Adenylyl cyclase system provides the action of hydrophilic hormones on the cell.
Initially, the hormone binds to the cell receptor. Further, the hormone-receptor complex interacts with the G-protein and changes its conformation. As a consequence, adenylyl cyclase is activated.
Adenylyl cyclase catalyzes the formation of cAMP from ATP. The activity of adenylyl cyclase depends on the alpha subunit of the G-protein, with which it is associated.
cAMP is an allosteric regulator of protein kinase A (protein kinase A consists of 2 catalytic and 2 regulatory subunits). When cAMP binds to the regulatory protein kinase A subunit, catalytic subunits are released. Catalytic subunits transfer the phosphate residue from ATP to specific protein substrates (for example: phosphorylase kinase). These phosphorylated protein substrates initiate the characteristic effects of cAMP inside the cell (for example: activation of glycogen phosphorylase, inhibition of glycogen synthetase).
With the help of adenylyl cyclase system, various hormones realize their effects. For example: glycogenolysis and release of glucose are activated under the influence of glucagon in the liver.