Poly-N-acetyllactosamine (Poly-LacNAc, [3Galb1,4GlcNAcb1] n ) glycans play an essential role in carbohydrate-protein interactions. The synthesis of poly-LacNAc, both chemical and enzymatic, is typically characterized by high losses of product during sequential synthesis, due to deprotection and/ or purification steps. In this work we present a onepot synthesis of poly-LacNAc oligosaccharides by combining recombinant glycosyltransferases. By fractionation of the poly-LacNAc glycan mixture we were able to isolate glycans with up to six N-acetyllactosamine (LacNAc) units. Activity measurements of the involved recombinant b1,4-galactosyltransferase-1 (b4GalT-1) and b1,3-N-acetylglucosaminyltransferase (b3GlcNAcT) with isolated glycan substrates of up to eight sugar units revealed a preference of b3GlcNAcT for the tetrasaccharide and no preference of b4GalT-1 for a specific glycan length.These findings led us to the optimization of combinatorial one-pot synthesis by variation of substrate and enzyme ratios, as well as starting the synthesis with various poly-LacNAc chain lengths. Consequently, we present here an optimized poly-LacNAc synthesis by the combination of two glycosyltransferases and a uridine-diphospho-glucose/N-acetylglucosamine 4'-epimerase as one-pot strategy resulting in long polyLacNAc glycans with up to six LacNAc units in high yields while minimizing reaction time and product loss. The obtained products are important ligands for the biofunctionalization of biomaterial surfaces and the construction of an artificial extracellular matrix for tissue engineering.
A new multivalent glycopolymer platform for lectin recognition is introduced in this work by combining the controlled growth of glycopolymer brushes with highly specific glycosylation reactions. Glycopolymer brushes, synthetic polymers with pendant saccharides, are prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) of 2-O-(N-acetyl-β-d-glucosamine)ethyl methacrylate (GlcNAcEMA). Here, the fabrication of multivalent glycopolymers consisting of poly(GlcNAcEMA) is reported with additional biocatalytic elongation of the glycans directly on the silicon substrate by specific glycosylation using recombinant glycosyltransferases. The bioactivity of the surface-grafted glycans is investigated by fluorescence-linked lectin assay. Due to the multivalency of glycan ligands, the glycopolymer brushes show very selective, specific, and strong interactions with lectins. The multiarrays of the glycopolymer brushes have a large potential as a screening device to define optimal-binding environments of specific lectins or as new simplified diagnostic tools for the detection of cancer-related lectins in blood serum.
The design of glycoclusters, glycodendrimers, glycopolymers and other complex glycostructures that mimic the multivalent carbohydrate display on the cell surface is of immense interest for diagnosis and therapy. This review presents a detailed insight into the exciting possibilities of multiple glycosylation using enzymes, particularly glycosyltransferases (EC 2.4). A representative choice of available scaffolds for the enzyme action is practically infinite and comprises synthetic polymers, carbosilane dendrimers, multiantennary glycans or hyperbranched conjugates. The introduced glyco-patterns range from common sialyl Lewis(x) and sialyl lacto-chains to chemically functionalized carbohydrate units for detection purposes. The possibilities of in vitro enzymatic production of N- and O-glycans and other natural polymers are also discussed. In harmony with their natural tasks, glycosyltransferases may in vitro complete the imperfect glycosylation pattern of proteins, recombinantly produced in pro- and eukaryotic hosts. What is more, the required enzymatic battery may be directly co-expressed with the protein, in order to elegantly accomplish the production of eukaryotic glycans. Ingenious metabolic labeling enables facile imaging of glycostructures. The boom of glycoarray technology opens vast possibilities in high-throughput screening for novel enzymes and substrate specificities as well as in the synthesis. Though there is still a long way until the Nature's ideal of multivalent glycans is achievable in the laboratory, the sketched pathways to multivalent glycostructures open tremendous possibilities for the future glycobiological research.
SummaryThe importance of glycans in biological systems is highlighted by their various functions in physiological and pathological processes. Many glycan epitopes on glycoproteins and glycolipids are based on N-acetyllactosamine units (LacNAc; Galβ1,4GlcNAc) and often present on extended poly-LacNAc glycans ([Galβ1,4GlcNAc]n). Poly-LacNAc itself has been identified as a binding motif of galectins, an important class of lectins with functions in immune response and tumorigenesis. Therefore, the synthesis of natural and modified poly-LacNAc glycans is of specific interest for binding studies with galectins as well as for studies of their possible therapeutic applications. We present the oxidation by galactose oxidase and subsequent chemical or enzymatic modification of terminal galactose and N-acetylgalactosamine residues of poly-N-acetyllactosamine (poly-LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) by galactose oxidase. Product formation starting from different poly-LacNAc oligomers was characterised and optimised regarding formation of the C6-aldo product. Further modification of the aldehyde containing glycans, either by chemical conversion or enzymatic elongation, was established. Base-catalysed β-elimination, coupling of biotin–hydrazide with subsequent reduction to the corresponding hydrazine linkage, and coupling by reductive amination to an amino-functionalised poly-LacNAc oligomer were performed and the products characterised by LC–MS and NMR analysis. Remarkably, elongation of terminally oxidised poly-LacNAc glycans by β3GlcNAc- and β4Gal-transferase was also successful. In this way, a set of novel, modified poly-LacNAc oligomers containing terminally and/or internally modified galactose residues were obtained, which can be used for binding studies and various other applications.
Lectins are proteins with a well-defined carbohydrate recognition domain. Many microbial proteins such as bacterial toxins possess lectin or lectin-like binding domains to interact with cell membranes that are decorated with glycan recognition motifs. We report a straightforward way to prepare monodisperse and biocompatible polyethylene glycol microgels, which carry glycan motifs for specific binding to lectins. The sugar-functionalized colloids exhibit a wide mesh size and a highly accessible volume. The microgels are prepared via drop-based microfluidics combined with radical polymerization. GSII and ECL are used as model lectins that bind specifically to the corresponding carbohydrates, namely, GlcNAc and LacNAc. LacNAc microgels bind ECL with a high capacity and high affinity (K ≈ 0.5 to 1 μM), suggesting multivalent binding of the lectin to the LacNAc-decorated flexible microgel network. Glycan-functionalized microgels present a useful tool for lectin scavenging in biomedical applications.
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