Q.32 Nitric oxide synthase is responsible for generation of Nitric oxide, an important signaling
molecule. The substrate for this enzyme is
(A)Glycine
(B) Lysine
(C) Histidine
(D)Arginine
Nitric oxide synthase (NOS) converts L-arginine into nitric oxide (NO) and L-citrulline, making arginine the correct substrate in this multiple-choice question. The right answer is (D) Arginine, as confirmed across biochemical sources detailing the enzyme’s reaction mechanism.
Correct Answer
The substrate for nitric oxide synthase is (D) Arginine. NOS catalyzes the oxidation of L-arginine to produce NO, a key signaling molecule involved in vasodilation, neurotransmission, and immune responses. This two-step process uses oxygen and NADPH, forming N-hydroxy-L-arginine as an intermediate before yielding NO and L-citrulline.
Option Explanations
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(A) Glycine: Incorrect. Glycine serves as a neurotransmitter and one-carbon donor but plays no role in NOS catalysis or NO production.
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(B) Lysine: Incorrect. Lysine is an essential amino acid involved in protein synthesis and collagen formation, not a substrate for NOS.
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(C) Histidine: Incorrect. Histidine acts as a precursor for histamine and coordinates metal ions in enzymes, but NOS specifically requires arginine’s guanidino group.
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(D) Arginine: Correct. L-Arginine’s guanidino nitrogen undergoes five-electron oxidation by NOS isoforms (nNOS, iNOS, eNOS).
Biological Relevance
NO from arginine regulates blood pressure and fights pathogens via iNOS in immune cells. Dysregulation links to diseases like hypertension, making NOS a therapeutic target. For students, this MCQ tests enzyme-substrate specificity in biochemistry exams.
