Why do many signal transduction pathways involve the protein kinase?
Protein kinase is like a switch. It can "turn on" (or off) a protein. They do this by changing the molecular configuration of the protein when the phosphate group is added to specific phosphorylation sites. This can expose (or close) sites that are active for specific reaction making the protein active (active-site cleft).
It is possible to change conformation of protein by adding phosphoryl group to a specific domain of the protein because phosphate can change a domain of the protein from hydrophobic to hydrophillic. Phosphoryl group is very hydrophillic at it have 2- charge and can form ion-dipole bond with water molecules.