Protein phosphorylation is a post-translational modification of proteins in which a serine, a threonine or a tyrosine reside is covalently modified by a protein kinase enzyme by the addition of a phosphate group. Regulation of proteins by phosphorylation is one of the most common modes of regulation of protein function, and is often termed "phosphoregulation". In almost all cases of phosphoregulation, the protein switches between a phosphorylated and an unphosphorylated form, and one of these two is an active form, while the other one is inactive.

Functions of phosphorylation

In some reactions, the purpose of phosphorylation is to "activate" or "energize" a molecule, increasing its energy so it is able to participate in a subsequent reaction with a negative free-energy change. All kinases require a divalent metal ion such as Mg²⁺ or Mn²⁺ to be present, which stabilizes the high-energy bonds of the donor molecule (usually ATP or ATP derivative) and allows phosphorylation to occur.

In other reactions, phosphorylation of a protein substrate can inhibit its activity (as when AKT phosphorylates the enzyme GSK-3). One common mechanism for phosphorylation-mediated enzyme inhibition was demonstrated in the tyrosine kinase called "src" (pronounced "sarc", see: Src (gene)). When src is phosphorylated on a particular tyrosine, it folds on itself, and thus masks its own kinase domain, and is thus shut "off".

In still other reactions, phosphorylation of a protein causes it to be bound to other proteins which have "recognition domains" for a phosphorylated tyrosine, serine, or threonine motif. As a result of binding a particular protein, a distinct signaling system may be activated or inhibited.

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.

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