Transfer of D-galactosyl groups to 6-O-substituted 2-acetamido-2-deoxy-D-glucose residues by use of bovine D-galactosyltransferase

Donor substrate promiscuity of bacterial β1-3-N-acetylglucosaminyltransferases and acceptor substrate flexibility of β1-4-galactosyltransferases

β1-3-N-Acetylglucosaminyltransferases (β3GlcNAcTs) and β1-4-galactosyltransferases (β4GalTs) have been broadly used in enzymatic synthesis of N-acetyllactosamine (LacNAc)-containing oligosaccharides and glycoconjugates including poly-LacNAc, and lacto-N-neotetraose (LNnT) found in the milk of human and other mammals. In order to explore oligosaccharides and derivatives that can be synthesized by the combination of β3GlcNAcTs and β4GalTs, donor substrate specificity studies of two bacterial β3GlcNAcTs from Helicobacter pylori (Hpβ3GlcNAcT) and Neisseria meningitidis (NmLgtA), respectively, using a library of 39 sugar nucleotides were carried out. The two β3GlcNAcTs have complementary donor substrate promiscuity and 13 different trisaccharides were produced. They were used to investigate the acceptor substrate specificities of three β4GalTs from Neisseria meningitidis (NmLgtB), Helicobacter pylori (Hpβ4GalT), and bovine (Bβ4GalT), respectively. Ten of the 13 trisaccharides were shown to be tolerable acceptors for at least one of these β4GalTs. The application of NmLgtA in one-pot multienzyme (OPME) synthesis of two trisaccharides including GalNAcβ1-3Galβ1-4GlcβProN3 and Galβ1-3Galβ1-4Glc was demonstrated. The study provides important information for using these glycosyltransferases as powerful catalysts in enzymatic and chemoenzymatic syntheses of oligosaccharides and derivatives which can be useful probes and reagents.

Investigations on β1,4-galactosyltransferase I using 6-sulfo-GlcNAc as an acceptor sugar substrate

6-sulfate modified N-acetylglucosamine (6-sulfo-GlcNAc) is often found as part of many biologically important carbohydrate epitopes such as 6-sulfo-Le(X). In these epitopes, the 6-sulfo-GlcNAc moiety is extended by a galactose sugar in a β1-4 linkage. The β4GalT1 enzyme transfers galactose (Gal) from UDP-Gal to N-acetylglucosamine (GlcNAc) in the presence of manganese. Here we report that the β4GalT1 enzyme transfers Gal to the 6-sulfo-GlcNAc and 4-methylumbelliferyl-6-sulfo-N-acetyl-β-D-glucosaminide (6-sulfo-βGlcNAc-MU) acceptor substrates, although with very low efficiency. To understand the effect that the 6-sulfate group on the GlcNAc acceptor has on the catalytic activity of the β4GalT1 molecule, we have determined the crystal structure of the catalytic domain of bovine β4GalT1 mutant enzyme M344H-β4GalT1 complex with the 6-sulfo-GlcNAc molecule. In the crystal structure, the 6-sulfo-GlcNAc is bound to the protein in a way that is similar to the GlcNAc molecule. However, the 6-sulfate group engages in additional interactions with the hydrophobic region, residues 276-285, of the protein molecule, and this group is found wedged between the aromatic side chains of Phe-280 and Trp314 residues. Since the side chain of the Trp314 residue undergoes conformational changes during the catalytic cycle of the enzyme, molecular interaction between Trp314 and the 6-sulfate group might hinder this conformational change. Therefore, the lack of a favorable binding environment, together with hindrance to the conformational changes, might be responsible for the poor catalytic activity.

Highly efficient chemoenzymatic synthesis of beta1-4-linked galactosides with promiscuous bacterial beta1-4-galactosyltransferases

Two bacterial beta1-4-galactosyltransferases, NmLgtB and Hp1-4GalT, exhibit promiscuous and complementary acceptor substrate specificity. They have been used in an efficient one-pot multienzyme system to synthesize LacNAc, lactose, and their derivatives including those containing negatively charged 6-O-sulfated GlcNAc and C2-substituted GlcNAc or Glc, from monosaccharide derivatives and inexpensive Glc-1-P.

UDP-Gal: GlcNAc-R beta1,4-galactosyltransferase--a target enzyme for drug design. Acceptor specificity and inhibition of the enzyme

Galactosyltransferases are important enzymes for the extension of the glycan chains of glycoproteins and glycolipids, and play critical roles in cell surface functions and in the immune system. In this work, the acceptor specificity and several inhibitors of bovine beta1,4-Gal-transferase T1 (beta4GalT, EC 2.4.1.90) were studied. Series of analogs of N-acetylglucosamine (GlcNAc) and GlcNAc-carrying glycopeptides were synthesized as acceptor substrates. Modifications were made at the 3-, 4- and 6-positions of the sugar ring of the acceptor, in the nature of the glycosidic linkage, in the aglycone moiety and in the 2-acetamido group. The acceptor specificity studies showed that the 4-hydroxyl group of the sugar ring was essential for beta4GalT activity, but that the 3-hydroxyl could be replaced by an electronegative group. Compounds having the anomeric beta-configuration were more active than those having the alpha-configuration, and O-, S- and C-glycosyl compounds were all active as substrates. The aglycone was a major determinant for the rate of Gal-transfer. Derivatives containing a 2-naphthyl aglycone were inactive as substrates although quinolinyl groups supported activity. Several compounds having a bicyclic structure as the aglycone were found to bind to the enzyme and inhibited the transfer of Gal to control substrates. The best small hydrophobic GlcNAc-analog inhibitor was found to be 1-thio-N-butyrylGlcNbeta-(2-naphthyl) with a K(i) of 0.01 mM. These studies help to delineate beta4GalT-substrate interactions and will aid in the development of biologically applicable inhibitors of the enzyme.

Synthesis and enzymatic evaluation of five-membered iminocyclitols and a pseudodisaccharide

Described here are the synthesis of five-membered iminocyclitols with galacto-configuration and a pseudodisaccharide, and their inhibitory activities against beta-galactosyltransferase, beta-galactosidase and alpha-mannosidase.

Galactosyl transfer ability of beta-(1-->4)-galactosyltransferase toward 5a-carba-sugars

Bovine beta-(1-->4)-galactosyltransferase was assayed with a series of 5a-carba-sugars, i.e., sugar analogues in which the ring oxygen of pyranose is replaced by a methylene group. The analogues are 5a-carba-sugar of 2-acetamido-2-deoxy-alpha-DL-galactopyranose, both alpha and beta anomers of 2-acetamido-2-deoxy-DL-glucopyranose (5a-carba-DL-GlcNAc), and 2-acetamido-2-deoxy-DL-mannopyranose. Of these analogues, both alpha and beta anomers of 5a-carba-DL-GlcNAc act as an acceptor. Enzymatic synthesis using the alpha and beta anomers of 5a-carba-DL-GlcNAc afforded the corresponding D-Gal-beta-(1-->4)-5a-carba-alpha-D-GlcNAc and D-Gal-beta-(1-->4)-5a-carba-beta-D-GlcNAc on a practical scale, and these structures were confirmed by NMR spectroscopy. These results indicate that the ring oxygen atom in the 5a-carba-D-GlcNAc is not used for specific recognition by bovine beta-(1-->4)-galactosyltransferase.

beta-1,4-Galactosyltransferase-catalyzed synthesis of the branched tetrasaccharide repeating unit of Streptococcus pneumoniae type 14

A chemoenzymatic approach is described towards the branched tetrasaccharide repeating unit, beta-D-Galp- (1-->4)-beta-D-Glcp-(1-->6)-[beta-D-Galp-(1-->4)]-beta-D-GlcpNac, of Streptococcus pneumoniae type 14 in a form suitable for conjugation. The linear trisaccharide acceptor, beta-D-Galp-(1-->4)-beta-D-Glcp-(1-->6)-beta-D-GlcpNAc-(1-->O)CH2CH++ + = CH2, was synthesized by coupling of peracetylated lactosyl trichloroacetimidate to a suitably protected glucosamine building block and subsequent deprotection steps. The obtained derivative was found to be a good acceptor for bovine milk beta-1,4-galactosyltransferase, and the resulting branched tetrasaccharide beta-allyl glycoside was isolated and characterized by NMR spectroscopy and FAB mass spectrometry. Reaction of the anomeric allyl function with cysteamine under UV-irradiation gave the beta-aminoethylthio-extended glycoside suitable for further coupling of the tetrasaccharide to protein carriers.

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.

Characterization of a rat liver Golgi sulphotransferase responsible for the 6-O-sulphation of N-acetylglucosamine residues in beta-linkage to mannose: role in assembly of sialyl-galactosyl-N-acetylglucosamine 6-sulphate sequence of N-linked oligosaccharides

Rat liver Golgi membranes were found to contain an enzyme that can transfer sulphate from 3'-phosphoadenosine 5'-phosphosulphate (PAPS) to C-6 of the terminal GlcNAc in beta-linkage to mannose and has properties indicating that it is involved in the synthesis of the NeuAc alpha 2-3(6)Gal beta 1-4GlcNAc(6-SO4) sequences observed in the N-linked carbohydrate units of various glycoproteins. Assays performed with [35S]PAPS (Km 0.67 microM) and GlcNAc beta 1-6Man alpha 1-O-Me (GnMaMe) acceptor (Km 0.71 mM) indicated that the sulphotransferase had a pH optimum of approx. 7.0 and is markedly stimulated by Mn2+ ions (maximum approx. 15 mM) and Triton X-100 (0.05-0.1%). Hydrazine/nitrous acid/NaBH4 treatment of the 35S-labelled product yielded radiolabelled 2,5-anhydromannitol(6-SO4). The sulphated GnMaMc product of the GlcNAc-6-O-sulphotransferase could be galactosylated by a rat liver Golgi enzyme that was shown to have the same properties as the UDP-Gal:GlcNAc beta-1,4-galactosyltransferase from bovine milk. Competition studies performed with GlcNAc and GlcNAc-6-SO4 furthermore indicated that the same liver enzyme acted on both acceptors to produce Gal beta 1-4GlcNAc and Gal beta 1-4GlcNAc(6-SO4) with Km values of 1.04 and 1.68 mM respectively. Because the sulphated N-acetyl-lactosaminc could in turn serve as an acceptor for rat liver sialyltransferase, it seems that this enzyme, together with the Golgi galactosyltransferase and the GlcNAc-6-O-sulphotransferase, could act in concert in assembling the NeuAc alpha 2-3(6)Gal beta 1-4GlcNAc(6-SO4) branches of complex N-linked oligosaccharides.

Synthesis of pentasaccharide analogues of the N-glycan substrates of N-acetylglucosaminyltransferases III, IV and V using tetrasaccharide precursors and recombinant beta-(1-->2)-N-acetylglucosaminyltransferase II

Recombinant human UDP-GlcNAc: alpha-Man-(1-->6)R beta-(1-->2)-N-acetylglucosaminyltransferase II (EC 2.4.1.143, GlcNAc-T II) was produced in the Sf9 insect cell/baculovirus expression system as a fusion protein with a (His)6 tag and partially purified by affinity chromatography on a metal chelating column. The partially purified enzyme was used to catalyze the transfer of GlcNAc from UDP-GlcNAc to R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha-Man(1-->3))beta-Man-O-octyl to form beta-GlcNAc(1-->2)R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha- Man(1-->3))beta-Man-O-octyl where there is either no modification of the alpha-Man(1-->6) residue (7), or where R is 3-deoxy (8), 4-deoxy (9) or 6-deoxy (10). The yields ranged from 64-80%. Products were characterized by 1H and 13C nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Compounds 7-10 are pentasaccharide analogues of the biantennary N-glycan substrates of N-acetylglucosaminyltransferases III, IV and V.

Applications of enzymes in synthetic carbohydrate chemistry

By photosynthesis 200 billion tons of glucose are formed per annum most of which remains in the sugar state. Thus, the well-known properties of saccharides as the basis for structural materials and energy stores is well established. Increasingly, focus is being addressed to their functions as recognition molecule. Therefore, carbohydrate chemistry became en vogue again as documented by classical syntheses of complex hetero-oligosaccharides employing protecting and activating group chemistry. In recent years enzymatic methods for acylation and deacylation in saccharide chemistry came into use. Novel saccharides could be approached following aldolase-catalyzed de novo syntheses. The very complex task of stereospecific glycosylation was addressed by use of enzymes. With glycohydrolases via transglycosylation as well as with glycosyltransferases including cofactor regenerating circles advances were made. Modified acceptor and also donor substrates allowed access to oligosaccharides in preparative quantities. This paper will discuss these modern developments and will focus on some selected examples from others' and our own laboratory.

Synthesis of tetrasaccharide analogues of the N-glycan substrate of beta-(1-->2)-N-acetylglucosaminyltransferase II using trisaccharide precursors and recombinant beta-(1-->2)-N-acetylglucosaminyltransferase I

Recombinant rabbit UDP-GlcNAc: alpha-Man-(1-->3R) beta-(1-->2)-N-acetylglucosaminyl-transferase I (EC 2.4.1.101, GlcNAc-T I) produced in the Sf9 insect cell/baculovirus expression system has been used to convert compounds of the form 3-R-alpha-Man(1-->6)(alpha-Man(1-->3)) beta-Man-O-octyl to 3-R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha-Man(1-->3)) beta-Man-O-octyl where R is OH (14), O-methyl (17), O-pentyl (18), O-(4,4-azo)pentyl (19), O-(5-iodoacetamido)pentyl (20) and O-(5-amino)pentyl (21); 2-deoxy-alpha-Man(1-->6)(beta-GlcNAc(1-->2) alpha-Man(1-->3)) beta-Man-O-octyl (16), 4-O-methyl-alpha-Man(1-->6) (beta-GlcNAc(1-->2) alpha-Man(1-->3)) beta-Man-O-octyl (22), 6-O-methyl-alpha-Man(1-->6)(beta-GlcNAc(1-->2) alpha Man(1-->3)) beta-Man-O-octyl (23) and alpha-Man(1-->6)[beta-GlcNAc(1-->2)(4-O-methyl) alpha-Man(1-->3)] beta-Man-O-octyl (15) were also synthesized by this procedure. The yields ranged from 80 to 99%. Products were characterized by high resolution 1H and 13C nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Compounds 14, 15, 17, 22, and 23 are excellent substrates for UDP-GlcNAc: alpha-Man(1-->6R) beta-(1-->2)-N-acetylglucosaminyltransferase II and the other compounds are inhibitors of this enzyme.

Synthetic substrate analogues for UDP-GlcNAc: Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II. Substrate specificity and inhibitors for the enzyme

UDP-GlcNAc:Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II (GlcNAc-T II; EC 2.4.1.143) is a key enzyme in the synthesis of complex N-glycans. We have tested a series of synthetic analogues of the substrate Man"'alpha 1-6(GlcNAc"beta 1-2Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T II. The enzyme attaches N-acetylglucosamine in beta 1-2 linkage to the 2"'-OH of the Man"'alpha 1-6 residue. The 2"'-deoxy analogue is a competitive inhibitor (Ki = 0.13 mM). The 2"'-O-methyl compound does not bind to the enzyme presumably due to steric hindrance. The 3"'-, 4"'- and 6"'-OH groups are not essential for binding or catalysis since the 3"'-, 4"'- and 6"'-deoxy and -O-methyl derivatives are all good substrates. Increasing the size of the substituent at the 3"'-position to pentyl and substituted pentyl groups causes competitive inhibition (Ki = 1.0-2.5 mM). We have taken advantage of this effect to synthesize two potentially irreversible GlcNAc-T II inhibitors containing a photolabile 3"'-O-(4,4-azo)pentyl group and a 3"'-O-(5-iodoacetamido)pentyl group respectively. The data indicate that none of the hydroxyls of the Man"'alpha 1-6 residue are essential for binding although the 2"'- and 3"'-OH face the catalytic site of the enzyme. The 4-OH group of the Man beta-O-octyl residue is not essential for binding or catalysis since the 4-deoxy derivative is a good substrate; the 4-O-methyl derivative does not bind.(ABSTRACT TRUNCATED AT 250 WORDS)

Further evidence for an active site polypeptide of galactosyl transferase

In a previous report it was shown that galactosyl transferase activity after blotting from acrylamide gel was present in a molecular weight range of less than 14 kDa, in Triton X-100 (1). Molecular sieve chromatography on Superose 12, in the presence of Triton X-100, gave the same result. The low molecular weight activity peak was eluted together with peptides as a part of the covalent structure of the enzyme or as absolutely requires effectors. Peptide mapping showed a new poly-lysine-like peptide and a new hydrophobic peptide in this low molecular weight activity peak as effectors of the enzyme inside its hydrophobic environment.

Large-scale synthesis of beta-L-fucopyranosyl phosphate and the preparation of GDP-beta-L-fucose

A practical 15-mmol large-scale synthesis of beta-L-fucopyranosyl dicyclohexylammonium phosphate from L-fucose in 63% overall yield was developed. The synthesis took advantage of a neighboring Bz-2 group participating in a Koenigs-Knorr-like glycosylation. The sugar phosphate was transformed into the activated sugar nucleoside, guanosine diphosphate beta-L-fucopyranose, on a gram scale.

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.

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.

Enzymic synthesis of oligosaccharides terminating in the tumor-associated sialyl-Lewis-a determinant

The isomeric sialyl-Lea-terminating pentasaccharide derivatives, alpha-Neup5Ac-(2----3)-beta-D-Galp-(1----3)-[alpha-L-Fucp-(1 ----4)]-beta- D-GlcpNAc-(1----3)-beta-D-Galp-O(CH2)8COOMe and alpha-Neup5Ac-(2----3)-beta-D-Galp-(1----3)-[alpha-L-Fucp-(1 ----4)]- beta-D-GlcpNAc-(1----6)-beta-D-Galp-O(CH2)8COOMe, have been prepared by the action in sequence of a porcine submaxillary (2----3)-alpha-sialyltransferase and a human-milk (1----3/4)-alpha-fucosyltransferase on the chemically synthesized trisaccharides beta-D-Galp-(1----3)-beta-D-GlcpNAc-(1----3)- and -(1----6)-beta-D-Galp- O(CH2)8COOMe, respectively.