What is the primary extracellular molecule involved in PKA activation?

The primary extracellular molecule involved in PKA (protein kinase A) activation is cyclic adenosine monophosphate, or cAMP. This molecule serves as a second messenger in various signaling pathways, particularly in response to hormone stimulation. When a signaling molecule, such as a hormone, binds to a cell surface receptor, it can activate an enzyme called adenylate cyclase. This enzyme catalyzes the conversion of ATP (adenosine triphosphate) into cAMP.

Once produced, cAMP interacts with the regulatory subunits of PKA, leading to a conformational change that releases the active catalytic subunits of PKA. The released catalytic subunits can then phosphorylate target proteins, modulating various cellular responses. This process is critical in many physiological functions, including metabolism and gene regulation.

The other molecules mentioned do not play this primary role in PKA activation. ADP, while involved in energy metabolism, does not directly interact with PKA. ATP serves as the substrate that is converted into cAMP but is not itself responsible for activating PKA. GTP is primarily involved in other signaling pathways and is not the main activator of PKA. Thus, cAMP is the correct and primary extracellular molecule that activates PKA