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Protein phosphorylation is the attachment of a phosphate (PO4) group to a protein. The new phosphorus group alters the role of the protein: it can activate, deactivate, or cause a change in function. Protein phosphorylation is fairly common in cells of 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 is dependent on another kinase. Sometimes a cell uses a string of reactions, called a phosphorylation cascade, to produce an outcome. The impetus for this type of event is usually a signal from outside the cell. Usually, the energy and the phosphate group for these operations comes from adenosine triphosphate, a ubiquitous feature of the cellular landscape.
Once the new phosphate group is added by protein phosphorylation, it changes the structure of its host protein. The shape of a whole protein—called its tertiary structure—is dependent on a variety of factors, including electrical charge. The negative charge of the phosphate group changes the tertiary structure sufficiently to alter the function of the whole protein. Some proteins can be phosphorylated at multiple locations, with different effects resulting from each. Phosphorylation only occurs at 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. Because most functions are carried out by proteins, the cell's usual way of performing some operation involves making more or fewer of the enzyme that carries it out. This system is relatively slow; it also is more difficult to undo, since most proteins will only stop functioning when they are destroyed.
The effects of protein phosphorylation can be undone by an enzyme called a phosphatase. This process is called dephosphorylation. Dephosphorylation works almost exactly like phosphorylation. Each process requires the other to be useful. It is the possibility of rapidly phosphorylating then dephosphorylating that makes this pathway a finer 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.