Glycome Atlas
Process
Also known as protein glycosylation, glycan attachment
GlcNAc (β1-N) → Asn; GlcNAc (β1-4) → GlcNAc; Man (β1-4) → GlcNAc; Man (α1-3) → Man; Man (α1-6) → Man; GlcNAc (β1-2) → Man; GlcNAc (β1-2) → Man; Gal (β1-4) → GlcNAc; Gal (β1-4) → GlcNAc; Neu5Ac (α2-6) → Gal; Neu5Ac (α2-6) → Gal
How to read these diagrams (SNFG)
Each shape is a class of sugar and each colour a specific one. Structures read right to left, with the reducing end (the point of attachment) on the right.
Plain-language answer
Glycosylation is the controlled process by which cells attach sugar chains to proteins and fats. Special enzymes add sugars one at a time as the protein is being made and packaged, building a coat of complex carbohydrates. This is how most proteins that end up on the cell surface or that are secreted into blood get their final, working form.12
Sugar coats decide how proteins fold, how long they last, and how cells recognize each other, so glycosylation shapes immunity, hormone signaling, and tissue structure. When the enzymes that build these sugar chains work incorrectly, the result can be inherited disorders and altered patterns seen in cancer and inflammation.1
Technical detail
Glycosylation is the enzyme-catalyzed, template-independent covalent attachment of glycans to proteins and lipids, carried out by membrane-bound glycosyltransferases and glycan-processing enzymes in the endoplasmic reticulum, Golgi, and cytosol using nucleotide sugar donors, producing the glycoproteins, glycolipids, and proteoglycans that dominate the cell surface and secretome.12
Unlike nucleic acid and protein synthesis, glycan assembly is not directed by a template. Instead, the structures produced are determined by the repertoire, expression level, and subcellular localization of glycosyltransferases and glycosidases, together with the availability of activated nucleotide sugar donors. Because many enzymes compete for shared substrates as a glycoprotein transits the secretory pathway, the same protein can carry a range of related glycan structures known as glycoforms.12
The principal forms include N-linked glycosylation, in which glycans are attached to asparagine within a sequon, and O-linked glycosylation, in which glycans are attached to serine or threonine hydroxyls. Additional pathways build glycosaminoglycan chains on proteoglycans, assemble glycosphingolipids, and attach glycosylphosphatidylinositol anchors that tether proteins to the outer leaflet of the plasma membrane.12
Glycosylation should not be confused with glycation. Glycosylation is enzyme-directed, regulated, and site-specific, whereas glycation is the spontaneous, non-enzymatic reaction of reducing sugars with protein amino groups that proceeds without catalysis. The two processes produce chemically and biologically distinct products, and only glycosylation is a programmed feature of normal cell biology.1
Human relevance
Inherited defects in the enzymes and transporters of the glycosylation machinery cause congenital disorders of glycosylation, multisystem conditions that illustrate how central enzyme-directed glycan assembly is to normal human development.1
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References