A protein kinase inhibitor (PKI) is a type of enzyme inhibitor that blocks the action of one or more protein kinases. Protein kinases are enzymes that phosphorylate (add a phosphate, or PO4, group) to a protein and can modulate its function.
The phosphate groups are usually added to serine, threonine, or tyrosine amino acids on the protein. Most kinases act on both serine and threonine, the tyrosine kinases act on tyrosine, and a number (dual-specificity kinases) act on all three. There are also protein kinases that phosphorylate other amino acids, including histidine kinases that phosphorylate histidine residues.[citation needed]
Phosphorylation regulates many biological processes, and protein kinase inhibitors can be used to treat diseases due to hyperactive protein kinases (including mutant or overexpressed kinases in cancer) or to modulate cell functions to overcome other disease drivers.