A High-Yielding Biocatalytic Process for the Production of 2-O-(α-D-glucopyranosyl)-sn-glycerol, a Natural Osmolyte and Useful Moisturizing Ingredient

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.

2-O-α-D-glucosyl glycerol production by whole-cell biocatalyst of lactobacilli encapsulating sucrose phosphorylase with improved glycerol affinity and conversion rate

BACKGROUND

2-O-α-D-glucosyl glycerol (2-αGG) is a valuable ingredient in cosmetics, health-care and food fields. Sucrose phosphorylase (SPase) is a favorable choice for biosynthesis of 2-αGG, while its glucosyl-acceptor affinity and thermodynamic feature remain largely unknown, limiting 2-αGG manufacturing.

RESULTS

Here, three SPases were obtained from lactobacilli and bifidobacteria, and the one encoded by Lb. reuteri SDMCC050455 (LrSP) had the best transglucosylation ability, with 2-αGG accounting for 86.01% in the total product. However, the LrSP exhibited an initial forward reaction rate of 11.83/s and reached equilibrium of 56.90% at 110 h, indicating low glycerol affinity and conversion rate. To improve catalytic efficiency, the LrSP was overexpressed in Lb. paracasei BL-SP, of which the intracellular SPase activity increased by 6.67-fold compared with Lb. reuteri SDMCC050455. After chemically permeabilized with Triton X-100, the whole-cell biocatalysis of Lb. paracasei BL-SP was prepared and showed the highest activity, with the initial forward reaction rate improved to 50.17/s and conversion rate risen to 80.79% within 17 h. Using the whole-cell biocatalyst, the final yield of 2-αGG was 203.21 g/L from 1 M sucrose and 1 M glycerol.

CONCLUSION

The food grade strain Lb. paracasei was used for the first time as cell factory to recombinantly express the LrSP and construct a whole-cell biocatalyst for the production of 2-αGG. After condition optimization and cell permeabilization, the whole-cell biocatalyst exhibited 23.89% higher equilibrium conversion and 9.10-fold of productivity compared with the pure enzyme catalytic system. This work would provide a reference for large-scale bioprocess of 2-αGG.

Spatially sequential co-immobilization of phosphorylases in tiny environments and its application in the synthesis of glucosyl glycerol

2-O-(α-d-glucopyranosyl)-sn-glycerol (2-αGG) has been applied in the food industry due to its numerous physiological benefits. The synthesis of 2-αGG can be achieved through a cascade catalytic reaction involving sucrose phosphorylase (SP) and 2-O-α-glucosylglycerol phosphorylase (GGP). However, the low substrate transfer rates between free enzymes have hindered the efficiency of 2-αGG synthesis. To address this issue, a novel technology was developed to prepare sequential multi-enzyme nanoflowers via chemical crosslinking and protein assembly, thus overcoming diffusion limitations. Specifically, spatially sequential co-immobilized enzymes, referred to as SP-GGP@Cap, were created through the targeted assembly of Bifidobacterium adolescentis SP and Marinobacter adhaerens GGP on Ca2+. This assembly was facilitated by the spontaneous protein reaction between SpyTag and SpyCatcher. Compared to free SP-GGP, SP-GGP@Cap demonstrated improved thermal and pH stability. Moreover, SP-GGP@Cap enhanced the biosynthesis of 2-αGG, achieving a relative concentration of 98 %. Additionally, it retained the ability to catalyze the substrate to yield 61 % relative concentration of 2-αGG even after ten cycles of recycling. This study presents a strategy for the spatially sequential co-immobilization of multiple enzymes in a confined environment and provides an exceptional biocatalyst for the potential industrial production of 2-αGG.

Advancements in the Heterologous Expression of Sucrose Phosphorylase and Its Molecular Modification for the Synthesis of Glycosylated Products

Sucrose phosphorylase (SPase), a member of the glycoside hydrolase GH13 family, possesses the ability to catalyze the hydrolysis of sucrose to generate α-glucose-1-phosphate and can also glycosylate diverse substrates, showcasing a wide substrate specificity. This enzyme has found extensive utility in the fields of food, medicine, and cosmetics, and has garnered significant attention as a focal point of research in transglycosylation enzymes. Nevertheless, SPase encounters numerous obstacles in industrial settings, including low enzyme yield, inadequate thermal stability, mixed regioselectivity, and limited transglycosylation activity. In-depth exploration of efficient expression strategies and molecular modifications based on the crystal structure and functional information of SPase is now a critical research priority. This paper systematically reviews the source microorganisms, crystal structure, and catalytic mechanism of SPase, summarizes diverse heterologous expression systems based on expression hosts and vectors, and examines the application and molecular modification progress of SPase in synthesizing typical glycosylated products. Additionally, it anticipates the broad application prospects of SPase in industrial production and related research fields, laying the groundwork for its engineering modification and industrial application.

Glycoside Phosphorylase Catalyzed Cellulose and β-1,3-Glucan Synthesis Using Chromophoric Glycosyl Acceptors

Glycoside phosphorylases are enzymes that are frequently used for polysaccharide synthesis. Some of these enzymes have broad substrate specificity, enabling the synthesis of reducing-end-functionalized glucan chains. Here, we explore the potential of glycoside phosphorylases in synthesizing chromophore-conjugated polysaccharides using commercially available chromophoric model compounds as glycosyl acceptors. Specifically, we report cellulose and β-1,3-glucan synthesis using 2-nitrophenyl β-d-glucopyranoside, 4-nitrophenyl β-d-glucopyranoside, and 2-methoxy-4-(2-nitrovinyl)phenyl β-d-glucopyranoside with Clostridium thermocellum cellodextrin phosphorylase and Thermosipho africanus β-1,3-glucan phosphorylase as catalysts. We demonstrate activity for both enzymes with all assayed chromophoric acceptors and report the crystallization-driven precipitation and detailed structural characterization of the synthesized polysaccharides, i.e., their molar mass distributions and various structural parameters, such as morphology, fibril diameter, lamellar thickness, and crystal form. Our results provide insights for the studies of chromophore-conjugated low molecular weight polysaccharides, glycoside phosphorylases, and the hierarchical assembly of crystalline cellulose and β-1,3-glucan.

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.

Acremonium sp. diglycosidase-aid chemical diversification: valorization of industry by-products

The fungal diglycosidase α-rhamnosyl-β-glucosidase I (αRβG I) from Acremonium sp. DSM 24697 catalyzes the glycosylation of various OH-acceptors using the citrus flavanone hesperidin. We successfully applied a one-pot biocatalysis process to synthesize 4-methylumbellipheryl rutinoside (4-MUR) and glyceryl rutinoside using a citrus peel residue as sugar donor. This residue, which contained 3.5 % [w/w] hesperidin, is the remaining of citrus processing after producing orange juice, essential oil, and peel-juice. The low-cost compound glycerol was utilized in the synthesis of glyceryl rutinoside. We implemented a simple method for the obtention of glyceryl rutinoside with 99 % yield, and its purification involving activated charcoal, which also facilitated the recovery of the by-product hesperetin through liquid-liquid extraction. This process presents a promising alternative for biorefinery operations, highlighting the valuable role of αRβG I in valorizing glycerol and agricultural by-products. KEYPOINTS: • αRβG I catalyzed the synthesis of rutinosides using a suspension of OPW as sugar donor. • The glycosylation of aliphatic polyalcohols by the αRβG I resulted in products bearing a single rutinose moiety. • αRβG I catalyzed the synthesis of glyceryl rutinoside with high glycosylation/hydrolysis selectivity (99 % yield).

Self-assembled multienzyme complex facilitates synthesis of glucosylglycerol from maltodextrin and glycerol

BACKGROUND

Compound 2-O-α-d-glucosylglycerol (2αGG) naturally serves as a compatible osmolyte in acclimation to environmental stresses, such as high osmolarity, dryness, and extreme temperature. It presents several bioactivities and has been used in the food, agriculture, and cosmetics areas.

RESULTS

In the present study, we attempted to synthesize the 2αGG from low-cost maltodextrin and glycerol by constructing an in vitro multi-enzyme system. The system contained two core enzymes, namely glucan phosphorylases (GPs) and glucosylglycerol phosphorylases (GGPs), and two auxiliary enzymes, namely isoamylase and 4-α-glucanotransferase. Several new GGPs from different organisms were characterized with the function of converting α-G1P and glycerol to sole stereo-configuration product 2αGG. Then, polypeptide SpyTag-SpyCatcher was employed to construct a self-assembled multienzyme complex, and different combinations between enzymes and peptides were constructed and tested. The best self-assembled multienzyme complex exhibited three-fold higher productivity compared to that of free enzyme. This reaction system also produced 240 mm (61 g L-1 ) 2αGG under high substrate concentration, with a conversion yield of 86%.

CONCLUSION

The present study provides an efficient approach for producing 2αGG. It also demonstrates that the SpyTag-SpyCatcher system could be applied to construct other multienzyme complexes for increased productivity and product titer. © 2023 Society of Chemical Industry.

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.

A Synergistic View on Osmolyte's Role against Salt and Cold Stress in Biointerfaces

We present our perspective on the role of osmolytes in mitigating abiotic stresses such as hypersalinity and sudden temperature changes. While the stabilizing effect of osmolytes on protein tertiary structures has been extensively studied, their direct impact on abiotic stress factors has eluded mainstream attention. Via highlighting a set of recent success stories of a joint venture of computer simulations and experimental measurements, we summarize the mechanistic insights into osmolytic action, particularly in the context of salt stress and combined cold-salt stress at the interface of biomolecular surfaces and saline environments. We stress the importance of chemical specificity in osmolytic activity, the interplay of differential osmolytic behaviors against heterogeneous salt stress, and the capability of osmolytes to adopt combined actions. Additionally, we discuss the potential of incorporating nanomaterial-based systems to enrich our understanding of osmolyte bioactions and facilitate their practical applications. We anticipate that this discourse will inspire interdisciplinary collaborations and motivate further investigations on osmolytes, ultimately broadening their applications in the fields of health and disease.

Efficient 2-O-α-D-glucopyranosyl-sn-glycerol production by single whole-cell biotransformation through combined engineering and expression regulation with novel sucrose phosphorylase from Leuconostoc mesenteroides ATCC 8293

2-O-α-D-glucopyranosyl-sn-glycerol (2-αGG) is a high value product with wide applications. Here, an efficient, safe and sustainable bioprocesses for 2-αGG production was designed. A novel sucrose phosphorylase (SPase) was firstly identified from Leuconostoc mesenteroides ATCC 8293. Subsequently, SPase mutations were processed with computer-aided engineering, of which the activity of SPaseK138C was 160% higher than that of the wild-type. Structural analysis revealed that K138C was a key functional residue moderating substrate binding pocket and thus influences catalytic activity. Furthermore, Corynebacterium glutamicum was employed to construct microbial cell factories along with ribosome binding site (RBS) fine-tuning and a two-stage substrate feeding control strategy. The maximum production of 2-αGG by these combined strategies reached 351.8 g·L^-1 with 98% conversion rate from 1.4 M sucrose and 3.5 M glycerol in a 5-L bioreactor. This was one of the best performance reported in single-cell biosynthesis of 2-αGG, which paved effective ways for industrial-scale preparation of 2-αGG.

β-1,3-Glucan synthesis, novel supramolecular self-assembly, characterization and application

β-1,3-Glucans are ubiquitously observed in various biological systems with diverse physio-ecological functions, yet their underlying assembly mechanism and multiscale complexation in vitro remains poorly understood. Here, we provide for the first-time evidence of unidentified β-1,3-glucan supramolecular complexation into intricate hierarchical architectures over several length scales. We mediated these unique assemblies using a recombinantly produced β-1,3-glucan phosphorylase (Ta1,3BGP) by fine-tuning solution conditions during particle nucleation and growth. We report a synthesis of interconnected parallel hexagonal lamellae composed of 8 nm thick sheets of highly expanded paracrystals. The architecture consists of β-1,3-glucan triple-helices with considerable inter-intra hydrogen bonding within, as well as in between adjacent triple-helices. The results extend our understanding of β-1,3-glucan molecular organization and shed light on different aspects of the crystallization processes of biomolecules into structures unseen by nature. The presented versatile synthesis yields new materials for diverse medical and industrial applications.

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.

Biological sources, metabolism, and production of glucosylglycerols, a group of natural glucosides of biotechnological interest

Glucosylglycerols (GGs) are a group of functional heterosides comprising glycerol and glucose. In nature, they are mainly produced by many moderately salt-tolerant cyanobacteria as compatible solutes in a salt-dependent manner and synthesized in a few higher plants and fermentation processes. Because of their many interesting physicochemical properties and biological activities, such as low sweetness, low hygroscopicity, high water-holding capacity, excellent biocompatibility, favorable performance in protecting macromolecules, and antitumor activity, GGs exhibit large application potential in the fields of cosmetics, health care, food service, enzyme production, and pharmaceuticals. Many in vitro systems using different members of the GH (glycoside hydrolase) family have been established for the enzymatic synthesis of GGs, and a few of them are in use for commercial production. Based on a good understanding of the genetic bases, biochemical processes, and regulatory mechanisms of GG metabolism in microorganisms (mainly cyanobacteria), in recent years GG production technologies with in vivo systems have also been developed by applying metabolic and bioprocess engineering to a few native or heterologous microbial cell factories. This successfully provides the market GG products with an alternative source. With the further elucidation of details about the biological functions of GGs and related mechanisms, the application scope of GGs will be greatly expanded. In the present review, the biological sources and physiological roles of GGs, the molecular bases and regulation of GG metabolism, and the recent progress in GG production and application are systematically summarized. A few new questions that have arisen in the basic research of GGs and perspectives on GG application are also discussed.

Sucrose phosphorylase from Lactobacillus reuteri: Characterization and application of enzyme for production of 2-O-α-d-glucopyranosyl glycerol

The enzymatic synthesis of 2-O-α-d-glucopyranosyl-glycerol (2-αGG) by transglycosylation activity of sucrose phosphorylase (SPase) is a promising method for 2-αGG manufacturing. However, there are only a few SPases available for 2-αGG production. Here, we report on the characterization and application of SPase from Lactobacillus reuteri (LrSPase). The results of transglycosylation properties assay showed that LrSPase was a potential glycerol glycosylating tool with high activity at pH 8.0 and 45 °C. And the transglycosylation activity of LrSPase was seriously inhibited by Fe³⁺, Zn²⁺ and Cu²⁺. Moreover, the result of substrate specificity assay showed LrSPase was able to catalyze the transglycosylation of 13 phenolic compounds. To produce commercially relevant concentrations of 2-αGG, we have developed a practical, efficient and scalable process for 2-αGG production using sucrose batch-feeding strategy by whole-cell catalyst. The maximum titer of 2-αGG was 237.68 g L^-1 with a productivity of 23.39 mM h^-1 and the molar conversion rate of glycerol reached 62.38%. To the best of our knowledge, this is the highest 2-αGG production level by using only SPase to synthesize 2-αGG until now. This study provides an effective way for industrial production of 2-αGG.

Biocatalytic Production of 2-α-d-Glucosyl-glycerol for Functional Ingredient Use: Integrated Process Design and Techno-Economic Assessment

Advanced biomanufacturing builds on production processes that are both profitable and sustainable. Integrated design of process unit operations, geared to output efficiency and waste minimization and guided by a rigorous techno-economic assessment, is essential for development aligned to these central aims. Here, we demonstrate such a development for the biocatalytic production of the biological extremolyte 2-O-α-d-glucosyl-glycerol (2-GG) for functional ingredient application. The process was aligned in scale over all steps (∼180 g product; ∼2.5 L reaction mixture) and involved continuous enzymatic synthesis from sucrose and glycerol interlinked with reactive extraction and nanofiltration for product isolation (purity of ∼80 wt %) and side stream recovery. Glycerol used in ∼6-fold excess over sucrose was recycled, and hydrothermal conversion into 5-(hydroxymethyl)furfural was evaluated for the fructose by-product released from sucrose. Based on a process mass intensity (total mass input/mass product) of 146, ∼80% of the total mass input was utilized and an E-factor (mass waste/mass product) of 28 was obtained. EcoScale analysis revealed a penalty point score of 44, suggesting an acceptable process from a sustainability point of view. Process simulation for an annual production of 10 tons 2-GG was used for the techno-economic assessment with discounted cash flow analysis. The calculated operating costs involved 35 and 47% contributions from materials and labor, respectively. About 91% of the material costs were due to chemicals for the reactive extraction-acidic stripping step, emphasizing the importance of material reuse at this step. Glycerol recycling involved a trade-off between waste reduction and energy use for the removal of water. Collectively, the study identifies options and boundaries of a profitable 2-GG process. The minimum selling price for 2-GG was calculated as ∼240 € kg^-1 or smaller. The framework of the methodology presented can be generally important in applied bio-catalysis: it facilitates closing of the gap between process design and implementation for accelerated development.

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.

Enzymatic synthesis of novel fructosylated compounds by Ffase from Schwanniomyces occidentalis in green solvents

The β-fructofuranosidase from the yeast Schwanniomyces occidentalis (Ffase) produces potential prebiotic fructooligosaccharides (FOS) by self-transfructosylation of sucrose, being one of the highest known producers of 6-kestose. The use of Green Solvents (GS) in biocatalysis has emerged as a sustainable alternative to conventional organic media for improving product yields and generating new molecules. In this work, the Ffase hydrolytic and transfructosylating activity was analysed using different GS, including biosolvents and ionic liquids. Among them, 11 were compatible for the net synthesis of FOS. Besides, two glycerol derivatives improved the yield of total FOS. Interestingly, polyols ethylene glycol and glycerol were found to be efficient alternative fructosyl-acceptors, both substantially decreasing the sucrose fructosylation. The main transfer product of the reaction with glycerol was a 62 g L-1 isomeric mixture of 1-O and 2-O-β-d-fructofuranosylglycerol, representing 95% of all chemicals generated by transfructosylation. Unexpectedly, the non-terminal 2-O fructo-conjugate was the major molecule catalysed during the process, while the 1-O isomer was the minor one. This fact made Ffase the first known enzyme from yeast showing this catalytic ability. Thus, novel fructosylated compounds with potential applications in food, cosmetics, and pharmaceutical fields have been obtained in this work, increasing the biotechnological interest of Ffase with innocuous GS.

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.

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.

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.

Discovering and efficiently promoting the extracellular secretory expression of Thermobacillus sp. ZCTH02-B1 sucrose phosphorylase in Escherichia coli

Sucrose phosphorylase (SPase, EC2.4.1.7) is a promising transglycosylation biocatalyst used for producing glycosylated compounds that are widely used in the food, cosmetics, and pharmaceutical industries. In this study, a recombinant SPase from the Thermobacillus sp. ZCTH02-B1 (rTSPase), which was previously reported to have high thermostability and the catalytic ability to synthesize ascorbic acid 2-glucoside, was attempted to be extracellularly expressed in Escherichia coli BL21(DE3) by fusion of endogenous osmotically-inducible protein Y. Unexpectedly, the rTSPase itself was produced outside the cells with an underestimated performance, although no typical signal peptide was predicted. Further N- and C-terminal truncation experiments revealed that both termini of rTSPase have an important role in protein folding and enzymatic activity, while its secretion was N-terminus associated. Extracellular protein concentration and rTSPase activity achieved 1.8 mg/mL and 6.2 U/mL after induction of 36 h in a 5-L fermenter. High-level extracellular rTSPase production could also be obtained from E. coli within 24 h by inducing overexpression of D, D-carboxypeptidase for cell lysis.

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.

High-Yield Biosynthesis of Glucosylglycerol through Coupling Phosphorolysis and Transglycosylation Reactions

Glucosylglycerol is a powerful osmolyte that has attracted attention as a useful moisturizing ingredient in the cosmetic industry. This study demonstrates two artificially designed synthetic routes for manufacturing glucosylglycerol by combining phosphorolysis and transglycosylation reactions. The overall Gibbs energy change of the synthetic routes was negative, indicating that they are thermodynamically favorable. In vitro biosystems were constructed through combining the phosphorolysis ability of sucrose/maltose phosphorylase and the transglycosylation capacity of glucosylglycerol phosphorylases from different organisms. A near-stoichiometric conversion of sucrose and glycerol with a high product yield of 98% was achieved under optimal reaction conditions. The large-scale glucosylglycerol production of this biosystem was investigated under a high concentration of substrates (2 mol/L sucrose and 2.4 mol/L glycerol), and the titer reached 1.78 mol/L (452 g/L) with a productivity of 24.3 g/L/h. To the best of our knowledge, this value presented the highest glucosylglycerol production level until now, which indicated a great industrial application potential for glucosylglycerol manufacturing.

Selective oxidation of alkyl and aryl glyceryl monoethers catalysed by an engineered and immobilised glycerol dehydrogenase

Enzymes acting over glyceryl ethers are scarce in living cells, and consequently biocatalytic transformations of these molecules are rare despite their interest for industrial chemistry. In this work, we have engineered and immobilised a glycerol dehydrogenase from Bacillus stearothermophilus (BsGlyDH) to accept a battery of alkyl/aryl glyceryl monoethers and catalyse their enantioselective oxidation to yield the corresponding 3-alkoxy/aryloxy-1-hydroxyacetones. QM/MM computational studies decipher the key role of D123 in the oxidation catalytic mechanism, and reveal that this enzyme is highly enantioselective towards S-isomers (ee > 99%). Through structure-guided site-selective mutagenesis, we find that the mutation L252A sculpts the active site to accommodate a productive configuration of 3-monoalkyl glycerols. This mutation enhances the k cat 163-fold towards 3-ethoxypropan-1,2-diol, resulting in a specific activity similar to the one found for the wild-type towards glycerol. Furthermore, we immobilised the L252A variant to intensify the process, demonstrating the reusability and increasing the operational stability of the resulting heterogeneous biocatalyst. Finally, we manage to integrate this immobilised enzyme into a one-pot chemoenzymatic process to convert glycidol and ethanol into 3-ethoxy-1-hydroxyacetone and (R)-3-ethoxypropan-1,2-diol, without affecting the oxidation activity. These results thus expand the uses of engineered glycerol dehydrogenases in applied biocatalysis for the kinetic resolution of glycerol ethers and the manufacturing of substituted hydroxyacetones.

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.

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

Sucrose phosphorylases are carbohydrate-active enzymes with outstanding potential for the biocatalytic conversion of common table sugar into products with attractive properties. They belong to the glycoside hydrolase family GH13, where they are found in subfamily 18. In bacteria, these enzymes catalyse the phosphorolysis of sucrose to yield α-glucose 1-phosphate and fructose. However, sucrose phosphorylases can also be applied as versatile transglucosylases for the synthesis of valuable glycosides and sugars because their broad promiscuity allows them to transfer the glucosyl group of sucrose to a diverse collection of compounds other than phosphate. Numerous process and enzyme engineering studies have expanded the range of possible applications of sucrose phosphorylases ever further. Moreover, it has recently been discovered that family GH13 also contains a few novel phosphorylases that are specialised in the phosphorolysis of sucrose 6F-phosphate, glucosylglycerol or glucosylglycerate. In this review, we provide an overview of the progress that has been made in our understanding and exploitation of sucrose phosphorylases and related enzymes over the past ten years.

Preparative and Kinetic Analysis of β-1,4- and β-1,3-Glucan Phosphorylases Informs Access to Human Milk Oligosaccharide Fragments and Analogues Thereof

The enzymatic synthesis of oligosaccharides depends on the availability of suitable enzymes, which remains a limitation. Without recourse to enzyme engineering or evolution approaches, herein we demonstrate the ability of wild-type cellodextrin phosphorylase (CDP: β-1,4-glucan linkage-dependent) and laminaridextrin phosphorylase (Pro_7066: β-1,3-glucan linkage-dependent) to tolerate a number of sugar-1- phosphate substrates, albeit with reduced kinetic efficiency. In spite of catalytic efficiencies of <1 % of the natural reactions, we demonstrate the utility of given phosphorylase-sugar phosphate pairs to access new-to-nature fragments of human milk oligosaccharides, or analogues thereof, in multi-milligram quantities.

Enzyme Immobilization in Wall-Coated Flow Microreactors

Flow microreactors are emergent engineering tools for the development of continuous biocatalytic transformations. Exploiting enzymes in continuous mode requires their retention for multiple rounds of conversions. To achieve this goal, immobilizing the enzymes on microchannel walls is a promising approach. However, protein immobilization within closed structures is difficult. Here, we describe a methodology based on the confluent design of enzyme and microreactor; fusion to the silica-binding module Z_basic2 is used to engineer enzymes for high-affinity-oriented attachment to the plain wall surface of glass microchannels. As a practical case, the methodology is described using a sucrose phosphorylase; the assayed reaction is synthesis of α-D-glucose 1-phosphate (αGlc 1-P) from sucrose and phosphate using the immobilized enzyme microreactor. Procedures of enzyme immobilization, reactor characterization, and operation are described. The methodology is applicable for any other enzymes fused to Z_basic2 and silica (glass)-based microfluidic reactors.

Structural Comparison of a Promiscuous and a Highly Specific Sucrose 6F-Phosphate Phosphorylase

In family GH13 of the carbohydrate-active enzyme database, subfamily 18 contains glycoside phosphorylases that act on α-sugars and glucosides. Because their phosphorolysis reactions are effectively reversible, these enzymes are of interest for the biocatalytic synthesis of various glycosidic compounds. Sucrose 6^F-phosphate phosphorylases (SPPs) constitute one of the known substrate specificities. Here, we report the characterization of an SPP from Ilumatobacter coccineus with a far stricter specificity than the previously described promiscuous SPP from Thermoanaerobacterium thermosaccharolyticum. Crystal structures of both SPPs were determined to provide insight into their similarities and differences. The residues responsible for binding the fructose 6-phosphate group in subsite +1 were found to differ considerably between the two enzymes. Furthermore, several variants that introduce a higher degree of substrate promiscuity in the strict SPP from I. coccineus were designed. These results contribute to an expanded structural knowledge of enzymes in subfamily GH13_18 and facilitate their rational engineering.

Chemoenzymatic Approach toward the Synthesis of 3-O-(α/β)-Glucosylated 3-Hydroxy-β-lactams

Glycosylation significantly alters the biological and physicochemical properties of small molecules. β-Lactam alcohols comprise eligible substrates for such a transformation based on their distinct relevance in the chemical and medicinal community. In this framework, the unprecedented enzymatic glycosylation of the rigid and highly strained four-membered β-lactam azaheterocycle was studied. For this purpose, cis-3-hydroxy-β-lactams were efficiently prepared in three steps by means of a classical organic synthesis approach, while a biocatalytic step was implemented for the selective formation of the corresponding 3-O-α- and -β-glucosides, hence overcoming the complexities typically encountered in synthetic glycochemistry and contributing to the increasing demand for sustainable processes in the framework of green chemistry. Two carbohydrate-active enzymes were selected based on their broad acceptor specificity and subsequently applied for the α- or β-selective formation of β-lactam-sugar adducts, using sucrose as a glucosyl donor.

Demystifying the Flow: Biocatalytic Reaction Intensification in Microstructured Enzyme Reactors

Continuous (flow) reactors have drawn a wave of renewed interest in biocatalysis. Many studies find that the flow reactor offers enhanced conversion efficiency. What the reported reaction intensification actually consists in, however, often remains obscure. Here, a canonical microreactor design for heterogeneously catalyzed continuous biotransformations, featuring flow microchannels that contain the enzyme immobilized on their wall surface are examined. Glycosylations by sucrose phosphorylase are used to assess the potential for reaction intensification due to microscale effects. Key variables are identified, and their corresponding relationship equations, to describe, and optimize, the interplay between reaction characteristics, microchannel geometry and reactor operation. The maximum space-time-yield (STY_max) scales directly with the enzyme activity immobilized on the available wall surface. Timescale analysis, comparing the characteristic times of reaction (τ_reac ) and diffusion (τ_diff ) to the mean residence time (τ_res ), reveals operational conditions for optimum reactor output. Theoretical insight into determinants of microreactor performance is applied to biocatalytic syntheses of α-d-glucose 1-phosphate and α-glucosyl glycerol. Process boundaries for enzyme showing, respectively, high (80 U mg^-1 ) and low (4 U mg^-1 ) specific activities are thus established and options for process design revealed. Opportunities, and limitations, of the application of principles of microscale flow chemistry to biocatalytic transformations are made evident.

Unravelling the Specificity of Laminaribiose Phosphorylase from Paenibacillus sp. YM-1 towards Donor Substrates Glucose/Mannose 1-Phosphate by Using X-ray Crystallography and Saturation Transfer Difference NMR Spectroscopy

Glycoside phosphorylases (GPs) carry out a reversible phosphorolysis of carbohydrates into oligosaccharide acceptors and the corresponding sugar 1-phosphates. The reversibility of the reaction enables the use of GPs as biocatalysts for carbohydrate synthesis. Glycosyl hydrolase family 94 (GH94), which only comprises GPs, is one of the most studied GP families that have been used as biocatalysts for carbohydrate synthesis, in academic research and in industrial production. Understanding the mechanism of GH94 enzymes is a crucial step towards enzyme engineering to improve and expand the applications of these enzymes in synthesis. In this work with a GH94 laminaribiose phosphorylase from Paenibacillus sp. YM-1 (PsLBP), we have demonstrated an enzymatic synthesis of disaccharide 1 (β-d-mannopyranosyl-(1→3)-d-glucopyranose) by using a natural acceptor glucose and noncognate donor substrate α-mannose 1-phosphate (Man1P). To investigate how the enzyme recognises different sugar 1-phosphates, the X-ray crystal structures of PsLBP in complex with Glc1P and Man1P have been solved, providing the first molecular detail of the recognition of a noncognate donor substrate by GPs, which revealed the importance of hydrogen bonding between the active site residues and hydroxy groups at C2, C4, and C6 of sugar 1-phosphates. Furthermore, we used saturation transfer difference NMR spectroscopy to support crystallographic studies on the sugar 1-phosphates, as well as to provide further insights into the PsLBP recognition of the acceptors and disaccharide products.

Production of the compatible solute α-D-glucosylglycerol by metabolically engineered Corynebacterium glutamicum

Background

α-d-Glucosylglycerol (αGG) has beneficial functions as a moisturizing agent in cosmetics and potential as a health food material, and therapeutic agent. αGG serves as compatible solute in various halotolerant cyanobacteria such as Synechocystis sp. PCC 6803, which synthesizes αGG in a two-step reaction: The enzymatic condensation of ADP-glucose and glycerol 3-phosphate by GG-phosphate synthase (GGPS) is followed by the dephosphorylation of the intermediate by the GG-phosphate phosphatase (GGPP). The Gram-positive Corynebacterium glutamicum, an industrial workhorse for amino acid production, does not utilize αGG as a substrate and was therefore chosen for the development of a heterologous microbial production platform for αGG.

Results

Plasmid-bound expression of ggpS and ggpP from Synechocystis sp. PCC 6803 enabled αGG synthesis exclusively in osmotically stressed cells of C. glutamicum (pEKEx2-ggpSP), which is probably due to the unique intrinsic control mechanism of GGPS activity in response to intracellular ion concentrations. C. glutamicum was then engineered to optimize precursor supply for αGG production: The precursor for αGG synthesis ADP-glucose gets metabolized by both the glgA encoded glycogen synthase and the otsA encoded trehalose-6-phosphate synthase. Upon deletion of both genes the αGG concentration in culture supernatants was increased from 0.5 mM in C. glutamicum (pEKEx3-ggpSP) to 2.9 mM in C. glutamicum ΔotsA IMglgA (pEKEx3-ggpSP). Upon nitrogen limitation, which inhibits synthesis of amino acids as compatible solutes, C. glutamicum ΔotsA IMglgA (pEKEx3-ggpSP) produced more than 10 mM αGG (about 2 g L⁻¹).

Conclusions

Corynebacterium glutamicum can be engineered as efficient platform for the production of the compatible solute αGG. Redirection of carbon flux towards αGG synthesis by elimination of the competing pathways for glycogen and trehalose synthesis as well as optimization of nitrogen supply is an efficient strategy to further optimize production of αGG.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0939-2) contains supplementary material, which is available to authorized users.

Switching enzyme specificity from phosphate to resveratrol glucosylation

Here we present a point mutation-triggered domain shift which switches the acceptor preference of a sucrose phosphorylase from phosphate to a variety of large polyphenolic compounds including resveratrol and quercetin, enabling their efficient glucosylation. The variant possesses a high affinity for aromatic substrates due to newly introduced π-π- and hydrophobic interactions in the altered active site. The domain shift brings about a substantially enlarged and multifunctional active site for polyphenol glucosylation and rare disaccharide production. The crystal structure of the variant with its product resveratrol-3-α-d-glucoside allows the prediction of the substrate scope and regioselectivity of the aromatic compounds' glucosylation sites.

Valorizing Dairy Waste: Thermophilic Biosynthesis of a Novel Ascorbic Acid Derivative

l-Ascorbic acid (l-AA) is an essential nutrient that is extremely unstable and cannot be synthesized by the human body. Therefore, attempts have been performed to develop biologically active l-AA derivatives with improved stability. This work presents a facile, scalable, and efficient enzymatic transgalactosylation of lactose to l-AA using β-glucosidase (TN0602) from Thermotoga naphthophila RKU-10. β-Glucosidase TN0602 displays high transgalactosylation activity at pH 5.0, 75 °C, and l-AA/lactose ratio of 2:1 to form a novel l-AA derivative [2-O-β-d-galactopyranosyl-l-ascorbic acid (l-AA-Gal)] with a maximal productivity of 138.88 mmol L^-1 in 12 h, which is higher than most reports of enzymatic synthesis of l-AA-α-glucoside. Synthetic l-AA-Gal retains most l-AA antioxidant capability and presents dramatically higher stability than l-AA in an oxidative environment (Cu²⁺). In conclusion, this work reports a new way to valorize dairy waste lactose into a novel molecule l-AA-Gal, which could be a promising l-AA derivative to be used in a wide range of applications.

Glucosylglycerate Phosphorylase, an Enzyme with Novel Specificity Involved in Compatible Solute Metabolism

Family GH13_18 of the carbohydrate-active enzyme database consists of retaining glycoside phosphorylases that have attracted interest with their potential for synthesizing valuable α-sugars and glucosides. Sucrose phosphorylase was believed to be the only enzyme with specificity in this subfamily for many years, but recent work revealed an enzyme with a different function and hinted at an even broader diversity that is left to discover. In this study, a putative sucrose phosphorylase from Meiothermus silvanus that resides in a previously unexplored branch of the family's phylogenetic tree was expressed and characterized. Unexpectedly, no activity on sucrose was observed. Guided by a thorough inspection of the genomic landscape surrounding other genes in the branch, the enzyme was found to be a glucosylglycerate phosphorylase, with a specificity never before reported. Homology modeling, docking, and mutagenesis pinpointed particular acceptor site residues (Asn275 and Glu383) involved in the binding of glycerate. Various organisms known to synthesize and accumulate glucosylglycerate as a compatible solute possess a putative glucosylglycerate phosphorylase gene, indicating that the phosphorylase may be a regulator of its intracellular levels. Moreover, homologs of this novel enzyme appear to be distributed among diverse bacterial phyla, a finding which suggests that many more organisms may be capable of assimilating or synthesizing glucosylglycerate than previously assumed.IMPORTANCE Glycoside phosphorylases are an intriguing group of carbohydrate-active enzymes that have been used for the synthesis of various economically appealing glycosides and sugars, and they are frequently subjected to enzyme engineering to further expand their application potential. The novel specificity discovered in this work broadens the diversity of these phosphorylases and opens up new possibilities for the efficient production of glucosylglycerate, which is a remarkably potent and versatile stabilizer for protein formulations. Finally, it is a new piece of the puzzle of glucosylglycerate metabolism, being the only known enzyme capable of catalyzing the breakdown of glucosylglycerate in numerous bacterial phyla.

Active-Site His85 of Pasteurella dagmatis Sialyltransferase Facilitates Productive Sialyl Transfer and So Prevents Futile Hydrolysis of CMP-Neu5Ac

Sialyltransferases of the GT-80 glycosyltransferase family are considered multifunctional because of the array of activities detected. They exhibit glycosyl transfer, trans-sialylation, and hydrolysis activities. How these enzymes utilize their active-site residues in balancing the different enzymatic activities is not well understood. In this study of Pasteurella dagmatis α2,3sialyltransferase, we show that the conserved His85 controls efficiency and selectivity of the sialyl transfer. A His85→Asn variant was 200 times less efficient than wild-type for sialylation of lactose, and exhibited relaxed site selectivity to form not only the α2,3- but also the α2,6-sialylated product (21 %). The H85N variant was virtually inactive in trans-sialylation but showed almost the same CMP-Neu5Ac hydrolase activity as wild-type. The competition between sialyl transfer and hydrolysis in the conversion of CMP-Neu5Ac was dependent on the lactose concentration; this was characterized by a kinetic partition ratio of 85 m^-1 for the H85N variant, compared to 17 000 m^-1 for the wild-type enzyme. His85 promotes the productive sialyl transfer to lactose and so prevents hydrolysis of CMP-Neu5Ac in the reaction.

Walking a Fine Line with Sucrose Phosphorylase: Efficient Single-Step Biocatalytic Production of l-Ascorbic Acid 2-Glucoside from Sucrose

The 2-O-α-d-glucoside of l-ascorbic acid (AA-2G) is a highly stabilized form of vitamin C, with important industrial applications in cosmetics, food, and pharmaceuticals. AA-2G is currently produced through biocatalytic glucosylation of l-ascorbic acid from starch-derived oligosaccharides. Sucrose would be an ideal substrate for AA-2G synthesis, but it lacks a suitable transglycosidase. We show here that in a narrow pH window (pH 4.8-6.0, with sharp optimum at pH 5.2), sucrose phosphorylases catalyzed the 2-O-α-glucosylation of l-ascorbic acid from sucrose with high efficiency and perfect site-selectivity. Optimized synthesis with the enzyme from Bifidobacterium longum at 40 °C gave a concentrated product (155 g L^-1 ; 460 mm), from which pure AA-2G was readily recovered in ∼50 % overall yield, thus providing the basis for advanced production. The peculiar pH dependence is suggested to arise from a "reverse-protonation" mechanism in which the catalytic base Glu232 on the glucosyl-enzyme intermediate must be protonated for attack on the anomeric carbon from the 2-hydroxyl of the ionized l-ascorbate substrate.