Final answer:
Adenylyl cyclase is the correct answer for the one that is not a second messenger. It is an enzyme that catalyzes the formation of the second messenger cAMP from ATP, while cAMP, cGMP, Ca2+, and IP3 are all examples of second messengers.
Step-by-step explanation:
To address the question of which one of the following is not a second messenger, the correct answer is A. Adenylyl cyclase. To clarify, second messengers are molecules that transmit signals from receptors on the cell surface to targets within the cell. Examples of second messengers include cyclic AMP (cAMP), cyclic GMP (cGMP), calcium ions (Ca2+), and inositol triphosphate (IP3).
Adenylyl cyclase is not a second messenger but rather an enzyme that catalyzes the conversion of ATP into the second messenger cAMP. The role of cAMP is to activate protein kinases like cAMP-dependent kinase (A-kinase), which phosphorylates proteins and initiates a cascade of cellular events. Similarly, other second messengers like Ca2+ and IP3 are involved in further signaling pathways, such as activating protein kinases and releasing calcium ions from internal stores, respectively.