Glycobiology studies the structure, biosynthesis, and biology of simple and complex carbohydrates (glycans) that occur widely in nature. Glycans cover essentially all cell surfaces, as well as most extracellular molecules, and are remarkably diverse. Glycans have a wide range of functions and can generally be divided into four categories: structure and regulation, interspecies recognition, intraspecies recognition, and molecular mimicry of host glycans. Glycans are ubiquitous in all cellular compartments, extracellular space, and body fluids. They can mediate many biological effects, such as the physical expulsion of pathogens and glycoprotein folding, based on their primary structural properties or by modulating the proteins and lipids to which they are attached. Glycans are involved in the recognition of many pathogens and their host species binding, such as antigen recognition, uptake, and processing. Glycans are also involved in intracellular glycoprotein folding, transport, degradation, cell-to-cell adhesion, and signal transduction. Given that the host immune system recognizes glycans found on many pathogens. Microbes have evolved ways to achieve molecular mimicry of host glycans. In conclusion, glycans are necessary for the life activities of organisms.
Fig.1 General classification of the biological roles of glycans. (Varki, 2017)
Glycosylation is a co-translational and post-translational modification of the addition of glycans to proteins or lipids to form glycoconjugates. Common glycosylation is N-linked glycosylation and O-linked glycosylation. N-Glycosylation is the attachment of N-glycans to the nitrogen atoms of the asparagine (Asn) side chain. O-Glycosylation is the attachment of O-glycans to the oxygen atoms of the side chains of serine (Ser) or threonine (Thr). Abnormal glycosylation is closely related to the occurrence and development of many diseases.
Glycosylation patterns are changed in many human diseases. In the Resource, several carbohydrate-related diseases are described, such as congenital disorders of glycosylation (CDG), autoimmune diseases, and chronic inflammation.
CDG is an inherited human disease caused by defects in glycan biosynthesis and metabolism, often embryonic lethal, which underlies the essential role of glycans. CDG mainly manifests as growth disorder, developmental delay, facial deformity, coagulation, and endocrine abnormalities. All CDGs are associated to some degree with changes in glycosylation. Therefore, Glycan Analysis is the most promising method for laboratory detection of CDG.
Cells of the immune system typically recognize and respond to foreign carbohydrate epitopes such as Lipopolysaccharide, Peptidoglycan, teichoic acid, capsular polysaccharide, and fungal mannan. Changes in protein glycosylation, such as altered sialylation, galactosylation, or fucosylation, affect the immune response to these glycan epitopes, leading to some autoimmune diseases and chronic inflammation.
We describe in detail defects of protein N-glycosylation, defects of protein O-glycosylation, defects of glycosphingolipid and glycosylphosphatidylinositol (GPI)-anchored glycosylation, and defects of multiple glycosylation and other pathways.
Changed glycosylation is a common feature of cancer cells. Frequently observed in tumor cells are Aberrant Sialylation, aberrant core fucosylation, increased N-glycan branching, or exposure of mucin-type O-glycan Tn antigens. For example, β-galactoside α-2,6-sialyltransferase 1 (ST6GalI) is used to synthesize N-glycans with terminal α2,6-sialylation. Upregulation of ST6GalI and increased α2,6-sialylation have been detected in pancreatic and colon cancers.
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