Hormones, second messengers and the reversible phosphorylation of proteins: An overview

The interconversion of key regulatory proteins between phosphorylated and dephosphorylated forms is an extremely versatile mechanism for reversible altering their activities, and in mammalian cells may be almost as common as allosteric regulation. It is now evident that protein phosphorylation is the basis of a complex network of interlocking systems which allow a variety of hormones and other extracellular signals, acting through just a few second messengers to coordinate biochemical functions.