An NMR study of the lactonization of alpha-N-acetylneuraminyl-(2 --> 3)-lactose

Binding of Glycans to the SARS CoV-2 Spike Protein, an Open Question: NMR Data on Binding Site Localization, Affinity, and Selectivity

We have used NMR experiments to explore the binding of selected glycans and glycomimetics to the SARS CoV-2 spike glycoprotein (S-protein) and to its receptor binding domain (RBD). STD NMR experiments confirm the binding of sialoglycans to the S-protein of the prototypic Wuhan strain virus and yield dissociation constants in the millimolar range. The absence of STD effects for sialoglycans in the presence of the Omicron/BA.1 S-protein reflects a loss of binding as a result of S-protein evolution. Likewise, no STD effects are observed for the deletion mutant Δ143-145 of the Wuhan S-protein, thus supporting localization of the binding site in the N-terminal domain (NTD). The glycomimetics Oseltamivir and Zanamivir bind weakly to the S-protein of both virus strains. Binding of blood group antigens to the Wuhan S-protein cannot be confirmed by STD NMR. Using 1 H,15 N TROSY HSQC-based chemical shift perturbation (CSP) experiments, we excluded binding of any of the ligands studied to the RBD of the Wuhan S-protein. Our results put reported data on glycan binding into perspective and shed new light on the potential role of glycan-binding to the S-protein.

The structure and role of lactone intermediates in linkage-specific sialic acid derivatization reactions

Sialic acids occur ubiquitously throughout vertebrate glycomes and often endcap glycans in either α2,3- or α2,6-linkage with diverse biological roles. Linkage-specific sialic acid characterization is increasingly performed by mass spectrometry, aided by differential sialic acid derivatization to discriminate between linkage isomers. Typically, during the first step of such derivatization reactions, in the presence of a carboxyl group activator and a catalyst, α2,3-linked sialic acids condense with the subterminal monosaccharides to form lactones, while α2,6-linked sialic acids form amide or ester derivatives. In a second step, the lactones are converted into amide derivatives. Notably, the structure and role of the lactone intermediates in the reported reactions remained ambiguous, leaving it unclear to which extent the amidation of α2,3-linked sialic acids depended on direct aminolysis of the lactone, rather than lactone hydrolysis and subsequent amidation. In this report, we used mass spectrometry to unravel the role of the lactone intermediate in the amidation of α2,3-linked sialic acids by applying controlled reaction conditions on simple and complex glycan standards. The results unambiguously show that in common sialic acid derivatization protocols prior lactone formation is a prerequisite for the efficient, linkage-specific amidation of α2,3-linked sialic acids, which proceeds predominantly via direct aminolysis. Furthermore, nuclear magnetic resonance spectroscopy confirmed that exclusively the C2 lactone intermediate is formed on a sialyllactose standard. These insights allow a more rationalized method development for linkage-specific sialic derivatization in the future.

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Analysis of Human Milk Neutral and Sialylated Free Oligosaccharides Using Girard's Reagent P On-Target Derivatization

The functional role of human milk oligosaccharides (HMOs) is closely associated with their type, composition, and structure. However, a detailed analysis of HMOs is difficult because neutral oligosaccharides (NHMOs) are mixed with sialylated oligosaccharides (SHMOs) in milk. Here, NHMOs were separated from SHMOs by DEAE-52 anion chromatography, and lactose was removed by graphite carbon solid-phase extraction. Lactose-free NHMOs were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) based on Girard's reagent P on-target derivatization (GPOD), and SHMOs were analyzed by MALDI-TOF-MS following selective sialic acid derivatization and GPOD. Sixty-four oligosaccharides were detected: 36 NHMOs, of which 28 were fucosylated, and 28 SHMOs, of which 8 with α-2,3-linked monosialic acid, 2 with α-2,3-linked disialic acid, 10 with α-2,6-linked monosialic acid, 2 with α-2,6-linked disialic acid, and 5 with both α-2,3- and α-2,6-linked disialic acid. These findings provide the groundwork for further characterization of the structure and activity of HMOs.

Regioselective chemoenzymatic synthesis of ganglioside disialyl tetrasaccharide epitopes

A novel chemoenzymatic approach for the synthesis of disialyl tetrasaccharide epitopes found as the terminal oligosaccharides of GD1α, GT1aα, and GQ1bα is described. It relies on chemical manipulation of enzymatically generated trisaccharides as conformationally constrained acceptors for regioselective enzymatic α2-6-sialylation. This strategy provides a new route for easy access to disialyl tetrasaccharide epitopes and their derivatives.

Chemical characterization of oligosaccharides in chimpanzee, bonobo, gorilla, orangutan, and siamang milk or colostrum

Neutral and acidic oligosaccharides were isolated from the milk or colostrum of four great ape species (chimpanzee (Pan troglodytes), bonobo (Pan paniscus), gorilla (Gorilla gorilla), and orangutan (Pongo pygmaeus)) and one lesser ape species (siamang (Symphalangus syndactylus)), and their chemical structures were characterized by (1)H-NMR spectroscopy. Oligosaccharides containing the type II unit (Gal(beta1-4)GlcNAc) were found exclusively (gorilla and siamang) or predominately (chimpanzee, bonobo, and orangutan) over those containing the type I unit (Gal(beta1-3)GlcNAc). In comparison, type I oligosaccharides predominate over type II oligosaccharides in human milk, whereas nonprimate milk almost always contains only type II oligosaccharides. The milk or colostrum of the great apes contained oligosaccharides bearing both N-glycolylneuraminic acid and N-acetylneuraminic acid, whereas human milk contains only the latter. Great ape milk, like that of humans, contained fucosylated oligosaccharides whereas siamang milk did not. Since these analyses are based on a limited number of individuals, further research on additional samples of great and lesser ape milk is needed to confirm phylogenetic patterns.

Structural determination of the oligosaccharides in the milk of an Asian elephant (Elephas maximus)

Milk of an Asian elephant (Elephas maximus), collected at 11 days post partum, contained 91 g/L of hexose and 3 g/L of sialic acid. The dominant saccharide in this milk sample was lactose, but it also contained isoglobotriose (Glc(alpha1-3)Gal(beta1-4)Glc) as well as a variety of sialyl oligosaccharides. The sialyl oligosaccharides were separated from neutral saccharides by anion exchange chromatography on DEAE-Sephadex A-50 and successive gel chromatography on Bio Gel P-2. They were purified by high performance liquid chromatography (HPLC) using an Amide-80 column and characterized by 1H-NMR spectroscopy. Their structures were determined to be those of 3'-sialyllactose, 6'-sialyllactose, monofucosyl monosialyl lactose (Neu5Ac(alpha2-3)Gal(beta1-4)[Fuc(alpha1-3)]Glc), sialyl lacto-N-neotetraose c (LST c), galactosyl monosialyl lacto-N-neohexaose, galactosyl monofucosyl monosialyl lacto-N-neohexaose and three novel oligosaccharides as follows: Neu5Ac(alpha2-3)Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc(beta1-3)Gal(beta1-4)Glc, Neu5Ac(alpha2-6)Gal(beta1-4)GlcNAc(beta1-3)Gal(beta1-4)GlcNAc(beta1-3)Gal(beta1-4)Glc, and Neu5Ac(alpha2-3)Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc(beta1-3)Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc(beta1-3)Gal(beta1-4)Glc. The higher oligosaccharides contained only the type II chain (Gal(beta1-4)GlcNAc); this finding differed from previously published data on Asian elephant milk oligosaccharides.

Lactones of disialyl lactose: characterisation by NMR and mass spectra

The lactonisation of alpha-Neup5Ac-(2-->8)-alpha-Neup5Ac-(2-->3)-beta-D-Galp-(1-->4)-D-Glc (disialyl lactose) was investigated. (1)H and (13)C NMR chemical shifts of disialyl lactose and alpha-Neup5Ac-(2-->8, 1-->9)-alpha-Neup5Ac-(2-->3, 1-->2)-beta-D-Galp-(1-->4)-D-Glc (disialyl lactose-dilactone) were assigned based on 1D and 2D NMR results, including edited HSQC, HSQC-TOSCY and HMBC. The time course of lactonisation was followed by thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC) with electrospray ionisation (ESI) mass spectrometry (MS) detection. The rate of lactonisation between alpha-(8)Neu5Ac and alpha-(3)Neu5Ac residues (lactonisation at the alpha-(2-->8) linkage) was faster than that of lactonisation between alpha-(3)Neu5Ac and Gal residues (lactonisation at the alpha-(2-->3) linkage). The mass spectra of disialyl lactose, its lactones, alpha-Neup5Ac-(2-->8)-alpha-Neup5Ac (alpha-(2-->8) disialic acid) and alpha-Neup5Ac-(2-->3)-beta-D-Galp-(1-->4)-D-Glc-lactone (3'-sialyllactose-lactone) showed that the alpha-(2-->8) linkage between Neu5Ac residues is difficult to cleave in the ESI-MS, compared with the alpha-(2-->3) linkage between Neu5Ac and Gal residues.

Formation of lactones from sialylated MUC1 glycopeptides

The tumor-associated carbohydrate antigens TN, T, sialyl TN and sialyl T are expressed on mucins in several epithelial cancers. This has stimulated studies directed towards development of glycopeptide-based anticancer vaccines. Formation of intramolecular lactones involving sialic acid residues and suitably positioned hydroxyl groups in neighboring saccharide moieties is known to occur for glycolipids such as gangliosides. It has been suggested that these lactones are more immunogenic and tumor-specific than their native counterparts and that they might find use as cancer vaccines. We have now investigated if lactonization also occurs for the sialyl TN and T antigens of mucins. It was found that the model compound sialyl T benzyl glycoside , and the glycopeptide Ala-Pro-Asp-Thr-Arg-Pro-Ala from the tandem repeat of the mucin MUC1, in which Thr stands for the 2,3-sialyl-T antigen, lactonized during treatment with glacial acetic acid. Compound gave the 1''--> 2' lactone as the major product and the corresponding 1''--> 4' lactone as the minor product. For glycopeptide the 1''--> 4' lactone constitued the major product, whereas the 1''--> 2' lactone was the minor one. When lactonized was dissolved in water the 1''--> 4' lactone underwent slow hydrolysis, whereas the 1''--> 2' remained stable even after a 30 days incubation. In contrast the corresponding 2,6-sialyl-TN glycopeptide did not lactonize in glacial acetic acid.

Occurrence of a unique sialyl tetrasaccharide in colostrum of a bottlenose dolphin (Tursiops truncatus)

Crude oligosaccharides were recovered from bottlenose dolphin (Tursiops truncatus) colostrum after chloroform/methanol extraction of lipids and protein precipitation, and purified using gel filtration, anion exchange chromatography and high performance liquid chromatography (HPLC). Their chemical structures characterized by NMR spectroscopy were as follows: GalNAc(beta1-4)[Neu5Ac(alpha2-3)]Gal(beta1-4)Glc, Neu5Ac(alpha2-3)Gal(beta1-4)Glc, Neu5Ac(alpha2-6)Gal(beta1-4)Glc and Gal(alpha1-4)Gal(beta1-4)Glc. The monosialyltetrasaccharide and neutral trisaccharide have not previously been found as free forms in any natural sources including milk or colostrum, although these structures have been found in the carbohydrate units of glycosphingolipids GM2 and Gb3.

Creatine phosphate--creatine kinase in enzymatic synthesis of glycoconjugates

[reaction: see text] Enzymatic production of glycoconjugates is hampered by expensive phosphagens such as acetyl phosphate (AcP) and phosphoenolpyruvate (PEP). Here, we introduce creatine phosphate--creatine kinase system as a novel and practical energy source in carbohydrate synthesis. This system was successfully demonstrated in the production of bioactive oligosaccharides with different sugar nucleotide regeneration systems.