Loop Protein Engineering for Improved Transglycosylation Activity of a β‐ N ‐Acetylhexosaminidase

Synthesis of fucosyllactose using α-L-fucosidases GH29 from infant gut microbial metagenome

Fucosyl-oligosaccharides (FUS) provide many health benefits to breastfed infants, but they are almost completely absent from bovine milk, which is the basis of infant formula. Therefore, there is a growing interest in the development of enzymatic transfucosylation strategies for the production of FUS. In this work, the α-L-fucosidases Fuc2358 and Fuc5372, previously isolated from the intestinal bacterial metagenome of breastfed infants, were used to synthesize fucosyllactose (FL) by transfucosylation reactions using p-nitrophenyl-α-L-fucopyranoside (pNP-Fuc) as donor and lactose as acceptor. Fuc2358 efficiently synthesized the major fucosylated human milk oligosaccharide (HMO) 2'-fucosyllactose (2'FL) with a 35% yield. Fuc2358 also produced the non-HMO FL isomer 3'-fucosyllactose (3'FL) and traces of non-reducing 1-fucosyllactose (1FL). Fuc5372 showed a lower transfucosylation activity compared to Fuc2358, producing several FL isomers, including 2'FL, 3'FL, and 1FL, with a higher proportion of 3'FL. Site-directed mutagenesis using rational design was performed to increase FUS yields in both α-L-fucosidases, based on structural models and sequence identity analysis. Mutants Fuc2358-F184H, Fuc2358-K286R, and Fuc5372-R230K showed a significantly higher ratio between 2'FL yields and hydrolyzed pNP-Fuc than their respective wild-type enzymes after 4 h of transfucosylation. The results with the Fuc2358-F184W and Fuc5372-W151F mutants showed that the residues F184 of Fuc2358 and W151 of Fuc5372 could have an effect on transfucosylation regioselectivity. Interestingly, phenylalanine increases the selectivity for α-1,2 linkages and tryptophan for α-1,3 linkages. These results give insight into the functionality of the active site amino acids in the transfucosylation activity of the GH29 α-L-fucosidases Fuc2358 and Fuc5372. KEY POINTS: Two α-L-fucosidases from infant gut bacterial microbiomes can fucosylate glycans Transfucosylation efficacy improved by tailored point-mutations in the active site F184 of Fuc2358 and W151 of Fuc5372 seem to steer transglycosylation regioselectivity.

Characterization of a GH20 β-N-Acetylhexosaminidase from Flavobacterium algicola Suitable to Synthesize Lacto-N-triose II

β-N-Acetylhexosaminidases have attracted much attention in the enzymatic synthesis of lacto-N-triose II (LNT2) as a backbone precursor of human milk oligosaccharides (HMOs). In this study, a novel glycoside hydrolase (GH) 20 family β-N-acetylhexosaminidase, FlaNag2353, from Flavobacterium algicola was biochemically characterized and applied to synthesize LNT2. FlaNag2353 displayed optimal activity to p-nitrophenyl N-acetyl-β-d-glucosaminide (pNP-GlcNAc) at 40 °C and pH 8.0. In addition to its excellent hydrolysis activity toward pNP-GlcNAc and chitooligosaccharides, FlaNag2353 showed trans-glycosylation activity. Under conditions of pH 9.0 and 55 °C for 2 h and utilizing 200 mM lactose and 10 mM pNP-GlcNAc, FlaNag2353 synthesized LNT2 with a conversion ratio of 4.15% calculated from pNP-GlcNAc. Moreover, when applied to LNT2 synthesis with 10 mM pNP-GlcNAc and 9.7% (w/v) industrial waste whey powder, FlaNag2353 achieved a conversion ratio of 2.39%. This study has significant implications for broadening the applications of GH20 β-N-acetylhexosaminidases and promoting the high-value utilization of whey powder.

Production of octyl butyrate using psychrophilic mutant lipase from Croceibacter atlanticus LipCA lipase developed by a molecular evolution technique

Lipases are used to synthesize a variety of industrially useful compounds. Among them, psychrophilic lipase can be used to synthesize thermo-labile compounds at low temperatures. In this study, random mutagenesis was introduced into Antarctic Croceibacter atlanticus lipase gene using error-prone PCR, resulting in changes in its protein sequence. Through two rounds of mutagenesis and screening, we found that a mutant R1 showed an enhanced activity at low temperatures. Mutant R1 had five mutations (F43L, S48G, S49G, D141K, and K297R) and higher kcat/KM value than the wild type (WT) at 10 °C. We immobilized this enzyme on methacrylate divinylbenzene resin and used it to synthesize octyl butyrate, a flavor compound. The esterification reaction proceeded even at 10 °C. Mutant R1 synthesized the ester compound faster than the WT. To determine which amino acids were responsible for the increase of activity, site-directed mutagenesis was performed to introduce five back mutations into mutant R1. Three back mutants (L43F, G48S, G49S) showed significant decreases of activity at low temperatures, indicating that these amino acids were closely related to the increase in activity. This psychrophilic mutant R1 is expected to be used in low-temperature enzyme conversion reactions in the food industry.

N-acetyl-d-glucosamine-based oligosaccharides from chitin: Enzymatic production, characterization and biological activities

Chitin, the second most abundant biopolymer, possesses diverse applications in the food, agricultural, and pharmaceutical industries due to its functional properties. However, the potential applications of chitin are limited owing to its high crystallinity and low solubility. N-acetyl chitooligosaccharides and lacto-N-triose II, the two types of GlcNAc-based oligosaccharides, can be obtained from chitin by enzymatic methods. With their lower molecular weights and improved solubility, these two types of GlcNAc-based oligosaccharides display more various beneficial health effects when compared to chitin. Among their abilities, they have exhibited antioxidant, anti-inflammatory, anti-tumor, antimicrobial, and plant elicitor activities as well as immunomodulatory and prebiotic effects, which suggests they have the potential to be utilized as food additives, functional daily supplements, drug precursors, elicitors for plants, and prebiotics. This review comprehensively covers the enzymatic methods used for the two types of GlcNAc-based oligosaccharides production from chitin by chitinolytic enzymes. Moreover, current advances in the structural characterization and biological activities of these two types of GlcNAc-based oligosaccharides are summarized in the review. We also highlight current problems in the production of these oligosaccharides and trends in their development, aiming to offer some directions for producing functional oligosaccharides from chitin.

The Engineered Hexosaminidase TtOGA-D120N Is an Efficient O-/N-/S-Glycoligase That Also Catalyzes Formation and Release of Oxazoline Donors for Cascade Syntheses with Glycosynthases or Transglycosylases

Engineering glycoside hydrolases is a major route to obtaining catalysts forming glycosidic bonds. Glycosynthases, thioglycoligases, and transglycosylases represent the main strategies, each having advantages and drawbacks. Here, we show that an engineered enzyme from the GH84 family, the acid-base mutant TtOGA-D120N, is an efficient O-, N-, and S-glycoligase, able to use Ssp3, Osp3, Nsp2, and Nsp nucleophiles. Moreover, TtOGA-D120N catalyzes the formation and release of N-acetyl-d-glucosamine 1,2-oxazoline, the intermediate of hexosaminidases displaying substrate-assisted catalysis. This release of an activated intermediate allows cascade synthesis by combination with transglycosylases or glycosynthases, here exemplified by synthesis of the human milk oligosaccharide lacto-N-triose II.

Directed evolution of a β-N-acetylhexosaminidase from Haloferula sp. for lacto-N-triose II and lacto-N-neotetraose synthesis from chitin

In our previous study, a β-N-acetylhexosaminidase (HaHex74) from Haloferula sp. showing high human milk oligosaccharides (HMOs) synthesis ability was identified and characterized. In this study, HaHex74 was further engineered by directed evolution and site-saturation mutagenesis to improve its transglycosylation activity for HMOs synthesis. A mutant (mHaHex74) with improved transglycosylation activity (HaHex74-Asn401Ile/His394Leu) was obtained and characterized. mHaHex74 exhibited maximal activity at pH 5.5 and 35 °C, respectively, which were distinct from that of HaHex74 (pH 6.5 and 45 °C). Moreover, mHaHex74 showed the highest LNT2 conversion ratio of 28.2% from N,N'-diacetyl chitobiose (GlcNAc₂), which is 2.2 folds higher than that of HaHex74. A three-enzyme cascade reaction for the synthesis of LNT2 and LNnT from chitin was performed in a 5-L reactor, and the contents of LNT2 and LNnT reached up to 15.0 g L¹ and 4.9 g L¹, respectively. Therefore, mHaHex74 maybe a good candidate for enzymatic synthesis of HMOs.

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.

Mutation Hotspot for Changing the Substrate Specificity of β-N-Acetylhexosaminidase: A Library of GlcNAcases

β-N-Acetylhexosaminidase from Talaromyces flavus (TfHex; EC 3.2.1.52) is an exo-glycosidase with dual activity for cleaving N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc) units from carbohydrates. By targeting a mutation hotspot of the active site residue Glu332, we prepared a library of ten mutant variants with their substrate specificity significantly shifted towards GlcNAcase activity. Suitable mutations were identified by in silico methods. We optimized a microtiter plate screening method in the yeast Pichia pastoris expression system, which is required for the correct folding of tetrameric fungal β-N-acetylhexosaminidases. While the wild-type TfHex is promiscuous with its GalNAcase/GlcNAcase activity ratio of 1.2, the best single mutant variant Glu332His featured an 8-fold increase in selectivity toward GlcNAc compared with the wild-type. Several prepared variants, in particular Glu332Thr TfHex, had significantly stronger transglycosylation capabilities than the wild-type, affording longer chitooligomers - they behaved like transglycosidases. This study demonstrates the potential of mutagenesis to alter the substrate specificity of glycosidases.

Enhancing the Catalytic Activity of Type II L-Asparaginase from Bacillus licheniformis through Semi-Rational Design

Low catalytic activity is a key factor limiting the widespread application of type II L-asparaginase (ASNase) in the food and pharmaceutical industries. In this study, smart libraries were constructed by semi-rational design to improve the catalytic activity of type II ASNase from Bacillus licheniformis. Mutants with greatly enhanced catalytic efficiency were screened by saturation mutations and combinatorial mutations. A quintuple mutant ILRAC was ultimately obtained with specific activity of 841.62 IU/mg and kcat/Km of 537.15 min−1·mM−1, which were 4.24-fold and 6.32-fold more than those of wild-type ASNase. The highest specific activity and kcat/Km were firstly reported in type II ASNase from Bacillus licheniformis. Additionally, enhanced pH stability and superior thermostability were both achieved in mutant ILRAC. Meanwhile, structural alignment and molecular dynamic simulation demonstrated that high structure stability and strong substrate binding were beneficial for the improved thermal stability and enzymatic activity of mutant ILRAC. This is the first time that enzymatic activity of type II ASNase from Bacillus licheniformis has been enhanced by the semi-rational approach, and results provide new insights into enzymatic modification of L-asparaginase for industrial applications.

Improvement of the Transglycosylation Efficiency of a Lacto-N-Biosidase from Bifidobacterium bifidum by Protein Engineering

The lacto-N-biosidase LnbB from Bifidobacterium bifidum JCM 1254 was engineered to improve its negligible transglycosylation efficiency with the purpose of enzymatically synthesizing lacto-N-tetraose (LNT; Gal-β1,3-GlcNAc-β1,3-Gal-β1,4-Glc) in one enzymatic step. LNT is a prebiotic human milk oligosaccharide in itself and constitutes the structural core of a range of more complex human milk oligosaccharides as well. Thirteen different LnbB variants were expressed and screened for transglycosylation activity by monitoring transglycosylation product formation using lacto-N-biose 1,2-oxazoline as donor substrate and lactose as acceptor substrate. LNT was the major reaction product, yet careful reaction analysis revealed the formation of three additional LNT isomers, which we identified to have a β1,2-linkage, a β1,6-linkage, and a 1,1-linkage, respectively, between lacto-N-biose (Gal-β1,3-GlcNAc) and lactose. Considering both maximal transglycosylation yield and regioselectivity as well as minimal product hydrolysis, the best variant was LnbB W394H, closely followed by W465H and Y419N. A high transglycosylation yield was also obtained with W394F, yet the substitution of W394 and W465 of the subsite −1 hydrophobic platform in the enzyme with His dramatically impaired the undesirable product hydrolysis as compared to substitution with Phe; the effect was most pronounced for W465. Using p-nitrophenyl-β-lacto-N-bioside as donor substrate manifested W394 as an important target position. The optimization of the substrate concentrations confirmed that high initial substrate concentration and high acceptor-to-donor ratio both favor transglycosylation.

Reprint of: Advanced glycosidases as ingenious biosynthetic instruments

Until recently, glycosidases, naturally hydrolyzing carbohydrate-active enzymes, have found few synthetic applications in industry, being primarily used for cleaving unwanted carbohydrates. With the establishment of glycosynthase and transglycosidase technology by genetic engineering, the view of glycosidases as industrial biotechnology tools has started to change. Their easy production, affordability, robustness, and substrate versatility, added to the possibility of controlling undesired side hydrolysis by enzyme engineering, have made glycosidases competitive synthetic tools. Current promising applications of engineered glycosidases include the production of well-defined chitooligomers, precious galactooligosaccharides or specialty chemicals such as glycosylated flavonoids. Other synthetic pathways leading to human milk oligosaccharides or remodeled antibodies are on the horizon. This work provides an overview of the synthetic achievements to date for glycosidases, emphasizing the latest trends and outlining possible developments in the field.

Advanced glycosidases as ingenious biosynthetic instruments

Until recently, glycosidases, naturally hydrolyzing carbohydrate-active enzymes, have found few synthetic applications in industry, being primarily used for cleaving unwanted carbohydrates. With the establishment of glycosynthase and transglycosidase technology by genetic engineering, the view of glycosidases as industrial biotechnology tools has started to change. Their easy production, affordability, robustness, and substrate versatility, added to the possibility of controlling undesired side hydrolysis by enzyme engineering, have made glycosidases competitive synthetic tools. Current promising applications of engineered glycosidases include the production of well-defined chitooligomers, precious galactooligosaccharides or specialty chemicals such as glycosylated flavonoids. Other synthetic pathways leading to human milk oligosaccharides or remodeled antibodies are on the horizon. This work provides an overview of the synthetic achievements to date for glycosidases, emphasizing the latest trends and outlining possible developments in the field.

Probing the determinants of the transglycosylation/hydrolysis partition in a retaining α-l-arabinofuranosidase

The use of retaining glycoside hydrolases as synthetic tools for glycochemistry is highly topical and the focus of considerable research. However, due to the incomplete identification of the molecular determinants of the transglycosylation/hydrolysis partition (t/h), rational engineering of retaining glycoside hydrolases to create transglycosylases remains challenging. Therefore, to understand better the factors that underpin transglycosylation in a GH51 retaining α-l-arabinofuranosidase from Thermobacillus xylanilyticus, the investigation of this enzyme's active site was pursued. Specifically, the properties of two mutants, F26L and L352M, located in the vicinity of the active site are described, using kinetic and 3D structural analyses and molecular dynamics simulations. The results reveal that the presence of L352M in the context of a triple mutant (also containing R69H and N216W) generates changes both in the donor and acceptor subsites, the latter being the result of a domino-like effect. Overall, the mutant R69H-N216W-L352M displays excellent transglycosylation activity (70 % yield, 78 % transfer rate and reduced secondary hydrolysis of the product). In the course of this study, the central role played by the conserved R69 residue was also reaffirmed. The mutation R69H affects both the catalytic nucleophile and the acid/base, including their flexibility, and has a determinant effect on the t/h partition. Finally, the results reveal that increased loop flexibility in the acceptor subsites creates new interactions with the acceptor, in particular with a hydrophobic binding platform composed of N216W, W248 and W302.

Carbohydrate-Active enZyme (CAZyme) enabled glycoengineering for a sweeter future

One of the stumbling blocks to advance the field of glycobiology has been the difficulty in synthesis of bespoke carbohydrate-based molecules like glycopolymers (e.g. human milk oligosaccharides) and glycoconjugates (e.g. glycosylated monoclonal antibodies). Recent strides towards using engineered Carbohydrate-Active enZymes (CAZymes) like glycosyl transferases, transglycosidases, and glycosynthases for glycans synthesis has allowed production of diverse glycans. Here, we discuss enzymatic routes for glycans biosynthesis and recent advances in protein engineering strategies that enable improvement of CAZyme specificity and catalytic turnover. We focus on rational and directed evolution methods that have been developed to engineer CAZymes. Finally, we discuss how improved CAZymes have been used in recent years to remodel and synthesize glycans for biotherapeutics and biotechnology related applications.

Comparative Characterization of Aspergillus Pectin Lyases by Discriminative Substrate Degradation Profiling

Fungal genomes often contain several copies of genes that encode carbohydrate active enzymes having similar activity. The copies usually have slight sequence variability, and it has been suggested that the multigenecity represents distinct reaction optima versions of the enzyme. Whether the copies represent differences in substrate attack proficiencies of the enzyme have rarely been considered. The genomes of Aspergillus species encode several pectin lyases (EC 4.2.2.10), which all belong to polysaccharide lyase subfamily PL1_4 in the CAZy database. The enzymes differ in terms of sequence identity and phylogeny, and exhibit structural differences near the active site in their homology models. These enzymes catalyze pectin degradation via eliminative cleavage of the α-(1,4) glycosidic linkages in homogalacturonan with a preference for linkages between methyl-esterified galacturonate residues. This study examines four different pectin lyases (PelB, PelC, PelD, and PelF) encoded by the same Aspergillus sp. (namely A. luchuensis), and further compares two PelA pectin lyases from two related Aspergillus spp. (A. aculeatus and A. tubingensis). We report the phylogeny, enzyme kinetics, and enzymatic degradation profiles of the enzymes' action on apple pectin, citrus pectin, and sugar beet pectin. All the pectin lyases exerted highest reaction rate on apple pectin [degree of methoxylation (DM) 69%, degree of acetylation (DAc) 2%] and lowest reaction rate on sugar beet pectin (DM 56%, DAc 19%). Activity comparison at pH 5-5.5 produced the following ranking: PelB > PelA > PelD > PelF > PelC. The evolution of homogalacturonan-oligomer product profiles during reaction was analyzed by liquid chromatography with mass spectrometry (LC-MS) detection. This analyses revealed subtle differences in the product profiles indicating distinct substrate degradation preferences amongst the enzymes, notably with regard to acetyl substitutions. The LC-MS product profiling analysis thus disclosed that the multigenecity appears to provide the fungus with additional substrate degradation versatility. This product profiling furthermore represents a novel approach to functionally compare pectin-degrading enzymes, which can help explain structure-function relations and reaction properties of disparate copies of carbohydrate active enzymes. A better understanding of the product profiles generated by pectin modifying enzymes has significant implications for targeted pectin modification in food and biorefinery processes.

β-N-Acetylhexosaminidases for Carbohydrate Synthesis via Trans-Glycosylation

β-N-acetylhexosaminidases (EC 3.2.1.52) are retaining hydrolases of glycoside hydrolase family 20 (GH20). These enzymes catalyze hydrolysis of terminal, non-reducing N-acetylhexosamine residues, notably N-acetylglucosamine or N-acetylgalactosamine, in N-acetyl-β-D-hexosaminides. In nature, bacterial β-N-acetylhexosaminidases are mainly involved in cell wall peptidoglycan synthesis, analogously, fungal β-N-acetylhexosaminidases act on cell wall chitin. The enzymes work via a distinct substrate-assisted mechanism that utilizes the 2-acetamido group as nucleophile. Curiously, the β-N-acetylhexosaminidases possess an inherent trans-glycosylation ability which is potentially useful for biocatalytic synthesis of functional carbohydrates, including biomimetic synthesis of human milk oligosaccharides and other glycan-functionalized compounds. In this review, we summarize the reaction engineering approaches (donor substrate activation, additives, and reaction conditions) that have proven useful for enhancing trans-glycosylation activity of GH20 β-N-acetylhexosaminidases. We provide comprehensive overviews of reported synthesis reactions with GH20 enzymes, including tables that list the specific enzyme used, donor and acceptor substrates, reaction conditions, and details of the products and yields obtained. We also describe the active site traits and mutations that appear to favor trans-glycosylation activity of GH20 β-N-acetylhexosaminidases. Finally, we discuss novel protein engineering strategies and suggest potential “hotspots” for mutations to promote trans-glycosylation activity in GH20 for efficient synthesis of specific functional carbohydrates and other glyco-engineered products.

Lacto-N-tetraose synthesis by wild-type and glycosynthase variants of the β-N-hexosaminidase from Bifidobacterium bifidum

Lacto-N-biose 1,2-oxazoline was prepared chemo-enzymatically and shown to be a donor substrate for β-1,3-glycosylation of lactose by the wild-type and glycosynthase variants (D320E, D320A, Y419F) of Bifidobacterium bifidum β-N-hexosaminidase. Lacto-N-tetraose, a core structure of human milk oligosaccharides, was formed in 20-60% yield of donor substrate (up to 8 mM product titre), depending on the degree of selectivity control by the enzyme used.

Converting a β-N-acetylhexosaminidase into two trans-β-N-acetylhexosaminidases by domain-targeted mutagenesis

We have recently derived a β-N-acetylhexosaminidase, BbhI, from Bifidobacterium bifidum JCM 1254, which could regioselectively synthesize GlcNAcβ1-3Galβ1-4Glc with a yield of 44.9%. Here, directed evolution of BbhI by domain-targeted mutagenesis was carried out. Firstly, the GH20 domain was selected for random mutagenesis using MEGAWHOP method and a small library of 1300 clones was created. A total of 734 colonies with reduced hydrolytic activity were isolated, and three mutants with elevated transglycosylation yields, GlcNAcβ1-3Galβ1-4Glc yields of 68.5%, 74.7%, and 81.1%, respectively, were obtained. Subsequently, nineteen independent mutants were constructed according to all the mutation sites in these three mutants. After transglycosylation analysis, Asp714 and Trp773 were identified as key residues for improvement in transglycosylation ability and were chosen for the second round of directed evolution by site-saturation mutagenesis. Two most efficient mutants D714T and W773R that acted as trans-β-N-acetylhexosaminidase were finally achieved. D714T with the substitution at the putative nucleophile assistant residue Asp714 by threonine showed high yield of 84.7% with unobserved hydrolysis towards transglycosylation product. W773R with arginine substitution at Trp773 residue locating at the entrance of catalytic cavity led to the yield up to 81.8%. The k_cat/K_m values of D714T and W773R for hydrolysis of pNP-β-GlcNAc displayed drastic decreases. NMR investigation of protein-substrate interaction revealed an invariable mode of the catalytic cavity of D714T, W773R, and WT BbhI. The collective motions of protein model showed the mutations Thr714 and Arg773 exerted little effect on the dynamics of the inside but a large effect on the dynamics of the outside of catalytic cavity.

β-N-Acetylhexosaminidases-the wizards of glycosylation

β-N-Acetylhexosaminidases (EC 3.2.1.52) are a unique family of glycoside hydrolases with dual substrate specificity and a particular reaction mechanism. Though hydrolytic enzymes per se, their good stability, easy recombinant production, absolute stereoselectivity, and a broad substrate specificity predestine these enzymes for challenging applications in carbohydrate synthesis. This mini-review aims to demonstrate the catalytic potential of β-N-acetylhexosaminidases in a range of unusual reactions, processing of unnatural substrates, formation of unexpected products, and demanding reaction designs. The use of unconventional media can considerably alter the progress of transglycosylation reactions. By means of site-directed mutagenesis, novel catalytic machineries can be constructed. Glycosylation of difficult substrates such as sugar nucleotides was accomplished, and the range of afforded glycosidic bonds comprises unique non-reducing sugars. Specific functional groups may be tolerated in the substrate molecule, which makes β-N-acetylhexosaminidases invaluable allies in difficult synthetic problems.

Synthesis of Human Milk Oligosaccharides: Protein Engineering Strategies for Improved Enzymatic Transglycosylation

Human milk oligosaccharides (HMOs) signify a unique group of oligosaccharides in breast milk, which is of major importance for infant health and development. The functional benefits of HMOs create an enormous impetus for biosynthetic production of HMOs for use as additives in infant formula and other products. HMO molecules can be synthesized chemically, via fermentation, and by enzymatic synthesis. This treatise discusses these different techniques, with particular focus on harnessing enzymes for controlled enzymatic synthesis of HMO molecules. In order to foster precise and high-yield enzymatic synthesis, several novel protein engineering approaches have been reported, mainly concerning changing glycoside hydrolases to catalyze relevant transglycosylations. The protein engineering strategies for these enzymes range from rationally modifying specific catalytic residues, over targeted subsite -1 mutations, to unique and novel transplantations of designed peptide sequences near the active site, so-called loop engineering. These strategies have proven useful to foster enhanced transglycosylation to promote different types of HMO synthesis reactions. The rationale of subsite -1 modification, acceptor binding site matching, and loop engineering, including changes that may alter the spatial arrangement of water in the enzyme active site region, may prove useful for novel enzyme-catalyzed carbohydrate design in general.

A chemo-enzymatic approach for the synthesis of human milk oligosaccharide backbone structures

The ability of an engineered β-N-acetylhexosaminidase to utilize a reactive oxazoline as donor molecule for transglycosylation reaction to synthesize human milk oligosaccharide backbone structures was studied. The human milk oligosaccharide precursor lacto-N-triose II and three regioisomers could be synthesized using the oxazoline, which was either in situ-generated resulting in a chemo-enzymatic sequential cascade or was used as a purified compound. The highest observed concentration of overall transglycosylation products in a cascade reaction was 13.7 mM after 18.5 h, whereas the use of purified oxazoline resulted in 25.0 mM of transglycosylation products after 6.5 h. Remarkably, the in situ-generated oxazoline could be used without any further purification and it was shown that the used enzyme tolerated significant amounts of reagents such as triethylamine, which is reported for the first time for an enzyme from the glycoside hydrolase family 20.