Enzymic methods in preparative carbohydrate chemistry

Aqueous Glycosylation of Unprotected Sucrose Employing Glycosyl Fluorides in the Presence of Calcium Ion and Trimethylamine

We report a synthetic glycosylation reaction between sucrosyl acceptors and glycosyl fluoride donors to yield the derived trisaccharides. This reaction proceeds at room temperature in an aqueous solvent mixture. Calcium salts and a tertiary amine base promote the reaction with high site-selectivity for either the 3'-position or 1'-position of the fructofuranoside unit. Because nonenzymatic aqueous oligosaccharide syntheses are underdeveloped, mechanistic studies were carried out in order to identify the origin of the selectivity, which we hypothesized was related to the structure of the hydroxyl group array in sucrose. The solution conformation of various monodeoxysucrose analogs revealed the co-operative nature of the hydroxyl groups in mediating both this aqueous glycosyl bond-forming reaction and the site-selectivity at the same time.

Multi-enzyme one-pot strategy for the synthesis of sialyl Lewis X-containing PSGL-1 glycopeptide

An enzymatic one-pot three-step glycosylation strategy was developed for the synthesis of sLex moiety of truncated PSGL-1 glycopeptide with and without sulfation. The method provided an efficient way to afford complex glycopeptides in a semi-preparative scale without further complicated and time-consuming purification process in each glycosylation step.

Chemical glycobiology

Chemical tools have proven indispensable for studies in glycobiology. Synthetic oligosaccharides and glycoconjugates provide materials for correlating structure with function. Synthetic mimics of the complex assemblies found on cell surfaces can modulate cellular interactions and are under development as therapeutic agents. Small molecule inhibitors of carbohydrate biosynthetic and processing enzymes can block the assembly of specific oligosaccharide structures. Inhibitors of carbohydrate recognition and biosynthesis can reveal the biological functions of the carbohydrate epitope and its cognate receptors. Carbohydrate biosynthetic pathways are often amenable to interception with synthetic unnatural substrates. Such metabolic interference can block the expression of oligosaccharides or alter the structures of the sugars presented on cells. Collectively, these chemical approaches are contributing great insight into the myriad biological functions of oligosaccharides.

UDP-N-Acetyl-alpha-D-glucosamine as acceptor substrate of beta-1,4-galactosyltransferase. Enzymatic synthesis of UDP-N-acetyllactosamine

The capacity of UDP-N-acetyl-alpha-D-glucosamine (UDP-GlcNAc) as an in vitro acceptor substrate for beta-1,4-galactosyltransferase (beta4GalT1, EC 2.4.1.38) from human and bovine milk and for recombinant human beta4GalT1, expressed in Saccharomyces cerevisiae, was evaluated. It turned out that each of the enzymes is capable to transfer Gal from UDP-alpha-D-galactose (UDP-Gal) to UDP-GlcNAc, affording Gal(beta1-4)GlcNAc(alpha1-UDP (UDP-LacNAc). Using beta4GalT1 from human milk, a preparative enzymatic synthesis of UDP-LacNAc was carried out, and the product was characterized by fast-atom bombardment mass spectrometry and 1H and 13C NMR spectroscopy. Studies with all three beta4GalTs in the presence of alpha-lactalbumin showed that the UDP-LacNAc synthesis is inhibited and that UDP-alpha-D-glucose is not an acceptor substrate. This is the first reported synthesis of a nucleotide-activated disaccharide, employing a Leloir glycosyltransferase with a nucleotide-activated monosaccharide as acceptor substrate. Interestingly, in these studies beta4GalT1 accepts an alpha-glycosidated GlcNAc derivative. The results imply that beta4GalT1 may be responsible for the biosynthesis of UDP-LacNAc, previously isolated from human milk.

Chemoenzymatic synthesis of spacer-linked oligosaccharides for the preparation of neoglycoproteins

In the present work, the combination of chemical and enzymatic methods to obtain neoglycoproteins is described. Three bovine serum albumin (BSA)-conjugates, BSA-[GalNAc alpha-], BSA-[Gal(beta 1-3)GalNAc(alpha-], and BSA-[Neu5Ac(alpha 2-3)Gal(beta 1-3)GalNAc(alpha-], were prepared. alpha GalNAc derivatives were galactosylated employing crude beta-galactosidase from bovine testes. The use of oversaturated donor solutions (pNP beta Gal) enhanced the yields up to 60%. This method was verified using divalent structures as acceptors, that rendered di- and tri-galactosylated products. Further treatment of the disaccharides with CMP-Neu5Ac and alpha 2-3 sialyltransferase from pork liver led to formation of trisaccharides. Finally, mono-, di-, and trisaccharides were coupled to BSA employing a thiolic group introduced into the protein for Michael addition to a maleinimide group in the spacer-arm of the saccharide components. The results were monitored by HPLC and MALDI-TOF.

High performance polymer supports for enzyme-assisted synthesis of glycoconjugates

Efficient and practical methodology for the construction of carbohydrates, including oligosaccharide derivatives and sphingoglycolipids, was established on the basis of a water-soluble polymer supports having unique linkers that can be cleaved by specific conditions. Novel glycomonomers for the construction of polymer supports were synthesized and copolymerized with acrylamide to give three types of water-soluble glycopolymers having primer sugars through the specific linkers containing (i) p-substituted benzyl group, (ii) L-phenylalanine residue, and (iii) ceramide-mimetic L-serine derivative, respectively. These glycopolymers were employed for sugar elongation reactions with glycosyl transferases such as GlcNAc beta 1,4-galactosyl transferase, beta Gall-->3/4GlcNAc alpha-2,6-sialyl transferase, and beta Gall-->3/4GlcNAc alpha-2,3-sialyl transferase in the presence of each sugar nucleotide as glycosyl donor to afford polymers having N-acetyllactosamine, sialyl alpha-(2-->6) N-acetyllactosamine, and sialyl alpha-(2-->3) lactose residues in excellent yield. Subsequent hydrogenolysis, hydrolysis with alpha-chymotrypsin, or transglycosylation to ceramide with ceramide glycanase proceeds smoothly to give N-acetyllactosamine, a versatile sialyl alpha-(2-->6) N-acetyllactosamine derivative having a terminal amino group, and ganglioside GM3 in high yield.

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.

Gram-scale synthesis of recombinant chitooligosaccharides in Escherichia coli

Cultivation of Escherichia coli harbouring heterologous genes of oligosaccharide synthesis is presented as a new method for preparing large quantities of high-value oligosaccharides. To test the feasibility of this method, we successfully produced in high yield (up to 2.5 g/L) penta-N-acetyl-chitopentaose (1) and its deacetylated derivative tetra-N-acetyl-chitopentaose (2) by cultivating at high density cells of E. coli expressing nodC or nodBC genes (nodC and nodB encode for chitooligosaccharide synthase and chitooligosaccharide N-deacetylase, respectively). These two products were easily purified by charcoal adsorption and ion-exchange chromatography. One important application of compound 2 could be its utilisation as a precursor for the preparation of synthetic nodulation factors by chemical acylation.

Porcine liver (2-->3)-alpha-sialyltransferase: substrate specificity studies and application of the immobilized enzyme to the synthesis of various sialylated oligosaccharide sequences

In search of substrate analogues for the porcine liver beta-D-Gal p-(1-->3)-D-Gal p-NAc: CMP-Neu5Ac-(2-->3')-alpha-sialyltransferase, three disaccharides beta-D-Gal p-(1-->3)-beta-D-Gal p-O-CH3 (5), beta-D-Gal p-(1-->3)-beta-D-(2-OAc)-Gal p-O-CH3 (7) and beta-D-Gal p-(1-->3)-beta-D-(2-OAc)-Gal p-O-Bn (11) were synthesized and tested with the enzyme. Disaccharide 7 turned out to be a very good substrate allowing a rapid access to the trisaccharide alpha-Neu5Ac-(2-->3)-beta-D-Gal p-(1-->3)-beta-D-(2-OAc)-Gal p-O-CH3 (13) on a preparative scale using the crude enzyme immobilized on cationic exchanger. Trisaccharide 13 was further exploited, first as a sialyl donor in Trypanosoma cruzi trans-sialidase catalyzed reaction and second through acetolysis reaction as a source for the synthon alpha-Neu5Ac-(2-->3)-D-Gal (16).

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.

A direct enzymatic synthesis of beta-D-galactopyranosyl-D-xylopyranosides and their use to evaluate rat intestinal lactase activity in vivo

By enzymatic beta-D-galactosylation of D-xylose a mixture of 4-, 3-, and 2-O-beta-D-galactopyranosyl-D-xyloses (1, 4, and 7, respectively) was obtained in 50% isolated yield. Disaccharides 1, 4, and 7 are substrates of intestinal lactase isolated from lamb small intestine with K(m) values of 250.0, 4.5, and 14.0 mM, respectively. The mixture was used to monitor the normal decline in lactase activity in rats that takes place after weaning. The data obtained by this method correlated with the levels of intestinal lactase activity in the same animals after being sacrificed.

In vitro glycosylation of proteins: an enzymatic approach

The glycosylation pathway is the most important post-translational modification of a protein and is moreover a highly specific process. The majority of proteins of pharmaceutical interest are glycoproteins. Therefore, it is necessary to identify the composition, the structure, the function and the biosynthesis of the glycoproteins. The present knowledge is described here. In addition, the performed studies about structure-function relationship of the glycoproteins have shown that the oligosaccharide part of a glycoprotein confers important and specific biological roles. Thus, the modification of the structure of the glycan chains can lead to a modification of the activity of the glycoprotein. This phenomenon is encountered at the time of the production of recombinant glycoprotein in a heterologous system. Indeed, the glycosylation profile of a protein is specific to both the host cell and the culture conditions of this cell. Thus, the advantages and the drawbacks of the different host cells used for the glycosylation engineering are presented. In this way, the identification of the different specific enzymes glycosyltransferases and glycosidases involved in the glycosylation pathway is now necessary to improve the production of recombinant glycoprotein. The structure and the characteristics of these enzymes, and more particularly the oligosaccharyltransferase and the galactosyltransferase, are also described.

Glycosidase-catalysed oligosaccharide synthesis: preparation of N-acetylchitooligosaccharides using the beta-N-acetylhexosaminidase of Aspergillus oryzae

The beta-N-acetylhexosaminidase of Aspergillus oryzae catalyses the formation of 2-acetamido-4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-deoxy-D- glucopyranose (di-N-acetylchitobiose) and 2-acetamido-6-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-deoxy-D- glucopyranose from p-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside and 2-acetamido-2-deoxy-D-glucopyranose. The ratio of the two disaccharides is time-dependent. The ratio of (1-->4)- to (1-->6)-isomers is a maximum (approximately 9:1) at the point of disappearance of the glycosyl donor. If left to evolve, the ratio changes to 92:8 in favour of the (1-->6)-isomer. Either the (1-->4)- or the (1-->6)-isomer can be isolated by treating the appropriately enriched dissaccharide mixture with the beta-N-acetylhexosaminidase of Jack bean (Canavalia ensiformis) or the beta-N-acetylhexosaminidase of A. oryzae, respectively. Di-N-acetylchitobiose [GlcNAc(beta 1-4)GlcNAc] is an efficient donor of 2-acetamido-2-deoxy-D-glucopyranosyl units in reactions catalysed by the N-acetylhexosaminidase of A. oryzae. Di-N-acetylchitobiose itself acts as acceptor to give tri-N-acetylchitotriose [GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc]. As the trisaccharide accumulates it, in turn, acts as acceptor giving tetra-N-acetylchitotetraose [GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc]. The product mixture consisting of mono-, di-, tri-, and tetrasaccharides is conveniently separated by charcoal-Celite chromatography.