<P> One such example of the regulatory role that phosphorylation plays is the p53 tumor suppressor protein . The p53 protein is heavily regulated and contains more than 18 different phosphorylation sites . Activation of p53 can lead to cell cycle arrest, which can be reversed under some circumstances, or apoptotic cell death . This activity occurs only in situations wherein the cell is damaged or physiology is disturbed in normal healthy individuals . </P> <P> Upon the deactivating signal, the protein becomes dephosphorylated again and stops working . This is the mechanism in many forms of signal transduction, for example the way in which incoming light is processed in the light - sensitive cells of the retina . </P> <P> Regulatory roles of phosphorylation include </P> <Ul> <Li> Biological thermodynamics of energy - requiring reactions <Ul> <Li> Phosphorylation of Na / K - ATPase during the transport of sodium (Na) and potassium (K) ions across the cell membrane in osmoregulation to maintain homeostasis of the body's water content . </Li> </Ul> </Li> <Li> Mediates enzyme inhibition <Ul> <Li> Phosphorylation of the enzyme GSK - 3 by AKT (Protein kinase B) as part of the insulin signaling pathway . </Li> <Li> Phosphorylation of src tyrosine kinase (pronounced "sarc") by C - terminal Src kinase (Csk) induces a conformational change in the enzyme, resulting in a fold in the structure, which masks its kinase domain, and is thus shut "off". </Li> </Ul> </Li> <Li> Important for protein - protein interaction via "recognition domains ." <Ul> <Li> Phosphorylation of the cytosolic components of NADPH oxidase, a large membrane - bound, multi-protein enzyme present in phagocytic cells, plays an important role in the regulation of protein - protein interactions in the enzyme . </Li> </Ul> </Li> <Li> Important in protein degradation . <Ul> <Li> In the late 1990s, it was recognized that phosphorylation of some proteins causes them to be degraded by the ATP - dependent ubiquitin / proteasome pathway . These target proteins become substrates for particular E3 ubiquitin ligases only when they are phosphorylated . </Li> </Ul> </Li> <Li> Important in glycolysis . </Li> </Ul>

The attachment of a phosphate to regulate the activity of an enzyme is referred to as