Correct option is D
Explanation-
Nitric oxide (NO) is a gaseous signaling molecule that acts as an important second messenger in various physiological processes, especially vasodilation (blood vessel relaxation). The main target of NO inside the cell is soluble guanylyl cyclase (sGC). NO diffuses across cell membranes because it’s a gas. NO binds to soluble guanylyl cyclase (sGC) in the cytoplasm. sGC gets activated and converts GTP → cGMP. cGMP (cyclic GMP) acts as a secondary messenger, leading to processes like smooth muscle relaxation, vasodilation, etc.
Incorrect Options-
Option a - Phosphodiesterase (PDE) - Enzyme that breaks down cyclic nucleotides (cAMP or cGMP). NO doesn't stimulate PDE. In fact, PDE lowers cGMP levels, while NO increases cGMP by activating guanylyl cyclase. PDE acts downstream, not as the NO target.
Option b - Nitric oxide synthase (NOS) - Enzyme that synthesizes NO from L-arginine. NOS is the producer of NO, not the enzyme that NO stimulates. The question asks about the downstream target of NO, not its source.
Option c - Adenylyl cyclase - This converts ATP → cAMP . This enzyme belongs to the cAMP signaling pathway, not the cGMP pathway used by NO. NO doesn't act on adenylyl cyclase at all.
Correct Answer:
Option d: Guanylyl cyclase
