Protein phosphorylation is an important post-translational modification on proteins, which is responsible for many biological functions. Phosphorylation alters the structural conformation of a protein, causing it to become activated, deactivated, or modifying its function. Many human proteins have phosphorylation sites. The reverse reaction of phosphorylation is known as dephosphorylation which is catalyzed by protein phosphatase. Protein kinases and phosphatases work independently and in a balance to regulate the function of proteins. The amino acids most commonly phosphorylated are serine, threonine, and tyrosine in eukaryotes, and histidine in prokaryotes and plants. These phosphorylations play important and well-characterized roles in signaling pathways and metabolism. However, other amino acids can also be phosphorylated, including arginine, lysine, aspartic acid, glutamic acid and cysteine. Protein phosphorylation was first reported in 1906, however, it was nearly 50 years until the enzymatic phosphorylation of proteins by protein kinases was discovered.
Figure 1. Model of a phosphorylated serine residue
Functions of phosphorylation
Phosphorylation introduces a charged and hydrophilic group in the side chain of amino acids, possibly changing a protein's structure by altering interactions with nearby amino acids. Some proteins such as p53 contain multiple phosphorylation sites, facilitating complex, multi-level regulation. Because of the ease with which proteins can be phosphorylated and dephosphorylated, this type of modification is a flexible mechanism for cells to respond to external signals and environmental conditions.
Reversible phosphorylation of proteins occurs in both prokaryotic and eukaryotic organisms. It is estimated that 230,000, 156,000 and 40,000 phosphorylation sites exist in human, mouse and yeast, respectively. Kinases phosphorylate proteins and phosphatases dephosphorylate proteins. Many enzymes and receptors are switched "on" or "off" by phosphorylation and dephosphorylation. Reversible phosphorylation will lead to a conformational change in the structure in most receptors and enzymes, causing them to become activated or deactivated. Phosphorylation often occurs on serine, threonine, tyrosine and histidine residues in eukaryotic proteins. Histidine phosphorylation of eukaryotic proteins occurs more frequent than tyrosine phosphorylation. In prokaryotic proteins phosphorylation occurs on the serine, threonine, tyrosine, histidine or arginine or lysine residues. The addition of a phosphate (PO4) molecule to a non-polar R group of an amino acid residue can turn a hydrophobic portion of a protein into a polar and extremely hydrophilic portion of a molecule. In this way, protein dynamics can induce conformational changes in protein structures through remote isomers through other hydrophobic and hydrophilic residues in the protein.
One example of the regulatory role that phosphorylation plays is that the p53 tumor suppressor protein. The p53 protein is heavily regulated and contains over 18 various phosphorylation sites. Activation of p53 will lead to cell cycle arrest, which may be reversed under some, apoptotic cell death or circumstances. This activity occurs only in situations wherein the cell is damaged or physiology is disturbed in normal healthy individuals.
Upon the deactivating signal, the protein becomes dephosphorylated once more and stops operating. 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. After deactivating the signal, the supramolecular dephosphorylates again and stops operating.
Regulatory roles of phosphorylation include:
Biological thermodynamics of energy-requiring reactions
During the osmotic adjustment process, sodium (Na+) and potassium (K+) ions pass through the cell membrane transport process to phosphorylate Na+/ K+ -ATPase to maintain the homeostasis of water in the body.
Mediates enzyme inhibition
Phosphorylation of the GSK-3 enzyme by AKT (protein kinase B) as part of the insulin signaling pathway.
Phosphorylation of src tyrosine kinase 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".
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