Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane, proteins of the transferase class of enzymes that play a role in signal transduction across the cellular membrane. The vast majority of HKs are homodimers that exhibit , phosphotransfer, and phosphatase activity. HKs can act as cellular receptors for signaling molecules in a way analogous to tyrosine kinase receptors (RTK). Multifunctional receptor molecules such as HKs and RTKs typically have portions on the outside of the cell (extracellular domain) that bind to hormone- or growth factor-like molecules, portions that span the cell membrane (transmembrane domain), and portions within the cell (intracellular domain) that contain the enzymatic activity. In addition to kinase activity, the intracellular domains typically have regions that bind to a secondary effector molecule or complex of molecules that further propagate signal transduction within the cell. Distinct from other classes of protein kinases, HKs are usually parts of a two-component signal transduction mechanisms in which HK transfers a phosphate group from ATP to a histidine residue within the kinase, and then to an aspartate residue on the receiver domain of a response regulator protein (or sometimes on the kinase itself). More recently, the widespread existence of protein histidine phosphorylation distinct from that of two-component histidine kinases has been recognised in human cells. It is worth nothing that, in marked contrast to Ser, Thr and Tyr phosphorylation, the analysis of phosphorylated Histidine using standard biochemical and mass spectrometric approaches is much more challenging.

In terms of enzymology, a histidine kinase (EC 2.7.13.3, EnvZ, histidine protein kinase, protein histidine kinase, protein kinase (histidine), HK1, HP165, Sln1p) is an enzyme that catalyzes the chemical reaction

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