Monday's Molecule #53 was phosphothreonine, one of several amino acid residues that can be phosphorylated.
UPDATE= The other major ones are serine and tyrosine but histidine, lysine, cysteine, aspartate and glutamate can also be phospohylated in some proteins as well as some modified amino acids like hydroxy-proline (Reinders, and Sickmann 2005).
Enzyme activity can be modified by covalent attachment of a phosphoryl group to an amino acid residue. In some cases the phosphorylated enzyme will be active while in other cases the phosphorylated enzyme will be inactive.
The classic examples of regulation by covalent modification are in the pathways for synthesis and degradation of glycogen [Regulating Glycogen Metabolism]. For example, the enzyme glycogen phosphorylase is responsible for breaking down glycogen and liberating glucose. Glycogen phosphorylase is active when phosphorylated and inactive when the phosphoryl group is removed. The phosphorylation is carried out by phosphorylase kinase and this enzyme is, itself, regulated by covalent modification. These enzymes are part of a larger signal transduction pathway involving several different phosphorylated enzymes.
Reinders, J. and Sickmann, A. (2005) State-of-the-art in phosphoproteomics. Proteomics 5:4052-4061.