Post-translational modification (PTM) is a process in protein biosynthesis that occurs after a protein has been translated from ribonucleic acid (RNA). The process of translation involves creating a chain of amino acids that corresponds to the RNA template. Once this chain has been formed the protein has been synthesized, but it must often undergo further changes before it becomes fully functional. These changes are known as post-translational modification processes, and include three-dimensional shaping, formation of disulfide bridges, phosphorylation, or the addition of other molecules.
One of the most simple post-translational modification actions a protein can undergo is adopting a stable, three-dimensional shape known as the native structure. This process often occurs directly after translation, and it is driven by hydrophobic interactions. Since the intracellular environment is aqueous, hydrophobic groups that repel water cluster together in the center of the protein away from the water, creating an energetically stable form. Additional proteins, known as chaperonins, can also help newly formed proteins fold into their correct shape.
Disulfide bridges and proteolytic cleavage reactions are other post-translational structural changes that may occur in proteins. If a protein contains two cysteine amino acid residues, it can form a covalent bond between the two if they are properly aligned, thereby altering the protein’s conformation. Similarly, some structural changes occur as a result of proteolytic cleavage, in which an enzyme cuts off a piece of the protein after it has been translated. An example of this process is the protein insulin, which remains in an inactive precursor form until undergoing proteolytic cleavage to form the active molecule.
The addition of functional groups, such as phosphate groups, sulfate groups, acyl groups, or methyl groups, is also a common post-translational modification. These groups can either activate a protein, inhibit it, or signal it to move elsewhere in the cell. For example, many enzymes switch between active and inactive states depending on whether or not they have been phosphorylated.
Ubiquitination is another form of post-translational modification that cells use to label proteins. The process involves the addition of ubiquitin, a small signaling protein, to an active protein to target it for degradation. Although ubiquitin can also sometimes act as a signaling molecule, it is generally used to modify active proteins that the cell is trying to degrade. In this way, a cell is able to control the levels of various enzymes and other proteins according to changes in the environment.
