Phosphorylation
- The most common type of reversible covalent modification is phosphorylation,
which involves the addition of a phosphoryl group on to the residues of enzymes
- For the most part, phosphoryl groups are added to to Ser, Thr and Tyr residues, transferred from ATP
- The enzymes which catalyze this phosphorylation are kinases. The importance of kinases
in regulation is demonstrated by the existence of over 500 different protein kinases in humans
(1.7% of genome). Collectively, they represent the 'kinome'
- Kinases regulate many kinds of activity in cells, including metabolism, signal transduction, cell division
and apoptosis
- The modification is reversible, through the complementary action of phosphatases, which are
enzymes that catalyze the removal of the phosphoryl groups