Phosphorylation is an important covalent post-translational modification (PTM) in cell signaling pathways. This modification is a reversible process, and is catalyzed by protein kinases. Study showed that over 30% of eukaryotic proteins are subject to phosphorylation. Phosphorylation modification has mainly two regulatory mechanisms for the target proteins: (1) phosphorylation may cause a conformational change in the structure of modified proteins, such as enzymes and receptors, turning “on” or “off” the function. (2) phosphorylation changes the proteins’ affinity to their effector, by doing so, phosphorylated proteins can recruit or release their downstream effectors. Hence, it is conceivable that phosphorylation modification regulates a broad range of biological activities, such as cell growth, cell metabolism, cell division, and so on.

source: http://www.creative-proteomics.com/services/phosphorylation.htm