Editing Protein biosynthesis (section)

===Addition of complex molecules===
[[File:Glycosylation of a polypeptide.png|400px|thumb|alt=Two polypeptide chains, one with an asparagine side chain exposed and a polysaccharide attached to the nitrogen atom within asparagine. The other polypeptide has a serine side chain exposed and the core of a polysaccharide attached to the oxygen atom within serine.| Illustrates the difference in structure between N-linked and O-linked glycosylation on a polypeptide chain.]]
Post-translational modifications can incorporate more complex, large molecules into the folded protein structure. One common example of this is [[glycosylation]], the addition of a polysaccharide molecule, which is widely considered to be most common post-translational modification.<ref name="Schubert 2015" />

In glycosylation, a [[polysaccharide]] molecule (known as a [[glycan]]) is covalently added to the target protein by [[glycosyltransferases]] enzymes and modified by [[glycoside hydrolases|glycosidases]] in the [[endoplasmic reticulum]] and [[Golgi apparatus]]. Glycosylation can have a critical role in determining the final, folded 3D structure of the target protein. In some cases glycosylation is necessary for correct folding. N-linked glycosylation promotes protein folding by increasing [[solubility]] and mediates the protein binding to [[chaperone (protein)|protein chaperones]]. Chaperones are proteins responsible for folding and maintaining the structure of other proteins.<ref name="Alberts 2015" />

There are broadly two types of glycosylation, [[N-linked glycosylation]] and [[O-linked glycosylation]]. N-linked glycosylation starts in the endoplasmic reticulum with the addition of a precursor glycan. The precursor glycan is modified in the Golgi apparatus to produce complex glycan bound covalently to the nitrogen in an [[asparagine]] amino acid. In contrast, O-linked glycosylation is the sequential covalent addition of [[Monosaccharide|individual sugars]] onto the oxygen in the amino acids serine and threonine within the mature protein structure.<ref name="Alberts 2015" />