A
protein kinase is a
kinase enzyme that modifies other
proteins by chemically adding
phosphate groups to them (
phosphorylation). Phosphorylation usually results in a functional change of the target protein (
substrate) by changing enzyme
activity, cellular location, or association with other proteins. The
human genome contains about 500 protein kinase genes and they constitute about 2% of all human genes. Up to 30% of all human proteins may be modified by kinase activity, and kinases are known to regulate the majority of cellular pathways, especially those involved in
signal transduction. Protein kinases are also found in bacteria and plants.