Ubiquitination is the process of binding ubiquitin to a targeted molecule, which can add activity sites or change the molecule’s role. It tags targeted proteins for degradation through the proteasome pathway. Polyubiquitination chains tag target proteins for degradation, while single instances of ubiquitination can alter the function of the target protein. Ubiquitin is found in almost all tissues of eukaryotic organisms and is evolutionarily beneficial for removing broken or subfunctional proteins from circulation.
Ubiquitination is the process of binding ubiquitin, a small functional regulatory protein, to another targeted molecule. Like phosphorylation, ubiquitination can add sites of activity to a molecule or change that molecule’s role within the body, although the most common association scientists have with this process is that it tags targeted proteins for degradation. After a molecule has gone through the ubiquitination process more than once, that molecule is then tagged for digestion through the proteasome. Ubiquitination helps an organism monitor the use of a molecule so that the molecule is more likely to be digested when it has been around for a long time.
Ubiquitin binds to the lysine groups of target proteins through the activity of ubiquitin ligase (E3) and a ubiquitin-conjugating enzyme (E2). These proteins facilitate the activation and adhesion of ubiquitin directly to the target protein or to other ubiquitin proteins that have already been attached to the target protein. Some proteins are ubiquinated only once, creating an additional functional group at the lysine site. Some enzymes are capable of binding with ubiquitin, so a single instance of ubiquitination on a protein does not necessarily mark it for degradation via the proteasome pathway. Rather, the function of the target protein can be altered by a single ubiquitin adherent due to the addition of a new functional group or enzyme binding site.
When a chain of more than one ubiquitin molecule attaches to the same target protein, whether the chain is attached en masse or attached through multiple steps of a single instance of ubiquitination, that protein is said to be polyubiquitinated. Polyubiquitination chains have no known purpose, except to tag target proteins for degradation. Single instances of ubiquitination on different parts of the same molecule do not form polyubiquitination chains and do not necessarily mark the target protein for digestion via the proteasome.
Ubiquitin is found in almost all tissues of eukaryotic organisms and can be found associated with almost any protein, hence it is given a name that loosely means “protein that is everywhere”. It can be tagged with any protein that contains an available lysine group. It is evolutionarily beneficial for all cells in eukaryotic organisms to take advantage of the ubiquitination process because it removes broken, worn out, or subfunctional proteins from the circulation before the proteins stop functioning completely. A system that preemptively removes proteins from use and flags protein recycling keeps cells healthy on an individual cell basis, providing the reason for the evolutionary conservation and ubiquity of this process.
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