Phosphorylation

Extracellular signals such as neurotransmitters, hormones, light, neurotrophic factors and cytokines produces physiological effects through regulation of phosphorylation of phosphoproteins. Over 95% of protein phosphorylation occurs on serine residues, 3-4% on threonine residues and less than 1% on tyrosine residues. Protein kinases are enzymes which transfer phosphate groups to proteins, a reaction that requires ATP as a phosphate donor and enzyme catalyst. Protein phophatases reverse this chemical reaction.