Polypeptides after synthesis on the cell’s protein-assembly apparatus require further elaboration for use. Both the peptide backbone and its side chains may need to be altered by post-translational modifications , the covalent attachment of chemical groups that change the properties, and hence the function, of newly generated proteins. Post-translational modifications also control the degradation of aberrant proteins and proteins at the end of their lifespan. Such modifications dramatically expand the compositional and functional complexity of these molecules.
Proteins may exist as a mixture of forms, each incorporating different post-translational modifications.
One intricate form of post-translational modifications is the attachment of carbohydrates, glycans, either to nitrogen atoms (N-linked) in the side chains of asparagine amino acids, or to oxygen atoms (O-linked) on the side chains of serine or threonine. In living cells, glycosidase and glycosyl transferase enzymes first trim N-linked glycans, then extend them with sugars that can branch in several directions, generating numerous variations on a theme. Glycosyl transferases also act on O-linked glycans, imposing similar extensions and modifications. To add to the complexity, each sugar can bear different chemical groups, and the linkages between sugars have specific orientations. Many biological processes, such as cellular differentiation and development, cell adhesion, immune surveillance and inflammation, rely to varying degrees on the correct decoration of proteins with such glycans.
Grotenbreg and Ploegh Nature 2007, vol 446, pp 993-995.

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