Editing Protein biosynthesis (section)

===Formation of covalent bonds===
[[File:Formation of disulphide covalent bonds.png|400px|thumb|alt=Formation of a disulfide bond between two cysteine amino acids within a single polypeptide chain and formation of a disulphide bond between two cysteine amino acids on different polypeptide chains, thereby joining the two chains.|Shows the formation of disulphide covalent bonds as a post-translational modification. Disulphide bonds can either form within a single polypeptide chain (left) or between polypeptide chains in a multi-subunit protein complex (right).]]
Many proteins produced within the cell are secreted outside the cell to function as [[extracellular]] proteins. Extracellular proteins are exposed to a wide variety of conditions. To stabilize the 3D protein structure, covalent bonds are formed either within the protein or between the different polypeptide chains in the quaternary structure. The most prevalent type is a [[disulfide|disulfide bond]] (also known as a disulfide bridge). A disulfide bond is formed between two [[cysteine]] amino acids using their side chain chemical groups containing a Sulphur atom, these chemical groups are known as [[thiol]] functional groups. Disulfide bonds act to stabilize the [[protein structure|pre-existing structure]] of the protein. Disulfide bonds are formed in an [[redox|oxidation reaction]] between two thiol groups and therefore, need an oxidizing environment to react. As a result, disulfide bonds are typically formed in the oxidizing environment of the endoplasmic reticulum catalyzed by enzymes called protein disulfide isomerases. Disulfide bonds are rarely formed in the cytoplasm as it is a reducing environment.<ref name="Alberts 2015" />