Protein phosphorylation is a post-translational modification of proteins in which a serine, a threonine or a tyrosine residue is phosphorylated by a protein kinase by the addition of a covalently bound 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 an inactive form. The reverse reaction of phosphorylation is called dephosphorylation which is catalyzed by protein phosphatases. Protein kinase and phosphatases work separately and in a balance to regulate the function of protein.
Functions of phosphorylation
The commonly occurring of protein kinase domain for phosphorylation on the serine, threonine, or tyrosine residue in eukaryotes genome such as yeast, worm and fly reveals the importance of phospho-signaling in higher organism. The phosphorylation mediated signaling is equally important in prokaryotes.
In some reactions, the purpose of phosphorylation is to "activate" or "volatize" a molecule, increasing its energy so it is able to participate in a subsequent reaction with a negative free-energy change. Glycogen phosphorylase, which accelerates the degradation of glycogen into glucose I-P, is the first enzyme found to be regulated by reversible phosphorylation. It works by altering allosteric conformation. 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 turned "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.
- Johnson L N, Lewis R J. Structural basis for control by phosphorylation[J]. Chemical reviews, 2001, 101(8): 2209-2242.
- Johnson L N, Barford D. The effects of phosphorylation on the structure and function of proteins[J]. Annual review of biophysics and biomolecular structure, 1993, 22(1): 199-232.