Phosphorylation is a ubiquitous cellular regulatory mechanism. Protein phosphorylation mediates signal transduction during development, transcription, immune response, metabolism, apoptosis, and cell differentiation. It is a reversible, covalent modification of a protein or lipid that serves to modify the activity of the phosphorylated molecule by inducing conformational changes within the molecule. This modification occurs either through the addition of phosphate groups via the transfer of the terminal phosphate from ATP to an amino acid residue and/or by their removal.
Families of specialized molecules catalyze the addition (kinases) or removal (phosphatases) of phosphate groups from proteins. Different classes of protein kinases act specifically on serine/threonine or tyrosine residues. Phosphorylation can alter substrate’s activity, subcellular localization, binding properties, or association with other proteins. The protein kinase complement of the human genome, known as the kinome, was completed in 2002, and 518 protein kinase genes were identified in a landmark Science paper, published by Manning et al. Aberrant regulation of kinases plays a causal role in many diseases, making these proteins very desirable drug targets. Cancer and other proliferative diseases, inflammatory diseases, metabolic disorders, and neurodegenerative diseases are among those in which protein phosphorylation plays an important role.