A tyrosine kinase is an enzyme which catalyzes the transfer of a phosphate group from adenosine triphosphate(ATP) to the hydroxyl oxygen atom on the amino acid tyrosine. This process is known as tyrosine phosphorylation. Tyrosines with a phosphate group are called phosphotyrosines. Tyrosines are present in several cellular proteins which are involved in cell signaling pathways (this includes tyrosine kinases themselves), usually in a cascade of phosphorylation events in which the phosphorylation of one protein leads to the phosphorylation of others downstream in the signalling pathway. These types of signaling pathways predominantly control cell growth, differentiation and migration, as well as metabolism and communication between cells.
Tyrosine phosphorylation is a highly controlled process under normal conditions. In non-dividing cells, tyrosine phosphorylation is at a very low level. In dividing cells, it is abundant. An extraordinarily high amount of tyrosine phosphorylation is also seen in cancer cells and in cases of inflammation due to oxidative stress or pathologic infections.