The use of immobilised glycosyltransferases in the synthesis of sialyloligosaccharides

Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis

Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.

Leloir Glycosyltransferases in Applied Biocatalysis: A Multidisciplinary Approach

Enzymes are nature’s catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.

Chemoselective ligation reaction of N-acetylglucosamine (NAG) with hydrazide functional probes to determine galactosyltransferase activity by MALDI mass spectrometry

A perfluorocarbon-modified gold surface is used to immobilize PF-β-NAG and allows quantification of β-GT enzymatic activity with MALDI-TOF/MS.

Chemo-enzymatic synthesis of C-9 acetylated sialosides

A chemo-enzymatic synthesis of [(5-acetamido-9-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylonic acid)-(2-->3)-O-(beta-D-galactopyranosyl)-(1-->3)-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)]-l-serine acetate (1) has been accomplished by a regioselective chemical acetylation of Neu5Ac (2) to give 9-O-acetylated sialic acid 3, which was enzymatically converted into CMP-Neu5,9Ac(2) (4) employing a recombinant CMP-sialic acid synthetase from Neisseria meningitis [EC 2.7.7.43]. The resulting compound was then employed for the enzymatic glycosylation of the C-3' hydroxyl of chemically prepared glycosylated amino acid 10 using recombinant rat alpha-(2-->3)-O-sialyltransferase expressed in Spodooptera frugiperda [EC 2.4.99.4] to give, after deprotection of the N(alpha)-benzyloxycarbonyl (CBz)-protecting group of serine, target compound 1. The N(alpha)-CBz-protected intermediate 11 can be employed for the synthesis of glycolipopeptides for immunization purposes.

Recombinant (2-->3)-alpha-sialyltransferase immobilized on nickel-agarose for preparative synthesis of sialyl Lewis(x) and Lewis(a) precursor oligosaccharides

The specificity of recombinant (2-->3)-alpha-sialyltransferase (ST3Gal-III), expressed in baculovirus-infected insect cells, has been determined with various oligosaccharide acceptors and sugar-nucleotide donors using a fluorescence based assay. Recombinant ST3Gal-III tagged with a polyhistidine tail was immobilized on Ni(2+)-NTA-Agarose as an active enzyme for use in the synthesis of three sialylated oligosaccharides: (i) the divalent molecule [alpha-Neu5Ac-(2-->3)-D-Galp-(1-->4)-beta-D-GlcpNAc-O-CH(2)](2)-C-(CH(2)OBn)(2) (12); (ii) the dansylated derivative, alpha-Neu5Ac-(2-->3)-D-Galp-(1-->3)-beta-D-GlcpNAc-O-(CH(2))(6)-NH-dansyl and; (iii) the tetrasacharide alpha-Neu5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-O-CH(3). Compound 12 was itself prepared from the divalent N-acetyllactosamine molecule built on pentaerythritol by a chemo-enzymatic route.

Up-regulation of the alpha2,6-sialyltransferase messenger ribonucleic acid increases glycoconjugates containing alpha2, 6-linked sialic acid residues in granulosa cells during follicular atresia of porcine ovaries

The sugar chains in cellular glycoconjugates have many biological functions. Extensive morphological development and remodeling occur in the ovary of female animals. This caused us to study glycobiological characteristics of ovarian cells, particularly granulosa cells that undergo apoptosis during follicular atresia. The lectin Sambucus sieboldiana agglutinin (SSA) specific for Siaalpha2,6Gal/GalNAc showed positive staining for granulosa cells only in atretic follicles of porcine ovaries by lectin histochemistry. Lectin blot analysis for SSA demonstrated specific glycoproteins only in atretic follicles. Furthermore, we performed analysis of backbone structures of SSA-positive glycans carried by granulosa cell glycoproteins increased during atresia by glycosidase treatment. Most of these structures were Siaalpha2,6Galbeta1,4GlcNAc on complex-type N-glycans, suggesting that only ST6Gal I of four distinct alpha2,6-sialyltransferases catalyzes alpha2,6-sialic acid transfer in most of the increased glycoproteins of granulosa cells during follicular atresia. Reverse transcription-polymerase chain reaction analysis demonstrated that the expression of ST6Gal I mRNA was up-regulated in granulosa cells during atresia. These results suggested that the alteration of glycoconjugates by ST6Gal I in granulosa cells during atresia is involved in some processes of ovarian follicular atresia and granulosa cell apoptosis.

Enzymatic synthesis of nucleotide sugars

The present review gives a survey on the biosynthetic pathways of nucleotide sugars which are important for the in vitro synthesis of mammalian glycoconjugates. With respect to the use of these enzymes in glycotechnology the availability as recombinant enzymes from different sources, the large-scale synthesis of nucleotide sugars and their in situ regeneration in combination with glycosyltransferases are summarized and evaluated.

Chemoenzymatic synthesis of a sialylated diantennary N-glycan linked to asparagine

A partial structure of many glycoproteins, a glycosylated asparagine carrying a complex type undecasaccharide N-glycan (Neu5Ac(alpha 2-6)Gal(beta 1-4)GlcNAc(beta 1-2)Man alpha 1-3) [Neu5Ac(alpha 2-6)Gal(beta 1-4)GlcNAc(beta 1-2)Man(alpha 1-6)]Man(beta 1-4) GlcNAc(beta 1-4)GlcNAc-Asn) was obtained by total synthesis. As a starting material served a chemically synthesized diantennary heptasaccharide azide which was deprotected in a three-step sequence in high yield. The reduction of the anomeric azide was accomplished with propanedithiol in methanol-ethyldiisopropylamine. Coupling of the glycosyl amine to an activated aspartic acid gave the benzyl protected asparagine conjugate. After removal of the six benzyl functions the resulting free heptasaccharide asparagine was elongated enzymatically in the oligosaccharide part. The use of beta-1,4-galactosyltransferase and alpha-2,6-sialytransferase in the presence of alkaline phosphatase allowed the efficient transfer of four sugar units to the acceptor resulting in a full length N-glycan, a sialyated diantennary undecasaccharide-asparagine of the complex type.

beta-Glucosylation of chitooligomers by galactosyltransferase

Galactosyltransferase from bovine milk was found to be able to utilise UDP-Glc to transfer Glc onto GlcNAc and chitooligomers[-beta-GlcNAc-(1-->4)-]n, n = 2-4. beta-Glucosylated products were used in binding studies with NKR-P1A protein cloned from rat natural killer cells.

Utilization of glycosyltransferases to change oligosaccharide structures

Carbohydrates on cell surfaces are important biomolecules in various biological recognition processes. Elucidation of the biological roles of complex oligosaccharides necessitates an efficient methodology to synthesize these compounds and their analogs. Enzymatic synthesis renders itself to be useful in the construction of an oligosaccharide structure owing to its mild reaction condition, high regio- and stereoselectivity. This review article focuses on the recent progress in oligosaccharide syntheses catalyzed by glycosyltransferases, namely sialyltransferase, galactosyltransferase, fucosyltransferase, and N-acetylglucosaminyltransferase. A survey of the latest patent and literature related to this field is also included.

Glycobiotechnology: enzymes for the synthesis of nucleotide sugars

Complex carbohydrates, as constituting part of glycoconjugates such as glycoproteins, glycolipids, hormones, antibiotics and other secondary metabolites, play an active role in inter- and intracellular communication. The aim of "glycobiotechnology" as an upcoming interdisciplinary research field is to develop highly efficient synthesis strategies, including in vivo and in vitro approaches, in order to bring such complex molecules into analytical and therapeutic studies. The enzymatic synthesis of glycosidic bonds by Leloir-glycosyltransferases is an efficient strategy for obtaining saccharides with absolute stereo- and regioselectivity in high yields and under mild conditions. There are, however, two obstacles hindering the realization of this process on a biotechnological scale, namely the production of recombinant Leloir-glycosyltransferases and the availability of enzymes for the synthesis of nucleotide sugars (the glycosyltransferase donor substrates). The present review surveys some synthetic targets which have attracted the interest of glycobiologists as well as recombinant expression systems which give Leloir-glycosyltransferase activities in the mU and U range. The main part summarizes publications concerned with the complex pathways of primary and secondary nucleotide sugars and the availability and use of these enzymes for synthesis applications. In this context, a survey of our work will demonstrate how enzymes from different sources and pathways can be combined for the synthesis of nucleotide deoxysugars and oligosaccharides.

Combined chemical and enzymatic synthesis of the sialylated non reducing terminal sequence of GM1b glycolylated ganglioside, a potential human tumor marker

N-Glycolylglucosamine 8 was synthesized in 4 steps from anisal glucosamine, via the new crystalline monochloracetyl derivatives 3, 4 and 7. N-Glycolylneuraminic acid 10 was prepared in 59% yield starting from pyruvate and a mixture of 8 and its manno epimer 9 in a 2:3 ratio, with immobilized sialic acid aldolase. Neu5Gc 10 was converted into CMP-NeuGc 11 in the presence of immobilized calf brain CMP-sialate synthetase. Finally 11 was used as a donor in the transfer to the acceptor beta-D-Gal-(1-3)-beta-D-GalNAc-OBn 12 catalyzed by a preparation of porcine liver (2-3)-alpha-sialyltransferase, roughly purified by a chromatography on Cibacron Blue-agarose. alpha-Neu5Gc-(2-3)-beta-D-Gal-(1-3)-beta-D-GalNac-OBn 13 isolated in 56% yield was deprotected to give the non-reducing terminal sequence of GM1b glycolylated ganglioside, which might be expressed in human tumors.

Primary structure of N-linked carbohydrate chains of a human chimeric plasminogen activator K2tu-PA expressed in Chinese hamster ovary cells

A recombinant human plasminogen activator hybrid variant K2tu-PA, expressed in Chinese hamster ovary cells, is partially glycosylated at Asn12 (A chain, kringle-2 domain) and completely glycosylated at Asn247 (B chain, protease domain). After release of the N-linked carbohydrate chains by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, the oligosaccharides were separated from the protein by gel permeation chromatography, then fractionated by FPLC on Mono Q, followed by HPLC on Lichrosorb-NH2, and analysed by 500-MHz 1H-NMR spectroscopy. The following types of carbohydrates occur: monosialylated diantennary (8%), disialylated diantennary (45%), disialylated tri- and tri'-antennary (1%), trisialylated tri- and tri'-antennary (28%), and tetrasialylated tetra-antennary (18%) structures, all having fucose in alpha(1-6)-linkage at the Asn-bound N-acetylglucosamine. Sialic acid occurred exclusively in alpha(2-3)-linkage to galactose, and consisted of N-acetylneuraminic acid (94%), N-glycolylneuraminic acid (3%), and N-acetyl-9-O-acetylneuraminic acid (3%). In addition, glycopeptide fragments corresponding with the A or B chain of K2tu-PA were analysed. The oligosaccharides attached to Asn12 are less processed than those attached to Asn247. Comparison of the glycosylation pattern of K2tu-PA with that of tissue-type plasminogen activator from different biological sources showed significant differences. Profiling studies on different K2tu-PA production batches demonstrated that the structures of N-linked oligosaccharides were identical, but that relative amounts vary with the applied isolation procedure of the chimeric glycoprotein.

Chemoenzymatic galactosialylation with integrated cofactor regeneration

As a precursor for the chemical synthesis of sialylated oligosaccharides, the trisaccharide glycoside Neu5Ac alpha (2-8)Gal beta (1-4)GlcNAc beta (1-O)-pent-4-ene was synthesized starting from GlcNAc beta (1-O)-pent-4-ene, UDP-glucose and N-acetylneuraminic acid in a one pot reaction employing galactosyltransferase and alpha (2-6)sialyltransferase in a complete cofactor regeneration system.

Preparative enzymatic synthesis of activated neuraminic acid by using a microbial enzyme

Using Escherichia coli K-235 as a production strain in a fed-batch fermentation process with an optimized sorbitol/yeast extract medium, we were able to produce 640 U of CMP-Neu5Ac synthetase in 10 l scale (64 U/l) and 9200 U (total enzyme) in 200 l scale (390 U/kg wet weight). By simple one-step purification procedures, enzyme preparations were obtained that could be used efficiently for the synthesis of CMP-Neu5Ac from CTP and Neu5Ac with over 90% yield, from Neu5Ac, CMP, and ATP or phosphoenolpyruvate by in situ generation of CTP, and from CTP, pyruvate, and ManNAc or GlcNAc by in situ generation of Neu5Ac.

Enzyme-catalyzed oligosaccharide synthesis

Cell-surface carbohydrates and their conjugates are involved in many types of molecular recognition. This review describes recent developments in enzyme-catalyzed oligosaccharide synthesis, with particular focus on glycosyltransferase and glycosidase reactions. With the increasing availability of glycosyltransferases via recombinant DNA technology, glycosyltransferase-catalyzed glycosylation with in situ regeneration of sugar nucleotides appears to be the most effective method for large-scale stereocontrolled oligosaccharide synthesis.

The use of porcine liver (2----3)-alpha-sialyltransferase in the large-scale synthesis of alpha-Neup5Ac-(2----3)-beta-D-Galp-(1----3)-D-GlcpNAc, the epitope of the tumor-associated carbohydrate antigen CA 50

alpha-Neup5Ac-(2----3)-beta-D-Galp-(1----3)-D-GlcpNAc (2) and, alpha-Neup5Ac-(2----3)-beta-D-Galp-(1----3)-beta-D-GlcpNAcOMBn+ ++ were prepared on a large scale by the action of beta-D-Galp-(1----3)-D-GalpNAc (2----3)-alpha-sialyltransferase (partially purified from porcine liver) on beta-D-Galp-(1----3)-D-GlcpNAc and beta-D-Galp-(1----3)-beta-D-GlcpNAcOMBn, respectively. The trisaccharide 2 is the epitope of the tumor-associated carbohydrate antigen CA 50, highly expressed in human pancreatic adenocarcinoma.