An enzyme inside cells that gets switched on when certain peptide hormones bind to the cell surface, triggering a chain reaction that changes what the cell does.Catalyzes the conversion of ATP to cyclic AMP (cAMP) upon Gs-coupled receptor activation. The cAMP it produces acts as a second messenger, activating protein kinase A (PKA) and downstream targets. Central to melanocortin, GLP-1, VIP, and secretin receptor signaling — which is why so many diverse peptide effects share a similar intracellular mechanism.