Phosphorylation is the addition or removal of a phosphate group on molecules, regulating proteins and pathways in cells. Kinases add phosphate groups, while phosphatases remove them. Serine/threonine protein kinases are important for extracellular signal-regulated kinases (ERKs), which transmit extracellular signals and amplify them inside the cell. Protein phosphorylation assays use antibodies to determine differences in structure. Other molecules, such as sugars, can also be phosphorylated, and phosphorylation of adenosine diphosphate (ADP) into ATP is essential for energy-intensive reactions.
Phosphorylation sites are particular areas of molecules that undergo the addition or removal of a phosphate group known as PO4. They are especially important for regulating proteins in the cell. Phosphorylation can turn the protein on or off and is an important way to regulate pathways in the cell. A single protein can have many phosphorylation sites, and any single cell can have thousands. Some results of phosphorylation gone wrong can include cancer and diabetes.
Proteins that can be phosphorylated include protein enzymes, which greatly speed up the rate of reactions. Phosphorylation of a protein can change its function or location in the cell. Individual enzymes vary in whether their active form has a phosphorylated or empty phosphorylation site.
Receptors are also important sites of phosphorylation. These signals transmit and signal transduction pathways are often regulated by phosphorylation sites. One factor that makes them advantageous for regulation is that signaling reaction times can vary from hours to less than a second. Phosphorylation pathways can be very complex with one set of proteins sequentially phosphorylating the next. This leads to amplification of a path.
A protein that adds a phosphate is called a kinase. These regulate a large number of reactions within the cell. Proteins that remove a phosphoryl group are called phosphatases.
Kinases get the phosphate group from adenosine triphosphate (ATP). They add the phosphate group to one of three amino acids: serine, threonine or tyrosine. Others may act on all three, or even on additional amino acids such as histidine. Some kinases have multiple specificities and can act on more than one target. Such broad target specificity allows for coordinated regulation of multiple pathways by a signal.
A very important subgroup of kinases is the serine/threonine protein kinase. Their phosphorylation site is the OH group of serine or threonine. Phosphorylation by these kinases can be regulated by chemical signals, as well as events such as DNA damage. MAP kinases are a well-studied group of this type, and a subgroup of MAP kinases is known as extracellular signal-regulated kinases (ERKs).
ERK phosphorylation is widely expressed as an intracellular signaling mechanism. What is important about these ERK kinases is that they transmit extracellular signals and amplify them inside the cell. The pathway is activated by many different extracellular factors, including growth factors, hormones and carcinogens. The ERK pathway is disrupted in many cancers.
Protein phosphorylation, and in particular the location of phosphorylation sites, is a very active field of study. Up to half of the proteins in a cell can be phosphorylated. Various companies specialize in predicting which areas of a protein can be phosphorylated.
The protein phosphorylation assay generally uses antibodies. These are proteins produced by an animal’s immune system that are specific to foreign invaders. There are hundreds of antibodies specific for phosphorylation-induced structural changes. The proteins are run on a gel that separates by size and charge and is known as two-dimensional electrophoresis. It is then treated with phospho-specific antibody to determine differences in structure.
It should be noted that other types of molecules can also be phosphorylated. For example, the phosphorylation of sugars is an important part of cellular metabolism. Glycolysis of the energy-producing metabolic pathway is one example. The first step in the breakdown of glucose is the phosphorylation of an OH group on the glucose molecule.
Phosphorylation of adenosine diphosphate (ADP) into ATP is essential for many of the energy-intensive reactions of the cell to occur. ATP is a high-energy molecule and gives off energy when it donates a phosphate group. Protein synthesis is among the many important cellular processes powered by ATP.
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