Phosphorylation plays a key role in the activation or deactivation of enzymes and requires ATP (adenosine triphosphate) as a source of the phosphate group. With the action of the enzyme ‘tyrosine kinases’, phosphorylation occurs in phenolic group of the residues of tyrosine in protein substrates.
On the other hand, the phosphorylation occurs in the alcoholic groups of residues of serine and threonine in the presence of enzyme serine-threonine kinases. All of these binding groups are capable of participating in hydrogen bonding. Addition of the bulky phosphate group to the alcoholic groups of serine and threonine residues in protein substrate leads to disruption in the hydrogen bonding.
The ionization of phosphate group usually takes place at a physiological pH. Thus negatively charged oxygen is introduced with phosphorylation. Phosphorylation in the presence of kinase enzymes result in desensitization of the receptors linked to the G-protein.
The phosphorylation of the residues of serine and threonine occurs, after prolonged binding of a ligand, on the intracellular carboxyl-terminal (C-terminal) chain. It results in a change in the conformations of protein in that region which, in turn, prevents the binding of the G-protein from binding.
Hence, the complex formed by the binding of receptor and ligand is unable to activate the G-protein. Therefore, the phosphorylation of the phenolic group of tyrosine residues of the tyrosine kinases suggests that the active sites of the enzyme contain a hydrophobic region.