In a β-adrenergic receptor signal-transduction pathway, to prevent the continuous activation of G-protein

In the first step, the intrinsic GTPase activity of the G-protein inactivates the stimulatory G-alpha subunit of the enzyme by forcing it to bind with GDP. After GTP hydrolysis, the phosphate is released and the G-alpha subunit reassociates with the G-beta, gama to form the heterotrimeric protein. Now, the inactivated G-protein no more activates adenylate cyclase and stops cyclic AMP formation, but this will not terminate the continuous activation of G-protein unless the ligand is permanently removed from the receptor. Therefore, the next important step is the resetting of the hormone-bound activated receptor. This is achieved by two processes. The first step marks the removal of hormone, and the receptor goes back to the inactivated state. The next step involves the phosphorylation of the serine and threonine residues in the carboxyl-terminal of the hormone-receptor complex. The final step includes the binding of B-arrestin to the phosphorylated receptor. This finally reduces the G-protein-activating ability of the receptor completely. Thus, the continuous activation of the G-protein is prevented by the phosphorylation and binding of B-arrestin that desensitize the receptor after continuous exposure to epinephrine hormone.