleD from Streptomyces antibioticus, 2.9 mU/mg; GT1 from Bacillus cereus, 60 mU/mg) produced mixtures of 15-hydroxy cinmethylin mono- and disaccharide glycosides. Using UDP-glucose recycling with sucrose synthase, 15-hydroxy cinmethylin conversion with UGT71E5 efficiently provided the β-mono-glucoside (≥95% yield; ∼9 mM) suitable for biological studies.

Engineering of a Thermostable Biocatalyst for the Synthesis of 2-O-Glucosylglycerol

2‐O‐Glucosylglycerol is accumulated by various bacteria and plants in response to environmental stress. It is widely applied as a bioactive moisturising ingredient in skin care products, for which it is manufactured via enzymatic glucosylation of glycerol by the sucrose phosphorylase from Leuconostoc mesenteroides. This industrial process is operated at room temperature due to the mediocre stability of the biocatalyst, often leading to microbial contamination. The highly thermostable sucrose phosphorylase from Bifidobacterium adolescentis could be a better alternative in that regard, but this enzyme is not fit for production of 2‐O‐glucosylglycerol due to its low regioselectivity and poor affinity for glycerol. In this work, the thermostable phosphorylase was engineered to alleviate these problems. Several engineering approaches were explored, ranging from site‐directed mutagenesis to conventional, binary, iterative or combinatorial randomisation of the active site, resulting in the screening of ∼3,900 variants. Variant P134Q displayed a 21‐fold increase in catalytic efficiency for glycerol, as well as a threefold improvement in regioselectivity towards the 2‐position of the substrate, while retaining its activity for several days at elevated temperatures.

Bacterial α-diglucoside metabolism: perspectives and potential for biotechnology and biomedicine

In a competitive microbial environment, nutrient acquisition is a major contributor to the survival of any individual bacterial species, and the ability to access uncommon energy sources can provide a fitness advantage. One set of soluble carbohydrates that have attracted increased attention for use in biotechnology and biomedicine is the α-diglucosides. Maltose is the most well-studied member of this class; however, the remaining four less common α-diglucosides (trehalose, kojibiose, nigerose, and isomaltose) are increasingly used in processed food and fermented beverages. The consumption of trehalose has recently been shown to be a contributing factor in gut microbiome disease as certain pathogens are using α-diglucosides to outcompete native gut flora. Kojibiose and nigerose have also been examined as potential prebiotics and alternative sweeteners for a variety of foods. Compared to the study of maltose metabolism, our understanding of the synthesis and degradation of uncommon α-diglucosides is lacking, and several fundamental questions remain unanswered, particularly with regard to the regulation of bacterial metabolism for α-diglucosides. Therefore, this minireview attempts to provide a focused analysis of uncommon α-diglucoside metabolism in bacteria and suggests some future directions for this research area that could potentially accelerate biotechnology and biomedicine developments. KEY POINTS: • α-diglucosides are increasingly important but understudied bacterial metabolites. • Kinetically superior α-diglucoside enzymes require few amino acid substitutions. • In vivo studies are required to realize the biotechnology potential of α-diglucosides.

Whole cell-based catalyst for enzymatic production of the osmolyte 2-O-α-glucosylglycerol

Background

Glucosylglycerol (2-O-α-d-glucosyl-sn-glycerol; GG) is a natural osmolyte from bacteria and plants. It has promising applications as cosmetic and food-and-feed ingredient. Due to its natural scarcity, GG must be prepared through dedicated synthesis, and an industrial bioprocess for GG production has been implemented. This process uses sucrose phosphorylase (SucP)-catalyzed glycosylation of glycerol from sucrose, applying the isolated enzyme in immobilized form. A whole cell-based enzyme formulation might constitute an advanced catalyst for GG production. Here, recombinant production in Escherichia coli BL21(DE3) was compared systematically for the SucPs from Leuconostoc mesenteroides (LmSucP) and Bifidobacterium adolescentis (BaSucP) with the purpose of whole cell catalyst development.

Results

Expression from pQE30 and pET21 plasmids in E. coli BL21(DE3) gave recombinant protein at 40–50% share of total intracellular protein, with the monomeric LmSucP mostly soluble (≥ 80%) and the homodimeric BaSucP more prominently insoluble (~ 40%). The cell lysate specific activity of LmSucP was 2.8-fold (pET21; 70 ± 24 U/mg; N = 5) and 1.4-fold (pQE30; 54 ± 9 U/mg, N = 5) higher than that of BaSucP. Synthesis reactions revealed LmSucP was more regio-selective for glycerol glycosylation (~ 88%; position O2 compared to O1) than BaSucP (~ 66%), thus identifying LmSucP as the enzyme of choice for GG production. Fed-batch bioreactor cultivations at controlled low specific growth rate (µ = 0.05 h⁻¹; 28 °C) for LmSucP production (pET21) yielded ~ 40 g cell dry mass (CDM)/L with an activity of 2.0 × 10⁴ U/g CDM, corresponding to 39 U/mg protein. The same production from the pQE30 plasmid gave a lower yield of 6.5 × 10³ U/g CDM, equivalent to 13 U/mg. A single freeze–thaw cycle exposed ~ 70% of the intracellular enzyme activity for GG production (~ 65 g/L, ~ 90% yield from sucrose), without releasing it from the cells during the reaction.

Conclusions

Compared to BaSucP, LmSucP is preferred for regio-selective GG production. Expression from pET21 and pQE30 plasmids enables high-yield bioreactor production of the enzyme as a whole cell catalyst. The freeze–thaw treated cells represent a highly active, solid formulation of the LmSucP for practical synthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01569-4.

Disaccharide phosphorylases: Structure, catalytic mechanisms and directed evolution

Disaccharide phosphorylases (DSPs) are carbohydrate-active enzymes with outstanding potential for the biocatalytic conversion of common table sugar into products with attractive properties. They are modular enzymes that form active homo-oligomers. From a mechanistic as well as a structural point of view, they are similar to glycoside hydrolases or glycosyltransferases. As the majority of DSPs show strict stereo- and regiospecificities, these enzymes were used to synthesize specific disaccharides. Currently, protein engineering of DSPs is pursued in different laboratories to broaden the donor and acceptor substrate specificities or improve the industrial particularity of naturally existing enzymes, to eventually generate a toolbox of new catalysts for glycoside synthesis. Herein we review the characteristics and classifications of reported DSPs and the glycoside products that they have been used to synthesize.

Biocatalysis: Enzymatic Synthesis for Industrial Applications

Biocatalysis has found numerous applications in various fields as an alternative to chemical catalysis. The use of enzymes in organic synthesis, especially to make chiral compounds for pharmaceuticals as well for the flavors and fragrance industry, are the most prominent examples. In addition, biocatalysts are used on a large scale to make specialty and even bulk chemicals. This review intends to give illustrative examples in this field with a special focus on scalable chemical production using enzymes. It also discusses the opportunities and limitations of enzymatic syntheses using distinct examples and provides an outlook on emerging enzyme classes.

Natural and engineered transglycosylases: Green tools for the enzyme-based synthesis of glycoproducts

An increasing number of transglycosylase-based processes provide access to oligosaccharides or glycoconjugates, some of them reaching performance levels compatible with industrial developments. Nevertheless, the full potential of transglycosylases has not been explored because of the challenges in transforming a glycoside hydrolase into an efficient transglycosylase. Advances in studying enzyme structure/function relationships, screening enzyme activity, and generating synthetic libraries guided by computational protein design or machine learning methods should considerably accelerate the development of these catalysts. The time has now come for researchers to uncover their possibilities and learn how to design and precisely refine their activity to respond more rapidly to the growing demand for well-defined glycosidic structures.

Glucosylation prevents plant defense activation in phloem-feeding insects

The metabolic adaptations by which phloem-feeding insects counteract plant defense compounds are poorly known. Two-component plant defenses, such as glucosinolates, consist of a glucosylated protoxin that is activated by a glycoside hydrolase upon plant damage. Phloem-feeding herbivores are not generally believed to be negatively impacted by two-component defenses due to their slender piercing-sucking mouthparts, which minimize plant damage. However, here we document that glucosinolates are indeed activated during feeding by the whitefly Bemisia tabaci. This phloem feeder was also found to detoxify the majority of the glucosinolates it ingests by the stereoselective addition of glucose moieties, which prevents hydrolytic activation of these defense compounds. Glucosylation of glucosinolates in B. tabaci was accomplished via a transglucosidation mechanism, and two glycoside hydrolase family 13 (GH13) enzymes were shown to catalyze these reactions. This detoxification reaction was also found in a range of other phloem-feeding herbivores.

Phosphorylase-catalyzed bottom-up synthesis of short-chain soluble cello-oligosaccharides and property-tunable cellulosic materials

Cellulose-based materials are produced industrially in countless varieties via top-down processing of natural lignocellulose substrates. By contrast, cellulosic materials are only rarely prepared via bottom up synthesis and oligomerization-induced self-assembly of cellulose chains. Building up a cellulose chain via precision polymerization is promising, however, for it offers tunability and control of the final chemical structure. Synthetic cellulose derivatives with programmable material properties might thus be obtained. Cellodextrin phosphorylase (CdP; EC 2.4.1.49) catalyzes iterative β-1,4-glycosylation from α-d-glucose 1-phosphate, with the ability to elongate a diversity of acceptor substrates, including cellobiose, d-glucose and a range of synthetic glycosides having non-sugar aglycons. Depending on the reaction conditions leading to different degrees of polymerization (DP), short-chain soluble cello-oligosaccharides (COS) or insoluble cellulosic materials are formed. Here, we review the characteristics of CdP as bio-catalyst for synthetic applications and show advances in the enzymatic production of COS and reducing end-modified, tailored cellulose materials. Recent studies reveal COS as interesting dietary fibers that could provide a selective prebiotic effect. The bottom-up synthesized celluloses involve chains of DP ≥ 9, as precipitated in solution, and they form ~5 nm thick sheet-like crystalline structures of cellulose allomorph II. Solvent conditions and aglycon structures can direct the cellulose chain self-assembly towards a range of material architectures, including hierarchically organized networks of nanoribbons, or nanorods as well as distorted nanosheets. Composite materials are also formed. The resulting materials can be useful as property-tunable hydrogels and feature site-specific introduction of functional and chemically reactive groups. Therefore, COS and cellulose obtained via bottom-up synthesis can expand cellulose applications towards product classes that are difficult to access via top-down processing of natural materials.

On the donor substrate dependence of group-transfer reactions by hydrolytic enzymes: Insight from kinetic analysis of sucrose phosphorylase-catalyzed transglycosylation

Chemical group-transfer reactions by hydrolytic enzymes have considerable importance in biocatalytic synthesis and are exploited broadly in commercial-scale chemical production. Mechanistically, these reactions have in common the involvement of a covalent enzyme intermediate which is formed upon enzyme reaction with the donor substrate and is subsequently intercepted by a suitable acceptor. Here, we studied the glycosylation of glycerol from sucrose by sucrose phosphorylase (SucP) to clarify a peculiar, yet generally important characteristic of this reaction: partitioning between glycosylation of glycerol and hydrolysis depends on the type and the concentration of the donor substrate used (here: sucrose, α-d-glucose 1-phosphate (G1P)). We develop a kinetic framework to analyze the effect and provide evidence that, when G1P is used as donor substrate, hydrolysis occurs not only from the β-glucosyl-enzyme intermediate (E-Glc), but additionally from a noncovalent complex of E-Glc and substrate which unlike E-Glc is unreactive to glycerol. Depending on the relative rates of hydrolysis of free and substrate-bound E-Glc, inhibition (Leuconostoc mesenteroides SucP) or apparent activation (Bifidobacterium adolescentis SucP) is observed at high donor substrate concentration. At a G1P concentration that excludes the substrate-bound E-Glc, the transfer/hydrolysis ratio changes to a value consistent with reaction exclusively through E-Glc, independent of the donor substrate used. Collectively, these results give explanation for a kinetic behavior of SucP not previously accounted for, provide essential basis for design and optimization of the synthetic reaction, and establish a theoretical framework for the analysis of kinetically analogous group-transfer reactions by hydrolytic enzymes.