[2] Chemical properties and separation of phosphoamino acids by thin-layer chromatography and/or electrophoresis

This chapter discusses the chemical properties and separation of phosphoamino acids by thin-layer chromatography and electrophoresis. Phosphorylated amino acid residues in proteins are commonly classified into three main groups: (1) O-phosphates or O-phosphomonoesters, which are formed by phosphorylation of the hydroxyamino acids serine, threonine, and tyrosine; (2) N-phosphates or phosphoramidates, which are produced by phosphorylation of the basic amino acids arginine, histidine, and lysine; and (3) acylphosphates or phosphate anhydrides, which are generated by phosphorylation of the acidic amino acids aspartic acid and glutamic acid. Two types of intracellular phosphoproteins are known: enzymes that are intermediately phosphorylated, usually at their active sites, and proteins that are phosphorylated by protein kinases. Phosphorylated intermediates in enzymatic mechanisms are either phosphoramidates or acyl phosphates, but in some cases, such as phosphoglucomutase and alkaline phosphatase, the active site of the enzyme is modified at a serine residue.