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Agonist binding to its receptor, such as epinephrine binding to theα1-adrenergic

receptor activates a heterotrimeric G protein, a single agonist–receptor interaction can activate more than one G protein, this step of the signal transduction pathway serves to amplify the original extracellular signal

The many types of ligand-receptor complexes may activate the G protein. These receptors experience a conformational change upon agonist engagement, which activates a related heterotrimeric G protein.

The G protein exchanges GDP for GTP and dissociates. Gαand Gβγunits may activate or inhibit targets such as adenylate cyclase, which produces the cAMP activator of protein kinase A (PKA).

A heterotrimeric G protein maintains its heterotrimeric state and its Gαsubunit binds GDP. However, the binding of such a Gα·GDP–Gβγcomplex to its cognate GPCR in a complex with an agonist induces the Gαsubunit to exchange its bound GDP for GTP. Thus, the ligand–GPCR complex functions as the Gαsubunit’s guanine nucleotide exchange factor (GEF).