Selective ß-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 ß-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 ß-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 ß-mono-glucoside (≥95% yield; ∼9 mM) suitable for biological studies.

Enzymes revolutionize the bioproduction of value-added compounds: From enzyme discovery to special applications.

Competitive sustainable production in industry demands new and better biocatalysts, optimized bioprocesses and cost-effective product recovery. Our review sheds light on the progress made for the individual steps towards these goals, starting with the discovery of new enzymes and their corresponding genes. The enzymes are subsequently engineered to improve their performance, combined in reaction cascades to expand the reaction scope and integrated in whole cells to provide an optimal environment for the bioconversion. Strain engineering using synthetic biology methods tunes the host for production, reaction design optimizes the reaction conditions and downstream processing ensures the efficient recovery of commercially viable products. Selected examples illustrate how modified enzymes can revolutionize future-oriented applications ranging from the bioproduction of bulk-, specialty- and fine chemicals, active pharmaceutical ingredients and carbohydrates, over the conversion of the greenhouse-gas CO2 into valuable products and biocontrol in agriculture, to recycling of synthetic polymers and recovery of precious metals.

Correction: Lacto-N-tetraose synthesis by wild-type and glycosynthase variants of the ß-N-hexosaminidase from Bifidobacterium bifidum.

Correction for 'Lacto-N-tetraose synthesis by wild-type and glycosynthase variants of the ß-N-hexosaminidase from Bifidobacterium bifidum' by Katharina Schmölzer et al., Org. Biomol. Chem., 2019, DOI: 10.1039/c9ob00424f.

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

Lacto-N-biose 1,2-oxazoline was prepared chemo-enzymatically and shown to be a donor substrate for ß-1,3-glycosylation of lactose by the wild-type and glycosynthase variants (D320E, D320A, Y419F) of Bifidobacterium bifidum ß-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.

Glycosyltransferase cascades made fit for chemical production: Integrated biocatalytic process for the natural polyphenol C-glucoside nothofagin.

Glycosyltransferase cascades are promising tools of biocatalysis for natural product glycosylation, but their suitability for actual production remains to be shown. Here, we demonstrate at a scale of 100 g isolated product the integrated biocatalytic production of nothofagin, the natural 3'-C-ß-D-glucoside of the polyphenol phloretin. A parallel reaction cascade involving coupled C-glucosyltransferase and sucrose synthase was optimized for the one-pot glucosylation of phloretin from sucrose via an UDP/UDP-glucose shuttle. Inclusion complexation with the highly water soluble 2-hydroxypropyl-ß-cyclodextrin pushed the phloretin solubility to its upper practical limit (∼120 mM) and so removed the main bottleneck on an efficient synthesis of nothofagin. The biotransformation thus intensified had excellent performance metrics of 97% yield and ∼50 gproduct /L at a space-time yield of 3 g/L/hr. The UDP-glucose was regenerated up to ∼220 times. A scalable downstream process for efficient recovery of nothofagin (≥95% purity; ≥65% yield) was developed. A tailored anion-exchange chromatography at pH 8.5 was used for capture and initial purification of the product. Recycling of the 2-hydroxypropyl-ß-cyclodextrin would also be possible at this step. Product precipitation at a lowered pH of 6.0 and re-dissolution in acetone effectively replaced desalting by size exclusion chromatography in the final step of nothofagin purification. This study therefore, reveals the potential for process intensification in the glycosylation of polyphenol acceptors by glycosyltransferase cascades. It demonstrates that, with up- and downstream processing carefully optimized and suitably interconnected, a powerful biocatalytic technology becomes available for the production of an important class of glycosides difficult to prepare otherwise.

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.

Integrated process design for biocatalytic synthesis by a Leloir Glycosyltransferase: UDP-glucose production with sucrose synthase.

Nucleotide sugar-dependent ("Leloir") glycosyltransferases (GTs), represent a new paradigm for the application of biocatalytic glycosylations to the production of fine chemicals. However, it remains to be shown that GT processes meet the high efficiency targets of industrial biotransformations. We demonstrate in this study of uridine-5'-diphosphate glucose (UDP-glc) production by sucrose synthase (from Acidithiobacillus caldus) that a holistic process design, involving coordinated development of biocatalyst production, biotransformation, and downstream processing (DSP) was vital for target achievement at ∼100 g scale synthesis. Constitutive expression in Escherichia coli shifted the recombinant protein production mainly to the stationary phase and enhanced the specific enzyme activity to a level (∼480 U/gcell dry weight ) suitable for whole-cell biotransformation. The UDP-glc production had excellent performance metrics of ∼100 gproduct /L, 86% yield (based on UDP), and a total turnover number of 103 gUDP-glc /gcell dry weight at a space-time yield of 10 g/L/h. Using efficient chromatography-free DSP, the UDP-glc was isolated in a single batch with ≥90% purity and in 73% isolated yield. Overall, the process would allow production of ∼0.7 kg of isolated product/L E. coli bioreactor culture, thus demonstrating how integrated process design promotes the practical use of a GT conversion. Biotechnol. Bioeng. 2017;114: 924-928. © 2016 Wiley Periodicals, Inc.

Combining expression and process engineering for high-quality production of human sialyltransferase in Pichia pastoris.

The human ß-galactoside α2,6-sialyltransferase I, ST6Gal-I has drawn considerable interest for its use as biocatalyst for in-vitro glycoengineering of recombinantly produced therapeutic proteins. By attaching sialic acid onto the terminal galactoses of biantennary protein N-glycans, ST6Gal-I facilitates protein remodeling towards a humanized glycosylation and thus optimized efficacy in pharmacological use. Secreted expression of ST6Gal-I in Pichia pastoris is promising, but proteolysis restricts both the yield and the quality of the enzyme produced. Focusing on an N-terminally truncated (Δ108) variant of ST6Gal-I previously shown to represent a minimally sized, still active form of ST6Gal-I, we show here that protein expression engineering and optimization of bioreactor cultivation of P. pastoris KM71H (pPICZαB) synergized to enhance the maximum enzyme titer about 57-fold to 17units/L. N-Terminal fusion to the Flag-tag plus deletion of a potential proteolytic site (Lys(114)-Asn→Gln(114)-Asn) improved the intrinsic resistance of Δ108ST6Gal-I to degradation in P. pastoris culture. A mixed glycerol/methanol feeding protocol for P. pastoris growth and induction was key for enzyme production in high yield and quality. The sialyltransferase was recovered from the bioreactor culture in a yield of 70% using a single step of anion-exchange chromatography. Its specific activity was 0.05units/mg protein.

Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development.

Sucrose synthase (SuSy, EC 2.4.1.13) is a glycosyltransferase (GT) long known from plants and more recently discovered in bacteria. The enzyme catalyzes the reversible transfer of a glucosyl moiety between fructose and a nucleoside diphosphate (NDP) (sucrose+NDP↔NDP-glucose+fructose). The equilibrium for sucrose conversion is pH dependent, and pH values between 5.5 and 7.5 promote NDP-glucose formation. The conversion of a bulk chemical to high-priced NDP-glucose in a one-step reaction provides the key aspect for industrial interest. NDP-sugars are important as such and as key intermediates for glycosylation reactions by highly selective Leloir GTs. SuSy has gained renewed interest as industrially attractive biocatalyst, due to substantial scientific progresses achieved in the last few years. These include biochemical characterization of bacterial SuSys, overproduction of recombinant SuSys, structural information useful for design of tailor-made catalysts, and development of one-pot SuSy-GT cascade reactions for production of several relevant glycosides. These advances could pave the way for the application of Leloir GTs to be used in cost-effective processes. This review provides a framework for application requirements, focusing on catalytic properties, heterologous enzyme production and reaction engineering. The potential of SuSy biocatalysis will be presented based on various biotechnological applications: NDP-sugar synthesis; sucrose analog synthesis; glycoside synthesis by SuSy-GT cascade reactions.

All-in-one assay for ß-d-galactoside sialyltransferases: Quantification of productive turnover, error hydrolysis, and site selectivity.

Sialyltransferases are important enzymes of glycobiology and the related biotechnologies. The development of sialyltransferases calls for access to quick, inexpensive, and robust analytical tools. We have established an assay for simultaneous characterization of sialyltransferase activity, error hydrolysis, and site selectivity. The described assay does not require expensive substrates, is very sensitive (limit of detection=0.3 µU), and is easy to perform. It is based on sialylation of nitrophenyl galactosides; the products thereof are separated and quantified by ion pair reversed phase high-performance liquid chromatography with ultraviolet detection.

Complete switch from α-2,3- to α-2,6-regioselectivity in Pasteurella dagmatis ß-D-galactoside sialyltransferase by active-site redesign.

Structure-guided active-site redesign of a family GT-80 ß-D-galactoside sialyltransferase (from Pasteurella dagmatis) to change enzyme regioselectivity from α-2,3 in the wild type to α-2,6 in a P7H-M117A double mutant is reported. Biochemical data for sialylation of lactose together with protein crystal structures demonstrate highly precise enzyme engineering.

High-quality production of human α-2,6-sialyltransferase in Pichia pastoris requires control over N-terminal truncations by host-inherent protease activities.

BACKGROUND: α-2,6-sialyltransferase catalyzes the terminal step of complex N-glycan biosynthesis on human glycoproteins, attaching sialic acid to outermost galactosyl residues on otherwise fully assembled branched glycans. This "capping" of N-glycans is critical for therapeutic efficacy of pharmaceutical glycoproteins, making the degree of sialylation an important parameter of glycoprotein quality control. Expression of recombinant glycoproteins in mammalian cells usually delivers heterogeneous N-glycans, with a minor degree of sialylation. In-vitro chemo-enzymatic glycoengineering of the N-glycans provides an elegant solution to increase the degree of sialylation for analytical purposes but also possibly for modification of therapeutic proteins. RESULTS: Human α-2,6-sialyltransferase (ST6Gal-I) was secretory expressed in P.pastoris KM71H. ST6Gal-I featuring complete deletion of both the N-terminal cytoplasmic tail and the transmembrane domain, and also partial truncation of the stem region up to residue 108 were expressed N-terminally fused to a His or FLAG-Tag. FLAG-tagged proteins proved much more resistant to proteolysis during production than the corresponding His-tagged proteins. Because volumetric transferase activity measured on small-molecule and native glycoprotein acceptor substrates did not correlate to ST6Gal-I in the supernatant, enzymes were purified and characterized in their action on non-sialylated protein-linked and released N-glycans, and the respective N-terminal sequences were determined by automated Edman degradation. Irrespective of deletion construct used (Δ27, Δ48, Δ62, Δ89), isolated proteins showed N-terminal processing to a highly similar degree, with prominent truncations at residue 108 - 114, whereby only Δ108ST6Gal-I retained activity. FLAG-tagged Δ108ST6Gal-I was therefore produced and obtained with a yield of 4.5 mg protein/L medium. The protein was isolated and shown by MS to be intact. Purified enzyme exhibited useful activity (0.18 U/mg) for sialylation of different substrates. CONCLUSIONS: Functional expression of human ST6Gal-I as secretory protein in P.pastoris necessitates that N-terminal truncations promoted by host-inherent proteases be tightly controlled. N-terminal FLAG-Tag contributes extra stability to the N-terminal region as compared to N-terminal His-Tag. Proteolytic degradation proceeds up to residues 108 - 114 and of the resulting short-form variants, only Δ108ST6Gal-I seems to be active. FLAG-Δ108ST6Gal-I transfers sialic acids to monoclonal antibody substrate with sufficient yields, and because it is stably produced in P.pastoris, it is identified here as an interesting glycoengineering catalyst.

Mechanistic study of CMP-Neu5Ac hydrolysis by α2,3-sialyltransferase from Pasteurella dagmatis.

Bacterial sialyltransferases of the glycosyltransferase family GT-80 exhibit pronounced hydrolase activity toward CMP-activated sialyl donor substrates. Using in situ proton NMR, we show that hydrolysis of CMP-Neu5Ac by Pasteurella dagmatis α2,3-sialyltransferase (PdST) occurs with axial-to-equatorial inversion of the configuration at the anomeric center to release the α-Neu5Ac product. We propose a catalytic reaction through a single displacement-like mechanism where water replaces the sugar substrate as a sialyl group acceptor. PdST variants having His(284) in the active site replaced by Asn, Asp or Tyr showed up to 10(4)-fold reduced activity, but catalyzed CMP-Neu5Ac hydrolysis with analogous inverting stereochemistry. The proposed catalytic role of His(284) in the PdST hydrolase mechanism is to facilitate the departure of the CMP leaving group.

Characterization of a multifunctional α2,3-sialyltransferase from Pasteurella dagmatis.

A new multifunctional α2,3-sialyltransferase has been discovered in Pasteurella dagmatis. The enzyme, in short PdST, was identified from the P. dagmatis genome by sequence similarity with sialyltransferases of glycosyltransferase family GT-80. In addition to its regioselective sialyltransferase activity (5.9 U/mg; pH 8.0), purified PdST is alternatively active at low pH as α2,3-sialidase (0.5 U/mg; pH 4.5) and α2,3-trans-sialidase (1.0 U/mg; pH 4.5). It also shows cytidine-5'-monophosphate N-acetyl-neuraminic (CMP-Neu5Ac) hydrolase activity (3.7 U/mg; pH 8.0) when no sialyl acceptor substrate is present in the reaction. After sialyltransferase PmST1 from P. multocida, PdST is the second member of family GT-80 to display this remarkable catalytic promiscuity. A unique feature of PdST, however, is a naturally occurring Ser-to-Thr substitution within a highly conserved Y(112)DDGS(116) sequence motif. In PmST1, the equivalent Ser(143) is involved in binding of the CMP-Neu5Ac donor substrate. Reversion of the natural mutation in a T116S-PdST variant resulted in a marked increase in α2,3-trans-sialidase side activity (4.0 U/mg; pH 4.5), whereas the major sialyltransferase activity was lowered (3.8 U/mg; pH 8.0). The Michaelis-Menten constant for CMP-Neu5Ac was decreased 4-fold in T116S mutant when compared with wild-type PdST (KM=1.1 mM), indicating that residue 116 of PdST contributes to a delicate balance between substrate binding and catalytic activity. D-Galactose and various ß-D-galactosides function as sialyl acceptors from CMP-Neu5Ac, whereas other hexoses (e.g. D-glucose) are inactive. Structure comparison was used to rationalize the particular acceptor substrate specificity of PdST in relation to other GT-80 sialyltransferases that show strict α2,3-regioselectivity, but are flexible in using α/ß-galactosides for sialylation.

Host cell and expression engineering for development of an E. coli ketoreductase catalyst: enhancement of formate dehydrogenase activity for regeneration of NADH.

BACKGROUND: Enzymatic NADH or NADPH-dependent reduction is a widely applied approach for the synthesis of optically active organic compounds. The overall biocatalytic conversion usually involves in situ regeneration of the expensive NAD(P)H. Oxidation of formate to carbon dioxide, catalyzed by formate dehydrogenase (EC 1.2.1.2; FDH), presents an almost ideal process solution for coenzyme regeneration that has been well established for NADH. Because isolated FDH is relatively unstable under a range of process conditions, whole cells often constitute the preferred form of the biocatalyst, combining the advantage of enzyme protection in the cellular environment with ease of enzyme production. However, the most prominent FDH used in biotransformations, the enzyme from the yeast Candida boidinii, is usually expressed in limiting amounts of activity in the prime host for whole cell biocatalysis, Escherichia coli. We therefore performed expression engineering with the aim of enhancing FDH activity in an E. coli ketoreductase catalyst. The benefit resulting from improved NADH regeneration capacity is demonstrated in two transformations of technological relevance: xylose conversion into xylitol, and synthesis of (S)-1-(2-chlorophenyl)ethanol from o-chloroacetophenone. RESULTS: As compared to individual expression of C. boidinii FDH in E. coli BL21 (DE3) that gave an intracellular enzyme activity of 400 units/g(CDW), co-expression of the FDH with the ketoreductase (Candida tenuis xylose reductase; XR) resulted in a substantial decline in FDH activity. The remaining FDH activity of only 85 U/g(CDW) was strongly limiting the overall catalytic activity of the whole cell system. Combined effects from increase in FDH gene copy number, supply of rare tRNAs in a Rosetta strain of E. coli, dampened expression of the ketoreductase, and induction at low temperature (18°C) brought up the FDH activity threefold to a level of 250 U/g(CDW) while reducing the XR activity by just 19% (1140 U/g(CDW)). The E. coli whole-cell catalyst optimized for intracellular FDH activity showed improved performance in the synthesis of (S)-1-(2-chlorophenyl)ethanol, reflected in a substantial, up to 5-fold enhancement of productivity (0.37 g/g(CDW)) and yield (95% based on 100 mM ketone used) as compared to the reference catalyst. For xylitol production, the benefit of enhanced FDH expression was observed on productivity only after elimination of the mass transfer resistance caused by the cell membrane. CONCLUSIONS: Expression engineering of C. boidinii FDH is an important strategy to optimize E. coli whole-cell reductase catalysts that employ intracellular formate oxidation for regeneration of NADH. Increased FDH-activity was reflected by higher reduction yields of D-xylose and o-chloroacetophenone conversions provided that mass transfer limitations were overcome.

Bioprocess design guided by in situ substrate supply and product removal: process intensification for synthesis of (S)-1-(2-chlorophenyl)ethanol.

We report herein on bioprocess development guided by the hydrophobicities of substrate and product. Bioreductions of o-chloroacetophenone are severely limited by instability of the catalyst in the presence of aromatic substrate and (S)-1-(2-chlorophenyl)ethanol. In situ substrate supply and product removal was used to protect the utilized Escherichia coli whole cell catalyst based on Candida tenuis xylose reductase during the reaction. Further engineering at the levels of the catalyst and the reaction media was matched to low substrate concentrations in the aqueous phase. Productivities obtained in aqueous batch reductions were 21-fold improved by addition of 20% (v/v) hexane, NAD(+), expression engineering, cell permeabilization and pH optimization. Reduction of 300 mM substrate was accomplished in 97% yield and use of the co-solvent hexane in subsequent extraction steps led to 88% recovery. Product loss due to high catalyst loading was minimized by using the same extractant in bioreduction and product isolation.