When a protein
is synthesized, the translational
machinery, and for that matter, the genetic template, is limited to 20 natural amino acids
. While these amino acids are able to cover a large spectrum of physical properties and chemical activities, some proteins require functional groups in addition to these. Many proteins, once synthesized, may undergo posttranslational modification
. As one can deduce from the name, this term describes processes that occur after translation.
One basic form of posttranslational modification is proteolytic processing. A number of proteins are synthesize in an inactive form. They can then be actived by another protein, a protease, which cuts the inactive protein at specific sites. This liberates a smaller part of the protein which is now active. The inactive protein is called a proenzyme or zymogen. Insulin is a classic example of this type of modification.
Other modifications are the addition of new chemical groups to a protein:
There are far more posttranslational modifications than the ones listed here, and I will continue to add them as I find them.