N-acetyllactosamine Derivatives Research Articles

Comparison of MM/GBSA calculations based on explicit and implicit solvent simulations

Molecular mechanics with generalised Born and surface area solvation (MM/GBSA) is a popular method to calculate the free energy of the binding of ligands to proteins. It involves molecular dynamics (MD) simulations with an explicit solvent of the protein-ligand complex to give a set of snapshots for which energies are calculated with an implicit solvent. This change in the solvation method (explicit → implicit) would strictly require that the energies are reweighted with the implicit-solvent energies, which is normally not done. In this paper we calculate MM/GBSA energies with two generalised Born models for snapshots generated by the same methods or by explicit-solvent simulations for five synthetic N-acetyllactosamine derivatives binding to galectin-3. We show that the resulting energies are very different both in absolute and relative terms, showing that the change in the solvent model is far from innocent and that standard MM/GBSA is not a consistent method. The ensembles generated with the various solvent models are quite different with root-mean-square deviations of 1.2-1.4 Å. The ensembles can be converted to each other by performing short MD simulations with the new method, but the convergence is slow, showing mean absolute differences in the calculated energies of 6-7 kJ mol(-1) after 2 ps simulations. Minimisations show even slower convergence and there are strong indications that the energies obtained from minimised structures are different from those obtained by MD.

ChemInform Abstract: Regiospecific Syntheses of N-Acetyllactosamine Derivatives and Application Toward a Highly Practical Synthesis of Lewis X Trisaccharide.

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1,6-anhydro-N-acetyl-β-D-glucosamine in oligosaccharide synthesis: II. The synthesis of the spacered Ley tetrasaccharide

3-Aminopropyl glycoside of 3,2'-di-O-alpha-L-fucosyl-N-acetyllactosamine (Ley tetrasaccharide) was synthesized. The glycosyl donor, 2-O-acetyl-3,4,6-tri-O-benzoyl-alpha-D-galactopyranosyl bromide, was coupled with glycosyl acceptor, 1,6-anhydro-2-acetamido-2-deoxy-beta-D-glucopyranose or its 3-O-acetyl derivative, to give the corresponding N-acetyllactosamine derivatives in 20 and 71% yields, respectively. The glycosyl donor was synthesized from 1,2-di-O-acetyl-3,4,6-tri-O-benzoyl-D-galactopyranose, which was obtained by the treatment of benzobromogalactose with sodium borohydride to yield 1,2-O-benzylidene derivative and subsequent removal of benzylidene group and acetylation. Acidic methanolysis of the disaccharide derivatives resulted in the selective removal of one or both acetyl groups to give the disaccharide acceptor bearing hydroxy groups at C3 of the glucosamine residue and C2 of the galactose residue. The introduction of fucose residues in these positions by the treatment with tetrabenzylfucopyranosyl bromide resulted in a tetrasaccharide derivative, which was converted into 3,2'-di-O-alphha-L-fucopuranosyl- 1,6-anhydro-N-acetyllactosamine peracetate after substitution of acetyl groups for benzoyl and benzyl groups. Opening of the anhydro ring by acetolysis resulted in peracetate, which was then converted into the corresponding oxazoline derivative in two steps. Glycosylation of the oxazoline derivative with 3-trifluoroacetamidopropan-1-ol and removal of O-acetyl and N-trifluoroacetyl protective groups resulted in a free spacered Ley tetrasaccharide.

Stepwise synthesis of chitooligosaccharides through a transition-state analogue substrate catalyzed by mutants of chitinase A1 from Bacillus circulans WL-12

AbstractSeveral mutants of chitinase A1 fromBacillus circulansWL-12 have been prepared by site-directed mutagenesis of tryptophan 433. The hydrolytic activity of these mutants towardp-nitrophenyl chitobioside was lower than that of wild-type chitinase A1. The mutants were found to catalyze the transglycosylation reaction of the 1,2-oxazoline derivative ofN-acetyllactosamine (Gal-GlcNAc-oxa) toN,N′′-diacetylchitobiose (GlcNAc-GlcNAc), giving rise to a transglycosylated tetrasaccharide product (Gal-GlcNAc-GlcNAc-GlcNAc) in high yield. Based on these results, a one-pot synthesis of chitooligosaccharides by combined use of mutant W433Y and β-galactosidase has been demonstrated. By controlling the reaction conditions, chitotriose and chitotetraose were obtained in 74% and 40% yield (determined by HPLC), respectively, without isolating any intermediate products.

Chemo-enzymatic Synthesis of Novel Oligo-N-acetyllactosamine Derivatives having a β(1-4)–β(1-6) Repeating Unit by Using Transition State Analogue Substrate

Chitinase-catalyzed hydrolytic and transglycosylating behavior of 1,2-oxazoline derivative of N-acetyllactosamine (LacNAc-oxa) 1 has been investigated. An extremely rapid hydrolysis (ring-opening of the oxazoline moiety) could be observed, suggesting that 1 behaves as a transition state analogue substrate for chitinase A1 (Bacillus circulans WL-12). This disaccharide monomer 1 was found to polymerize under basic conditions, giving rise to novel oligosaccharides having a β(1-4)–β(1-6) repeating unit in the main chain. The degree of polymerization of the resulting oligosaccharides was up to 5. This is the first example of enzymatic glycosylation reaction forming a β(1-6) bond catalyzed by chitinase.

In vitro sulfation of N-acetyllactosaminide by soluble recombinant human β-Gal-3′-sulfotransferase

Membrane-bound β-Gal-3′-sulfotransferase (GP3ST) was expressed and used for in vitro sulfation of Tamm–Horsfall glycoprotein. Further, the regioselective transfer of sulfate to an N-acetyllactosamine derivative could be realised with soluble chimeric GP3ST, also in combination with Lac transglycosylation by means of β-galactosidase. Two alternative straightforward chemical syntheses for the target compound could be elaborated.

Thioureido N-acetyllactosamine derivatives as potent galectin-7 and 9N inhibitors

Derivatives of N-acetyllactosamine carrying structurally diverse thioureido groups at galactose C3 were prepared from a C3′-azido N-acetyllactosamine derivative in a three-step reaction sequence involving azide reduction and isothiocyanate formation by thiophosgene treatment of the C3-amine, followed by reaction of the isothiocyanate with a panel of amines. Evaluation of the N-acetyllactosamine thioureas as inhibitors against galectins-1, 3, 7, 8N (N-terminal domain), and 9N (N-terminal domain) revealed thiourea-mediated affinity enhancements for galectins-1, 3, 7, and 9N. In particular, good inhibitors were discovered against galectin-7 and 9N ( K d 23 and 47 μM, respectively, for a 3-pyridylmethylthiourea derivative), which represents more than an order of magnitude affinity enhancement over the parent natural N-acetyllactosamine.

Structure-Based Engineering of E. coli Galactokinase as a First Step toward In Vivo Glycorandomization

In vitro glycorandomization is a rapid chemoenzymatic strategy to diversify complex natural product scaffolds. The glycorandomization sugar activation pathway is dependent upon the efficient construction of diverse sugar-1-phosphate libraries. In the context of the previously evolved GalK Y371H "gatekeeper" mutation, the active site M173L mutation described herein presents a kinase with remarkably broadened substrate range to include 28 diverse natural and unnatural sugars. Among these new substrates, 6-azido-6-deoxy-galactose and 6-azido-6-deoxy-glucose present unique chemical probes to assess the utility of an E. coli Y371H/M173L-GalK-overproducing strain to generate unnatural sugar-1-phosphates in vivo. Remarkably, the in vivo conversion of both unnatural sugars rival that demonstrated in vitro. This notable in vivo success stands as the first step toward constructing short sugar-activation pathways in vivo and, ultimately, in vivo natural-product glycorandomization.

Glycosyltransferase activity can be selectively modulated by chemical modifications of acceptor substrates

A range of N-acetyllactosamine derivatives, which are modified by a wide range of functionalities at C-2′ and C-6, have been synthesised and the kinetic parameters of transfer catalysed by recombinant α-2,6-sialyltransferase and α-1,3-fucoyltransferase VI determined. Several of the chemical modifications led to selective modulate the activity the enzymes and offer promising lead compounds for the development of oligosaccharide primers for selective metabolic inhibition of oligosaccharide biosynthesis.

Chemo‐enzymatic Synthesis of Conformationally Constrained Oligosaccharides.

N-Acetyllactosamine derivative 4, which has a methylene amide tether between C-6 and C-2′, was enzymatically glycosylated using rat liver α-2,6-sialyltransferase (ST6GalI) or recombinant human fucosyltansferase V (FucT-V) to give conformationally constrained trisaccharides 5 and 6, respectively. The methylene amide linker of 4 was installed by a two-step procedure, which involved acylation of a C-6 amino function of a LacNAc derivative with chloroacetic anhydride followed by macrocyclization by nucleophilic displacement of the chloride by a C-2′ hydroxyl. The conformational properties of 4 were determined by a combination of NOE and trans-glycosidic heteronuclear coupling constant measurements and molecular mechanics simulations and these studies established that the glycosidic linkage of 4 is conformationally constrained and resides in only one of the several energy minima accessible to LacNAc. The apparent kinetic parameters of transfer to LacNAc and conformationally constrained saccharides 3 and 4 indicates that fucosyltransferase V recognize LacNAc in its A-conformer whereas α-2,6-sialyltransferase recongizes the B-conformer of LacNAc.

Glycosyltransferase activity can be modulated by small conformational changes of acceptor substrates.

A range of N-acetyllactosamine derivatives (compounds 4-7) that have restricted mobilities around their glycosidic linkages have been employed to determine how small changes in conformational properties of an oligosaccharide acceptor affect catalytic efficiencies of glycosylations by alpha-2,6- and alpha-2,3-sialyltransferases and alpha-1,3-fucosyltransferases IV and VI. Restriction of conformational mobility was achieved by introducing tethers of different length and chemical composition between the C-6 and C-2' hydroxyl of LacNAc. Compound 4 is a 2',6-anhydro derivative which is highly constrained and can adopt only two unusual conformations at the LacNAc glycosidic linkage. Compound 5 is modified by a methylene acetal tether and can exist in a larger range of conformations; however, the Phi dihedral angle is restricted to values smaller than 30 degrees, which are not entirely similar to minimum energy conformations of LacNAc. The ethylene-tethered 6 can attain conformations in the relatively large energy plateau of LacNAc that include syn conformations A and B, whereas compound 7, which is modified by a methylamide tether, can only reside in the B-conformer. 2',6-Dimethoxy derivative 2 was employed to determine the effect of alkylation of the C-6 and C-2' hydroxyls of 5 and 6 whereas 3 was used to reveal the effects of the C-6 amide and C-2' alkylation of 7. The apparent kinetic parameters of transfer to the conformationally constrained 4-7 and reference compounds 1-3 catalyzed by alpha-2,6- and alpha-2,3-sialyltransferases and alpha-1,3-fucosyltransferases IV and VI were determined, and the results correlated with their conformational properties. The data for 4-6 showed that each enzyme recognizes N-acetyllactosamine in a low minimum energy conformation. A small change in conformational properties such as in compound 5 resulted in a significant loss of catalytic activity. Larger conformational changes such as in compound 4 abolished all activity of the sialyltransferases whereas the fucosyltransferases showed some activity, albeit very low. The kinetic data for compounds 4 and 5 demonstrate clearly that different glycosyltransferases respond differently to conformational changes, and the fucosyltransferases lost less activity than the sialyltransferases. Correlating apparent kinetic parameters of conformationally constrained 6 and 7 and their reference compounds 2 and 3 further supports the fact that different enzymes respond differently and indicates that sialyltransferases and fucosyltransferases recognize N-acetyllactosamine in a different conformation. Collectively, the data presented here indicate that small conformational changes of an oligosaccharide acceptor induced by, for example, the protein structure can be employed to modulate the patterns of protein glycosylation.

Synthesis of a useful anomeric thioacetate of an N-acetyllactosamine derivative and its application

A novel anomeric β-thioacetate of an N-acetyllactosamine derivative was efficiently synthesized in high yield from the known 2-azido glycosyl chloride using thioacetic acid as a convenient reagent. The synthesis involved not only an S N2 replacement of the chloride by a carbothiolate anion but also a reductive acetamidation of the azide group. Applications of the thioacetate for glycosidation were demonstrated to provide both O- and S-glycosides in high yields. Furthermore, both intermediates gave a new class of glycoclusters that included thioglycosidic linkages.

Chemo-enzymatic synthesis of conformationally constrained oligosaccharides

N-Acetyllactosamine derivative 4, which has a methylene amide tether between C-6 and C-2', was enzymatically glycosylated using rat liver alpha-2,6-sialyltransferase (ST6GalI) or recombinant human fucosyltransferase V (FucT-V) to give conformationally constrained trisaccharides 5 and 6, respectively. The methylene amide linker of 4 was installed by a two-step procedure, which involved acylation of a C-6 amino function of a LacNAc derivative with chloroacetic anhydride followed by macrocyclization by nucleophilic displacement of the chloride by a C-2' hydroxyl. The conformational properties of 4 were determined by a combination of NOE and trans-glycosidic heteronuclear coupling constant measurements and molecular mechanics simulations and these studies established that the glycosidic linkage of 4 is conformationally constrained and resides in only one of the several energy minima accessible to LacNAc. The apparent kinetic parameters of transfer to LacNAc and conformationally constrained saccharides 3 and 4 indicates that fucosyltransferase V recognize LacNAc in its A-conformer whereas alpha-2,6-sialyltransferase recognizes the B-conformer of LacNAc.

Efficient Method for the Elongation of the N-Acetylglucosamine Unit by Combined Use of Chitinase and -Galactosidase

A novel strategy for the regio- and stereoselective synthesis by two enzymatic steps of oligosaccharides having an N-acetylglucosamine unit at the nonreducing end was developed. The first step involves a chitinase-catalyzed highly selective β-N-acetyllactosamination of an oligosaccharide acceptor with a 4,5-dihydrooxazole derivative of N-acetyllactosamine as the glycosyl donor. The usage of a transition-state-analogue substrate for the chitinase under basic conditions allows the reaction to proceed only in the synthetic direction while suppressing hydrolysis of the product in aqueous media. Several chitinase mutants also catalyzed the glycosylation efficiently under neutral conditions. The second step is a regioselective cleavage of the glycosidic bond between the terminal galactose unit and the adjacent N-acetylglucosamine unit by the action of a β-galactosidase. This constitutes a very useful method to add an N-acetylglucosamine unit to the nonreducing end of chito- and cello-oligosaccharide derivatives in a regio- and stereoselective manner.

Thermodynamic analysis of the binding of galactose and poly- N-acetyllactosamine derivatives to human galectin-3

Galectin-3, with a wide tissue distribution and marked developmental regulation, provides significant insights into the progression of various disease and developmental stages. Recognized by its specificity for galactose, a detailed characterization of its sugar binding ability has been investigated by isothermal titration calorimetry. The results presented here complement well with the earlier studies utilizing hapten inhibition assays. Among the various lactose derivatives studied, A-tetrasaccharide emerged with the highest affinity for binding to galectin-3 combining site. This blood group saccharide exhibited a binding affinity 37-fold higher and a 102 kJ/mol more favorable change in enthalpy over lactose at 280 K indicating the existence of additional subsites for both the α1-3-linked N-acetylgalactosamine at the non-reducing end and the α1-2-linked L-fucosyl residue. The thermodynamic parameters evaluated for other ligands substantiate further the carbohydrate recognition domain to be part of an extended binding site. Binding thermodynamics of galectin-3 with the galactose derivatives are essentially enthalpically driven and exhibit compensatory changes in Δ H° and TΔ S owing to solvent reorganization.

Design of Fluorogenic Substrates for Continuous Assay of Sialyltransferase by Resonance Energy Transfer

Glycosyltransferases are important synthetic enzymes for the construction of naturally occurring glycoconjugates as well as for the design of neoglycoconjugates. The assay methods currently available for these enzymes require tedious and time-consuming procedures for separation of products and do not permit continual assay of enzyme activities. As a set of convenient fluorogenic substrates for continuous monitoring of sialyltransferase activities, we designed and synthesized a novel CMP-Neu5Ac derivative with a naphthylmethyl group at the C-9 position and N-acetyllactosamine derivative containing a dansyl group at the terminal position of aglycon. In such substrates, the emission peak of the naphthylmethyl group (λem = 340 nm) of the glycosyl donor is successfully overlapped with the excitation peak due to the dansyl group (λex = 335 nm) of the glycosyl acceptor. A coupling reaction of these two substrates catalyzed by rat liver 2,6-sialyltransferase caused an increase of dansyl fluorescence (λem = 525 nm) and a decrease of naphthylmethyl fluorescence on the basis of resonance energy transfer between two fluorescence probes. The substrates presented here permit continuous fluorescent monitoring of enzymatic sugar combining reactions. Actually, using this time course of enzymatic reactions, kinetic constants of rat liver 2,6-sialyltransferase against glycosyl donor substrates were estimated to be Km = 4.85 μM and Vmax = 0.119 μmol/min, respectively. This strategy allows precise and efficient analyses of enzyme kinetics not possible with the conventional assay methods for the glycosyltransferases that usually require separation of products from the reaction mixture.

Regiospecific Syntheses of N-Acetyllactosamine Derivatives and Application Toward a Highly Practical Synthesis of Lewis X Trisaccharide

ABSTRACT An efficient methodology for regiospecific glycosylation was developed by using 6-O-tert-butyldiphenylsilyl N-acetylglucosamine derivatives 3 and 5 which bear two free hydroxyl groups as acceptors. The regiospecificity was attributed to the presence of the tert-butyldiphenylsilyl group at the O-6 position of the N-acetylglucosamine derivatives. Glycosylation of suitably protected galactoside donors 10-14 with acceptors 3 and 5 gave only β (1→4) linked disaccharides 15-19 in good yields. Fucosylation of disaccharide 18 led to Lewis X (Lex) trisaccharide 21 in high yield.

Dual affinity labeling of the active site of human lysozyme with an N-acetyllactosamine derivative: first ligand assisted recognition of the second ligand.

Among the three kinds of the 2',3'-epoxypropyl beta-glycoside of disaccharides (GlcNAc-beta1,4-GlcNAc, Gal-beta1,4-GlcNAc, and Man-beta1,4-GlcNAc), the derivative of N-acetyllactosamine (Gal-beta1,4-GlcNAc-Epo) caused the dual labeling of human lysozyme (HL) most efficiently. The labeled HL was crystallized and analyzed by X-ray diffraction methodology. The X-ray analysis located the two Gal-beta1,4-GlcNAc-Epo moieties inside the catalytic cleft of HL. The attachment sites were the side-chain carboxylate groups of the catalytic residues Glu35 and Asp53 in HL. The first Gal-beta1, 4-GlcNAc-Epo moiety occupied virtually the same position as observed in the HL labeled with single Gal-beta1,4-GlcNAc-Epo molecule. The second Gal-beta1,4-GlcNAc-Epo moiety was recognized via the carbohydrate-carbohydrate interaction with the first Gal-beta1, 4-GlcNAc-Epo moiety in addition to the protein-carbohydrate interaction with the "right-side" catalytic cleft of HL through a number of hydrogen bonds including water-mediated ones as well as many van der Waals contacts. The two N-acetylglucosamine residues stacked with each other, while the two rings of galactose residues approximately shared the same plane. The dual labeling with two Gal-beta1,4-GlcNAc-Epo molecules was supposed to have occurred sequentially, which was accompanied with the alteration to the pKa of Glu35 derived from the esterification of Asp53 in the first labeling. Both asymmetric carbons in the connection parts between HL and N-acetyllactosamine moieties showed the same stereoconfiguration derived from the reaction with (2'R) stereoisomer concerning the epoxide group in the labeling reagent. The results demonstrated that the HL labeled with single Gal-beta1,4-GlcNAc-Epo was functional as a novel N-acetyllactosamine-binding protein, and the second labeling was performed by way of the first-ligand assisted recognition of the second ligand.

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 watersoluble 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 β1,4-galactosyl transferase, βGall → 3/4GlcNAc α-2,6-sialyl transferase, and βGall → 3/4GlcNAc α-2,3-sialyl transferase in the presence of each sugar nucleotide as glycosyl donor to afford polymers having N-acetyllactosamine, sialyl α-(2 → 6) N-acetyllactosamine, and sialyl α-(2 → 3) lactose residues in excellent yield. Subsequent hydrogenolysis, hydrolysis with α-chymotrypsin, or transglycosylation to ceramide with ceramide glycanase proceeds smoothly to give N-acetyllactosamine, a versatile sialyl α-(2 → 6) N-acetyllactosamine derivative having a terminal amino group, and ganglioside GM3 in high yield.

Monitoring Biosynthetic Transformations of N-Acetyllactosamine Using Fluorescently Labeled Oligosaccharides and Capillary Electrophoretic Separation

Capillary electrophoresis with laser-induced fluorescence detection was used to monitor biosynthetic transformations of a labeled derivative of N-acetyllactosamine (β Gal(1 → 4)βGlcNAc, LacNAc) in crude microsomal extracts. For this purpose, six authentic standards including LacNAc-O-TMR itself (-TMR, a linker arm attached to tetramethylrhodamine), the tri-and tetrasaccharides that would form by either α(1 → 2) or α(1 → 3) fucosylation of LacNAc or both (i.e,, H-type 2, LeX and LeY sequences) were chemically synthesized. The potential degradation products produced by galactosidase and hexosaminidase were also prepared. All of the standards were kinetically competent substrates for the enzymes present in HT-29 cells and could be baseline separated in a single run requiring 11 min. The action of competing enzymes acting on the common LacNAc sequence could thus be monitored in a single run with sensitivity routinely as low as a few thousand molecules.