Nitric oxide (NO) acts as a second messenger by activating soluble guanylyl cyclase. The mechanism involves:
- A NO binds the ATP-binding domain of guanylyl cyclase, acting as a competitive inhibitor
- B NO binds the haem prosthetic group of guanylyl cyclase, displacing O2 and activating the enzyme to produce cGMP ✓
- C NO activates PKA, which phosphorylates guanylyl cyclase
- D NO oxidises guanylyl cyclase cysteine residues via S-nitrosylation, activating it
Correct answer: B. NO binds the haem prosthetic group of guanylyl cyclase, displacing O2 and activating the enzyme to produce cGMP
Explanation
Soluble guanylyl cyclase (sGC) contains a haem group coordinated to histidine. NO binds to the ferrous iron of this haem, causing a conformational change that dramatically increases enzymatic activity, converting GTP to cGMP. The resulting cGMP activates PKG, leading to smooth muscle relaxation and vasodilation. This is the basis for nitrate therapy in angina.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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