Based on Figure 1, which of the following is the first step in the activation of cAMP-dPK?
(A) The exchange of GTP for GDP
(B) The binding of epinephrine
(C) The separation of a-GTP
(D) Phosphorylation of intracellular enzymes

G-protein coupled receptors (GPCRs) are a common type of protein receptor containing seven transmembrane segments and bearing intracellular and extracellular portions. The structure of a typical GPCR and its associated partners is shown below. GPCRs are coupled to G-proteins consisting of three subunits (a, β, and γ) that bind the nucleotides GTP or GDP on the intracellular side of the GPCR. The presence or absence of a ligand on the extracellular side of the GPCR determines whether GDP or GTP will bind to the intracellular G-protein. Extracellular ligand binding initiates GDP being exchanged for GTP and the separation of the G-protein. One subunit will travel to join with a secondary partner and will initiate a cascade of signaling effects within the cell. In the example shown below, the secondary partner is adenylyl cyclase and the signaling is cAMP upregulation.

Transcribed Image Text:

JAMAM ooooo -Ligand (epinephrine) G-protein-linked receptor α GTP EXTRACELLULAR oooooo nos ma βγ GDP INTRACELLULAR lo v v W X Y l W X WV V α ALLAHLAOK GTP enzyme enzyme-PO α GTP CAMP MAMAX vy oooooo Adenylyl Cyclase ATP CAMP-dPK