Protein kinases transfer phosphate groups from ATP to proteins. The human genome encodes for over 500 structurally similar protein kinases that transfer the gamma-phosphate of ATP to serine, threonine, or tyrosine residues of proteins.
From Åberg, E. et al., "Structural origins of AGC protein kinase inhibitor selectivities: PKA as a drug discovery tool", Biol. Chem. (2012) 393, 1121-1129. PMID: 23092797.