The molecule cAMP usually directly activates __________.

• A. phosphodiesterase
• B. receptor tyrosine kinases
• C. G proteins
• D. adenylyl cyclase
• E. protein kinase A

Answer: (E)

Verified step by step guidance

1. Understand the role of cAMP (cyclic adenosine monophosphate) in cellular signaling. It is a second messenger important in many biological processes.
2. Recognize that cAMP is involved in the signal transduction pathway, where it relays signals from extracellular molecules to intracellular targets.
3. Identify that cAMP directly activates protein kinase A (PKA), which is a key step in the signaling pathway. PKA is an enzyme that phosphorylates various target proteins, leading to a cellular response.
4. Differentiate between the roles of other molecules listed: phosphodiesterase breaks down cAMP, receptor tyrosine kinases are activated by ligands, G proteins are involved in signal transduction but are not directly activated by cAMP, and adenylyl cyclase is the enzyme that synthesizes cAMP from ATP.
5. Conclude that the direct activation by cAMP is specific to protein kinase A, which then continues the signaling cascade by phosphorylating other proteins.