Mass spectrometry-based glycomics provides an inventory of glycans present in the sample, but requires liberating glycans from their underlying scaffold.
However, relatively few antibodies against defined glycans are available. For the few glycan species with high-affinity anti-glycan antibodies, the pace of discovery is higher than for those without such reagents. These profiling experiments typically rely on mass spectrometry, fluorophore-conjugated lectins or antibodies. Research in the field hinges on accurate methods for identifying and quantifying glycans in a sample. However, the development of glycan-targeted therapies has been hindered by many factors, beginning with our lack of tools for understanding basic glycobiology.
Last, certain glycans are themselves ligands for lectins, which are carbohydrate-specific receptors.īecause glycans are essential for organism health, defects in glycosylation are important contributors to human disease. Second, glycans can regulate the function or properties of the entity to which they are attached, for instance by controlling protein stability or receptor dimerization. First, some glycans form structures with unique physical properties. Additional complexity arises from modification of glycans by sulfation, methylation, phosphorylation, acetylation and O-acylation. Considering these factors alone, there are 20 different ways of linking together glucose and galactose in their ring forms through a glycosidic bond, 19 of which do not make lactose. Therefore, the notation for lactose, Galβ1–4Glc, for example, refers to a galactose linked though a β-glycosidic bond to the hydroxyl group on C4 of glucose. The orientation of the glycosidic bond relative to the anomeric carbon (α versus β) affects the overall shape of the glycan. Monosaccharides are linked together through a glycosidic bond between the anomeric carbon of one sugar and a hydroxyl group of the other. The assembly of these monosaccharides into glycans is performed by enzymes associated with the endoplasmic reticulum and Golgi apparatus. In humans, glycans are primarily constructed from ten monosaccharides: glucose (Glc), galactose (Gal), N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc), fucose (Fuc), xylose (Xyl), sialic acid (Neu5Ac), glucuronic acid (GlcA), mannose (Man) and iduronic acid (IdoA). Glycans can be conjugated to proteins (to form glycoproteins, proteoglycans and glycosylphosphatidylinositol (GPI)-anchored proteins) and lipids (to form glycolipids), or they can be secreted without conjugation to other macromolecules (in the form of glycosaminoglycans such as hyaluronan). The varied functions of glycans are matched by their diverse structures. Cell surface proteins are therefore embedded in a matrix of glycans. Indeed, the dense layer of glycans on the cell surface (the glycocalyx) can extend more than 30 nm from the plasma membrane on some cells 1. Glycans are involved in fundamental aspects of cell and organismal biology, such as the receptor-mediated cell to cell interactions that underlie both normal and pathological processes.