Proteins are capable of executing numerous functions based on the versatility of their 20 different amino acids. Proteins are also covalently modified to change their functions via attachment of other groups besides amino acids. For example, to make proteins more resistant to degradation, acetyl groups can attach to the amino termini of proteins. Also, hydroxyl groups can attach to different proline residues which stabilizes fibers of newly synthesized collagen. Furthermore, many proteins, specifically those present on the surfaces of cells, acquire carbohydrate units from attachment of specific asparagine residues. Carbohydrates in the form of aspargine-linked (N-linked) or serine/threonine (O-linked) oligosaccharides are major structural components of many cell surface and secreted proteins. The process of attachment of sugar molecules (such as carbohydrate) to the protein is called glycosylation. Protein glycosylation is known as one of the major post-translational modifications with significant effects on protein folding, conformation distribution, stability and activity. This addition of sugars makes the protein more hydrophilic and therefore able to better participate in protein-protein interactions. Besides the addition of certain groups to amino acids, other special groups can also be generated by chemical rearrangements of side chains and the peptide backbone, for example by spontaneous formation and oxidation.
Reference: Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer, eds. "Biochemistry." New York: W.H. Freeman and Co., 2007: 57-58.