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Tyrosine kinase is an enzyme that activates or deactivates other proteins in the cells of animals. It does this by transferring a phosphate molecule to tyrosine, a specific amino acid on the target protein, which either activates or deactivates the target's biological function. This process controls many aspects of metabolism and normal cell division. There are many types of tyrosine kinase, many of which require outside signals before they become chemically active. In some forms of cancer, tumor growth is enhanced by the uncontrolled activity of kinases.
Phosphorylation is the addition of a phosphate group to a protein, typically an enzyme, causing structural changes. Tyrosine kinase is a subtype of protein kinase, an enzyme that phosphorylates an amino acid — or converts the amino acid to an organic phosphate. These catalysts are in turn activated or deactivated by the process, since their activities depend on changes to their chemical structures. Many of the normal and even diseased biochemical processes are controlled by phosphorylation. Tyrosine kinase transfers the phosphate group from the ubiquitous energy molecule ATP to a site on the amino acid tyrosine.
In many organisms, including humans and other mammals, tyrosine phosphorylation is essential to the processes of organ and tissue development, cellular movement, and cell proliferation. The genes that encode the many different tyrosine kinase enzymes are well-represented within the human genome since kinases effect changes in nearly a third of the proteins in the body. Tyrosine kinases regulate enzymes by altering the catalytic ability of the protein through phosphorylation. This changes the rate of a cellular biochemical reaction, such as metabolism, and can prevent it from occurring at all.
Like many biologically important molecules, some tyrosine kinases require activation by other proteins before they begin to phosphorylate an amino acid. This subgroup of enzymes are called receptor tyrosine kinases, compounds which will not catalyze the phosphate group transfer inside of a cell until after being bound by a specialized molecule called a ligand. At this point, a chain reaction begins at the moment of phosphorylation, triggering a cascade of chemical messengers to reach the cell's nucleus. There, they cause the genes within the nucleus to make more or or less of a desired protein product, which is usually another enzyme.
Malfunctioning receptor tyrosine kinases are linked to the development of some cancers. Mutation of the gene that encodes these proteins can make them over-active, constantly phosphorylating the tyrosine in a cell regardless of whether or not this is needed by the surrounding tissues. Normally, these kinases require a ligand to activate them, but the mutant forms function without any signals from other parts of the cell. In some research programs, a tyrosine kinase inhibitor has been used to prevent the growth of cancers, bolstering the evidence supporting a connection between uncontrolled tyrosine phosphorylation and malignant cell growth.