Phosphorylation occurs only on specific amino acids, such as serine.
Protein phosphorylation is the attachment of a phosphate group (PO 4 ) to a protein. The new phosphorus group changes the protein’s role: it can activate, deactivate or cause a change in function. Protein phosphorylation is quite common in cells of both prokaryotic and eukaryotic organisms. It provides a way for the cell to regulate biological functions without having to change the actual amount of proteins available to perform them.
A molecule called a protein kinase, sometimes called a phosphotransferase, is responsible for inducing protein phosphorylation. There are many different protein kinases, all with different target proteins. Often, the activity of a protein kinase is itself dependent on phosphorylation. This process depends on another kinase. Sometimes a cell uses a sequence of reactions, called a phosphorylation cascade, to produce a result. The impetus for this type of event is usually a signal from outside the cell. Typically, the energy and phosphate group for these operations come from adenosine triphosphate, a ubiquitous feature in the cell landscape.
Once the new phosphate group is added by protein phosphorylation, it changes the structure of its host protein. The shape of an entire protein – called a tertiary structure – depends on a variety of factors, including electrical charge. The negative charge of the phosphate group changes the tertiary structure enough to alter the function of the entire protein. Some proteins can be phosphorylated at multiple sites, with different effects resulting from each. Phosphorylation occurs only on specific amino acids: serine, threonine and tyrosine.
Protein phosphorylation is a crucial element of biological homeostasis. Most cellular processes are stochastic – they rely on a set of partially random interactions that can only be managed statistically. Since most functions are performed by proteins, the cell’s usual way of performing some operation involves producing more or less of the enzyme that performs it. This system is relatively slow; it is also more difficult to break down, as most proteins only stop working when they are destroyed.
The effects of protein phosphorylation can be undone by an enzyme called phosphatase. This process is called dephosphorylation. Dephosphorylation works almost exactly like phosphorylation. Each process requires the other to be useful. It is the ability to rapidly phosphorylate and then dephosphorylate that makes this pathway a more refined means of control than the process of generating new proteins from DNA and RNA. The sum of the two processes, including the signals involved in completing them, is called phosphoregulation.