| Protein phosphorylation-dephosphorylation is one of the major signaling mechanisms for modulating the functional properties of proteins involved in gene expression, cell adhesion, cell cycle, cell proliferation, and differentiation. Proteins can be phosphorylated on specific serine, threonine, or tyrosine residues by the action of protein kinases. Protein kinases catalyze the transfer of a phosphoryl group from a high-energy compound, such as ATP, to a nucleophilic acceptor group located on the amino acid side-chain(s) of proteins. Protein phosphatases are phosphoesterases that catalyze the hydrolytic removal of a phosphate group from hydroxylated amino acid residues on proteins. In contrast to permanent covalent modifications of proteins, this reversible phosphorylation-dephosphorylation reaction is a fast and efficient way to change the properties of proteins in the compartmentalized cellular environment. In order to sustain target specificity among hundreds of phosphoproteins within a cell, cells maintain a vast network of delicately regulated protein kinases and phosphatases. |
