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.