Any enzyme that catalyses the transfer of a phosphate group from a high-energy molecule, e.g. ATP, to a substrate molecule (usually another protein). It is a common mechanism for propagating intra-cellular signalling, allowing amplification of signals, activation (or inhibition) of more than one downstream target and allowing feedback control and modulation by other pathways.
The reverse process, i.e. removal of phosphate, is catalysed by a phosphatases.
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