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An enzyme that phosphorylates another protein, that is, adds phosphate groups to proteins.
By phosphorlyating a specific protein a specific protein kinase modifies the activity of that protein, such as activating an enzyme. Protein kinases play important roles in signal transduction pathways since, by activating an enzyme such as another protein kinase, they can great amplify a signal. Protein phosphatases, by contrast, dephosphorylate proteins, that is, remove phosphate groups.
Signal transduction pathways can consist of series of protein kinases, call them, for example, protein kinase A, protein kinase B, and protein kinase C. Upon activation of protein kinase A it can phosphorylate protein kinase B, activating that enzyme, Protein kinase B can then activate protein C.
Because each activated enzyme can in turn activate many individual enzyme copies, the result is a rapid amplification of the number of molecules involved in a signal transduction pathway. At the same time, protein phosphorylases are dephosphorylating these same enzymes, inactivating them. These properties contribute to the on-off nature of signal transduction pathways, which can rapidly go from all the way on (here when protein kinase A is active) to all the way off (when whatever signal that is keeping protein kinase A active is no longer present).
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