Both epinephrine (a tyrosine derivative) and glucagon (a peptide hormone) increase glucose export from the liver into the bloodstream. Each ligand binds a different receptor, but both lead to an activation of PKA. How does this happen? Choose one: A. Each activated receptor interacts with a unique GTP binding protein that activates PKA. B. Both receptors bind and activate the same Ga subunit, Gsa, which indirectly leads to PKA activation. C. Both receptors bind directly to and activate PKA after the Gsa protein dissociates from the receptor. D. The receptors for the two ligands interact before binding to the heterotrimeric G proteins.

1. Both epinephrine and glucagon stimulate liver cells to increase blood glucose levels. 2. Epinephrine is a derivative of tyrosine, while glucagon is a peptide hormone. Despite their structural differences, both hormones initiate signals that culminate in the activation of protein kinase A (PKA). 3. Each hormone binds to its specific receptor on the cell surface. However, the receptors activate the same downstream signaling molecule, the heterotrimeric G protein \( \mathrm{G}_s \). 4. Upon ligand binding, \( \mathrm{G}_s \) activates adenylate cyclase, an enzyme that converts ATP to cyclic AMP (\( \text{cAMP} \)). 5. The increased \( \text{cAMP} \) serves as a second messenger, binding to the regulatory subunits of PKA. This binding causes the catalytic subunits to be released. 6. Once released, the catalytic subunits phosphorylate various proteins that lead to increased glucose export from the liver. Thus, the correct answer is: B. Both receptors bind and activate the same Ga subunit, \( G_{s} \), which indirectly leads to PKA activation.