In the cAMP second messenger system, the activated G protein switches on which enzyme?

1. A water-soluble hormone binds a membrane receptor associated with an intracellular G protein. 2. Hormone binding activates the G protein, which activates adenylyl cyclase. 3. Adenylyl cyclase then converts ATP into cAMP, the second messenger. 4. cAMP activates a protein kinase, so the kinase sits one step later in the chain, not next to the G protein. 5. Phosphodiesterase ends the signal by degrading cAMP, and phospholipase C belongs to the calcium pathway instead. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 17 "The Endocrine System", section 17.2 Hormones_