After a signal is received, signal transduction involves altering the behavior of proteins in the cascade, in effect turning them on or off like a switch. Adding or removing phosphates is a fundamental mechanism for altering the shape, and therefore the behavior, of a protein. Phosphorylation may open up an enzyme's active site, allowing it to perform chemical reactions,
Phosphorylation may open the active site of an enzyme.
or it may frequently generate a binding site allowing a specific interaction (may make a bulge in one side preventing the protein from fitting together) with a molecular partner.
Enzymes that add phosphate groups to other molecules are called kinases, and the molecules the enzymes act on are called substrates. Protein kinases are a family of enzymes that use ATP to add phosphate groups on to other proteins, thereby altering the properties of these substrate proteins. Protein kinases themselves are frequently turned on or off by phosphorylation performed by other protein kinases; thus a kinase can be both enzyme and substrate.
Protein kinases can be broadly divided into two groups based on the amino acids to which they add phosphate groups. The serine/threonine kinases (ser/thr) are found in all eukaryotic cells and tend to be involved in regulation of metabolic and cytoskeletal activity. The tyrosine (tyr) kinases are found in all animals but not in yeast, protozoa, or plants, and appear to have evolved specifically to deal with the complex challenges of signaling in animals.
Molecular switches are useful only if they can also be flipped back to their original state. This is achieved by specific protein phosphatases, which can remove phosphate groups from kinase substrates.
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