Information AboutPhosphorylation |
| CATEGORIES ABOUT PHOSPHORYLATION | |
| cell biology | |
| signal transduction | |
| posttranslational modification | |
| phosphorus | |
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PROTEIN PHOSPHORYLATION Function In Eukaryotes , protein phosphorylation is probably the most important regulatory event. Many Enzyme s and Receptors are switched "on" or "off" by phosphorylation and dephosphorylation. Phosphorylation is catalyzed by various specific Protein Kinase s, whereas ''' Phosphatases ''' ''de''phosphorylate. Adding a phosphoryl (PO3) to a polar R group of an Amino Acid might not seem like it would do much to a protein, but it can actually turn a Nonpolar Hydrophobic protein into a Polar and extremely Hydrophilic molecule. An example of the important role that phosphorylation plays is the P53 Tumor Suppressor Gene , which—when active—stimulates Transcription of genes that suppress the Cell Cycle , even to the extent that it undergoes Apoptosis . However, this activity should be limited to situations where the cell is damaged or Physiology is disturbed. To this end, the p53 protein is extensively regulated. In fact, p53 contains more than 18 different ''phosphorylation'' sites. Upon the deactivating signal, the protein becomes dephosphorylated again and stops working. 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 . Signaling networks The network underlying phosphorylation can be very complex. In some cellular signalling pathways, a protein A phosphorylates B, and B phosphorylates C, but A also phosphorylates C directly, and B can phosphorylate D, which may in turn phosphorylate A. Types of phosphorylation ''See also Protein Kinase for more details on the different types of phosphorylation'' Within a protein, phosphorylation can occur on several Amino Acid s. Phosphorylation on Serine is the most common, followed by Threonine . Tyrosine phosphorylation is relatively rare. However, since tyrosine phosphorylated proteins are relatively easy to purify using Antibodies , tyrosine phosphorylation sites are relatively well understood. Histidine and Aspartate phosphorylation occurs in Prokaryotes as part of two-component signalling. OTHER KINDS ATP , the "high-energy" exchange medium in the cell, is synthesized in the Mitochondrion by addition of a third phosphate group to ADP in a process referred to as Oxidative Phosphorylation . ATP is also synthesized by Substrate Level Phosphorylation during Glycolysis . ATP is synthesized at the expense of solar energy by Photophosphorylation in the Chloroplast s of plant cells. Phosphorylation of Sugar s is often the first stage of their Catabolism . It allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across their transporter. |
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