Most proteins require post-translational modifications to function properly. These modifications can be transient or long-lasting. Glycosylation, which is an ordered assembly of carbohydrate structures, represents by far the most complex type of protein modification. The glycosylation machinery comprises hundreds of enzymes and transporters, which shape a tremendous diversity of carbohydrate chains also called glycans. Different cell types express different sets of glycosylation genes, meaning that a given protein usually carries different glycans when produced in the liver or in the brain. In fact, various cell markers used in biology are glycan structures, such as the ABO blood groups and the stage-specific embryonic antigens SSEA-1 to -4. Glycans affect the properties and functions of proteins, thereby regulating signaling cascades, cell adhesion and differentiation. Glycosylation is essential to life. Even slight changes of glycosylation can disrupt complex biological pathways. In accord with the broad involvement of glycans, diseases of glycosylation are diverse, encompassing neurological disorders, muscular dystrophies, immune deficiencies and skeletal abnormalities among others.
Glycans cannot be sequenced like DNA and cannot be synthesized like peptides. Assigning functions to a specific glycan requires the combined application of complex analytical techniques and of model organisms.
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