Modification of oligosaccharide antenna flexibility induced by exoglycosidase trimming

Identification of the binding roles of terminal and internal glycan epitopes using enzymatically synthesized N-glycans containing tandem epitopes

Glycans play diverse roles in a wide range of biological processes. Research on glycan-binding events is essential for learning their biological and pathological functions. However, the functions of terminal and internal glycan epitopes exhibited during binding with glycan-binding proteins (GBPs) and/or viruses need to be further identified. Therefore, a focused library of 36 biantennary asparagine (Asn)-linked glycans with some presenting tandem glycan epitopes was synthesized via a combined Core Isolation/Enzymatic Extension (CIEE) and one-pot multienzyme (OPME) synthetic strategy. These N-glycans include those containing a terminal sialyl N-acetyllactosamine (LacNAc), sialyl Lewis x (sLex) and Siaα2-8-Siaα2-3/6-R structures with N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc) sialic acid form, LacNAc, Lewis x (Lex), α-Gal, and Galα1-3-Lex; and tandem epitopes including α-Gal, Lex, Galα1-3-Lex, LacNAc, and sialyl LacNAc, presented with an internal sialyl LacNAc or 1-2 repeats of an internal LacNAc or Lex component. They were synthesized in milligram-scale, purified to over 98% purity, and used to prepare a glycan microarray. Binding studies using selected plant lectins, antibodies, and viruses demonstrated, for the first time, that when interpreting the binding between glycans and GBPs/viruses, not only the structure of the terminal glycan epitopes, but also the internal epitopes and/or modifications of terminal epitopes needs to be taken into account.

The effect of membrane fluidity on FRET parameters: an energy transfer study inside small unilamellar vesicle

The fluorescence resonance energy transfer (FRET) in a lipid bilayer system containing two different donors and one common acceptor at below and above transition temperature has been studied and all the FRET parameters are analyzed using steady state and time-resolved fluorescence spectroscopy. Using dynamic light scattering measurement, we have followed the process of preparation of small unilamellar vesicles, and by following the FRET parameters of C-153-Rh6G and C-151-Rh6G pairs inside SUVs at 16 °C and 33 °C (T(m) = 23.9 °C) we have noticed that there is greater effect of temperature on the FRET parameters in case of the C-153-Rh6G pair than that of the C-151-Rh6G pair. Finally we have concluded that this difference is due to their different location inside the lipid bilayer in which fluidity of the long alkyl chain markedly affects the FRET parameters for C-153-Rh6G pair embedded inside a small unilamellar vesicle of size 20-50 nm.

Complex N-glycans are the major ligands for galectin-1, -3, and -8 on Chinese hamster ovary cells

Galectins are implicated in a large variety of biological functions, many of which depend on their carbohydrate-binding ability. Fifteen members of the family have been identified in vertebrates based on binding to galactose (Gal) that is mediated by one or two, evolutionarily conserved, carbohydrate-recognition domains (CRDs). Variations in glycan structures expressed on glycoconjugates at the cell surface may, therefore, affect galectin binding and functions. To identify roles for different glycans in the binding of the three types of mammalian galectins to cells, we performed fluorescence cytometry at 4 degrees C with recombinant rat galectin-1, human galectin-3, and three forms of human galectin-8, to Chinese hamster ovary (CHO) cells and 12 different CHO glycosylation mutants. All galectin species bound to parent CHO cells and binding was inhibited >90% by 0.2 M lactose. Galectin-8 isoforms with either a long or a short inter-CRD linker bound similarly to CHO cells. However, a truncated form of galectin-8 containing only the N-terminal CRD bound only weakly to CHO cells and the C-terminal galectin-8 CRD exhibited extremely low binding. Binding of the galectins to the different CHO glycosylation mutants revealed that complex N-glycans are the major ligands for each galectin except the N-terminal CRD of galectins-8, and also identified some fine differences in glycan recognition. Interestingly, increased binding of galectin-1 at 4 degrees C correlated with increased propidium iodide (PI) uptake, whereas galectin-3 or -8 binding did not induce permeability to PI. The CHO glycosylation mutants with various repertoires of cell surface glycans are a useful tool for investigating galectin-cell interactions as they present complex and simple glycans in a natural mixture of multivalent protein and lipid glycoconjugates anchored in a cell membrane.

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 (lambdaem = 340 nm) of the glycosyl donor is successfully overlapped with the excitation peak due to the dansyl group (lambdaex = 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 (lambdaem = 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 microM and Vmax. = 0.119 micromol/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.

Molecular dynamics simulation and nuclear magnetic resonance studies of the terminal glucotriose unit found in the oligosaccharide of glycoprotein precursors

The trisaccharide alpha-d-Glcp-(1 --> 2)-alpha-d-Glcp-(1 --> 3)-alpha-d-Glcp-OMe, a model for the terminal glucotriose in Glc(3)Man(9)GlcNAc(2) in glycoprotein precursors, has been investigated by computer simulations and NMR spectroscopy. Molecular dynamics simulations were performed for 1 ns in aqueous solution and 20 ns in vacuo using the CHARMM-based force fields PARM22 and CHEAT95. An additional Monte Carlo simulation with the HSEA force field was also carried out. Experimental NMR data in water solution was obtained from measurement of long-range (1)H,(13)C heteronuclear trans-glycosidic coupling constants, (3)J(H,C), using one-dimensional Hadamard spectroscopy. Calculation of the (3)J(H,C) values from the simulations showed a varying degree of agreement to experimental data. It could be shown from simulation that the φ torsion angles differed, which was corroborated by the NMR measurements. Analyses were done of radial distribution functions and of hydrogen bonds. It was suggested that intermolecular hydrogen bonds were present, but in contrast to simulation the results from NMR spectroscopy did not support any major contribution. Hence, their influence on the conformation of the trisaccharide is rather small. Comparison of (1)H NMR chemical shifts for the trisaccharide and the glucotriose in Glc(3)Man(8)GlcNAc revealed high similarity. However, the derived conformation of the model substance in this work differed at one glycosidic torsion angle compared to the glucotriose on a large oligosaccharide.

O-mannosyl glycans in mammals

Most proteins within living organisms contain glycans. Glycan structures can modulate the biological properties and functions of glycoproteins. The major glycans of glycoproteins can be classified into two groups, N-glycans and O-glycans, according to their glycan-peptide linkage regions. Developments in glycobiology have revealed a new type of glycosidic linkage to the peptide portion, the O-mannosyl linkage, in mammals, while so far it had been thought to be specific to yeast. This review will give an outline of the O-mannosyl glycans of mammalian glycoproteins. Since one of the most well known O-mannosyl-modified mammalian glycoproteins is dystroglycan, the functional aspects of the O-mannosyl glycan of dystroglycan will be described to help understand this new glycobiological field.

A conformational study of the vicinally branched trisaccharide beta-D-glcp-(1 --> 2)[beta-D-glcp-(1 --> 3)]alpha-D-Manp-OMe by nuclear Overhauser effect spectroscopy (NOESY) and transverse rotating-frame Overhauser effect spectroscopy (TROESY) experiments: comparison to Monte Carlo and Langevin dynamics simulations

The trisaccharide beta-D-Glcp-(1 --> 2)[beta-D-Glcp-(1 --> 3)]alpha-D-Manp-OMe, a model for branching regions in oligosaccharides, has been investigated by one-dimensional DPFGSE (1)H, (1)H nuclear Overhauser effect spectroscopy (NOESY) and transverse rotating-frame Overhauser effect spectroscopy (TROESY) experiments at 30 degrees C in water and in the solvent mixture water : dimethyl sulfoxide (7 : 3). Cross-relaxation rates were obtained from the nmr experiments and interpreted as proton-proton distances. From Metropolis Monte Carlo and Langevin dynamics simulations, distances were calculated and compared to those obtained from experiment. Using the previously determined dynamics from carbon-13 nmr relaxation measurements of the trisaccharide in the solvent mixture, intraresidue proton distances could be obtained that were in excellent to reasonable agreement with those calculated from simulations. In water, the isolated spin-pair approximation was used for comparison of interproton distances. The experimentally derived distances in both solvents showed that the trans-glycosidic distances were shorter between the anomeric proton of the glucosyl group and the proton at the linkage position, respectively, than to the proton on the adjacent carbon on the mannosyl residue. The interresidue distances calculated from the computer simulations, performed with three different force fields, namely HSEA, PARM22, and CHEAT95, resulted in the reverse order in all cases but one.

Structures of sialylated O-linked oligosaccharides of bovine peripheral nerve alpha-dystroglycan. The role of a novel O-mannosyl-type oligosaccharide in the binding of alpha-dystroglycan with laminin

alpha-Dystroglycan is a heavily glycosylated protein, which is localized on the Schwann cell membrane as well as the sarcolemma, and links the transmembrane protein beta-dystroglycan to laminin in the extracellular matrix. We have shown previously that sialidase treatment, but not N-glycanase treatment, of bovine peripheral nerve alpha-dystroglycan greatly reduces its binding activity to laminin, suggesting that the sialic acid of O-glycosidically-linked oligosaccharides may be essential for this binding. In this report, we analyzed the structures of the sialylated O-linked oligosaccharides of bovine peripheral nerve alpha-dystroglycan by two methods. O-Glycosidically-linked oligosaccharides were liberated by alkaline-borotritide treatment or by mild hydrazinolysis followed by 2-aminobenzamide-derivatization. Acidic fractions obtained by anion exchange column chromatography that eluted at a position corresponding to monosialylated oligosaccharides were converted to neutral oligosaccharides by exhaustive sialidase digestion. The sialidases from Arthrobacter ureafaciens and from Newcastle disease virus resulted in the same degree of hydrolysis. The neutral oligosaccharide fraction, thus obtained, gave a major peak with a mobility of 3.8-3.9 glucose units upon gel filtration, and its reducing terminus was identified as a mannose derivative. Based on the results of sequential exoglycosidase digestion, lectin column chromatography, and reversed-phase high-performance liquid chromatography, we concluded that the major sialylated O-glycosidically-linked oligosaccharide of the alpha-dystroglycan was a novel O-mannosyl-type oligosaccharide, the structure of which was Siaalpha2-3Galbeta1-4GlcNAcbeta1-2Man-Ser/Thr (where Sia is sialic acid). This oligosaccharide constituted at least 66% of the sialylated O-linked sugar chains. Furthermore, a laminin binding inhibition study suggested that the sialyl N-acetyllactosamine moiety of this sugar chain was involved in the interaction of the alpha-dystroglycan with laminin.

Solution conformations of a biantennary glycopeptide and a series of its exoglycosidase products from sequential trimming of sugar residues

Linkages between sugar residues in branched oligosaccharides exhibit various degrees of flexibility. This flexibility, together with other forces, determines the overall solution conformation of oligosaccharides. We used the method of time-resolved resonance energy transfer to study the solution conformations of a biantennary glycopeptide and its partially trimmed products by exoglycosidases. The N-terminal of the glycopeptide was labeled with 2-naphthyl acetic acid as a fluorescent donor. Either terminal sugar residue, Gal6', on the branch bearing 6-linked Man (antenna 6'), or Neu5Ac on the branch bearing 3-linked Man (antenna 6) was labeled with 5-dimethylaminophthalene-1-sulfonyl as an acceptor. The distance and distance distributions between the terminals were measured. In the intact biantennary glycopeptide, the donor-acceptor distance distribution of antenna 6' is bimodal with a majority of the population in the extended conformation and that of antenna 6 in one very broad population. The Neu5Ac on antenna 6 is oriented toward the N-terminal at low temperature and adopts a more extended form at high temperature. The removal of individual sugar residues along one of the two antenna in the biantennary oligosaccharide has a small effect on the distance distribution of the remaining antenna for both antennae 6 and 6'. Together with previous studies of the triantennary glycopeptides (Rice, K. G., Wu, P. G., Brand, L., and Lee, Y.C. (1993) Biochemistry 32, 7264-7270), our results suggest that both steric hindrance and inter-residue hydrogen bonding are very important in the folding pattern in oligosaccharide structures.

A bi-fluorescence-labeled substrate for ceramide glycanase based on fluorescence energy transfer

An alkyl lactoside containing two different fluorescence probes as an energy donor and an energy acceptor was synthesized as a substrate for ceramide glycanase. n-Pentenyl beta-lactoside was converted into its 4',6'-O-(2-naphthylmethylidene) derivative with subsequent benzoylation of all remaining OH groups. The fully protected lactoside was treated with borane-trimethylamine complex and aluminum chloride in tetrahydrofuran [P.J. Garegg, Pure Appl. Chem., 56 (1984) 845-858] for selective opening of the 4',6'-acetal group to give the 6'-O-(2-naphthylmethyl) derivative in high yield. After O-debenzoylation, the omega-alkenyl group at the reducing end was extended by Michael addition with HS(CH2)2NH2.HCl to provide an amino group at the terminal position. The amino group was then dansylated to give the target lactoside, which has two different fluorescent probes at each end. Excitation at 290 nm (of the 2-naphthyl group) of the bi-fluorescence-labeled lactoside showed emissions at 335 nm (2-naphthyl) and at 540 nm (dansyl). The distance between the naphthyl group and the dansyl group was estimated to be 12 A by the Förster relationship. Digestion of this lactoside with American leech (Macrobdella decora) ceramide glycanase [B. Zhou et al., J. Biol. Chem., 264 (1989) 12,272-12,277] resulted in an increase in the naphthyl emission with a concomitant decrease in the dansyl emission. These changes can be used for continuous monitoring of the ceramide glycanase activity.

Three-dimensional structures of oligosaccharides

Oligosaccharides represent a particularly challenging class of molecules for conformational analysis. Recent advances in experimental and theoretical methods have begun to yield further insight into their conformational behavior; however, general rules governing their conformational preferences have not yet emerged. X-ray and NMR techniques may provide vital insights into protein-bound oligosaccharide conformations, but these do not necessarily represent highly populated solution conformations. Moreover, an oligosaccharide's inherent flexibility and lack of strong intermolecular interactions places extreme demands on theoretical methods.

Fluorescence resonance energy transfer

In the past year, a number of studies have demonstrated the utility of fluorescence resonance energy transfer as a technique for probing complex intermolecular interactions and for determining the spatial extension and geometrical characteristics of multicomponent structures composed of diverse molecular constituents, such as proteins, lipids, carbohydrates, nucleic acids, and even cells with viruses. The benefits of fluorescence resonance energy transfer are becoming increasingly evident to researchers who require measurements with high sensitivity, specificity, non-invasiveness, rapidity, and relative simplicity.