Question 1

Which enzyme is responsible for converting tyrosine to L-Dopa in the catecholamine biosynthesis pathway, and what is its required cofactor?

Accepted Answer

Tyrosine hydroxylase; cofactor: BH4 (tetrahydrobiopterin)

Question 2

A patient presents with elevated urinary levels of vanillylmandelic acid (VMA). Which neurotransmitter's metabolism is most likely increased?

Accepted Answer

Norepinephrine and epinephrine

Question 3

Which of the following steps in catecholamine biosynthesis requires Vitamin C as a cofactor?

Accepted Answer

Dopamine to norepinephrine conversion by dopamine β-hydroxylase

Question 4

Which enzyme is responsible for the degradation of serotonin to 5-hydroxyindoleacetic acid (5-HIAA), and why is 5-HIAA clinically significant?

Accepted Answer

MAO-A; 5-HIAA in urine is used to diagnose serotonin-secreting tumors

Question 5

Which neurotransmitter is synthesized from tryptophan and is involved in regulating circadian rhythm?

Accepted Answer

Melatonin

Question 6

A deficiency in PLP (Vitamin B6) would most directly impair the synthesis of which neurotransmitter(s)?

Accepted Answer

Dopamine, serotonin, histamine, and GABA

Question 7

Which enzyme catalyzes the conversion of acetyl-CoA and choline to acetylcholine, and in which types of neurons is this reaction most prominent?

Accepted Answer

Choline acetyltransferase (ChAT); motor, autonomic, and CNS cholinergic neurons

Question 8

Which of the following is the correct sequence of reactions in catecholamine biosynthesis?

Accepted Answer

Hydroxylation → decarboxylation → hydroxylation → methylation

Question 9

Which neurotransmitter is synthesized from glutamate and acts as the primary inhibitory neurotransmitter in the CNS?

Accepted Answer

GABA

Question 10

What is the role of nitric oxide (NO) in vascular smooth muscle relaxation, and which enzyme does it activate?

Accepted Answer

NO activates guanylate cyclase, increasing cGMP and causing smooth muscle relaxation

Question 11

Which enzyme is responsible for the inactivation of acetylcholine in the synaptic cleft, and what are the products of this reaction?

Accepted Answer

Acetylcholinesterase; products: acetate and choline

Question 12

A patient with a mutation in the gene encoding dopamine β-hydroxylase would have decreased levels of which neurotransmitter(s)?

Accepted Answer

Norepinephrine and epinephrine

Question 13

Which cofactor is required for the methylation step in both catecholamine and melatonin synthesis?

Accepted Answer

SAM (S-adenosylmethionine)

Question 14

Which neurotransmitter is produced by mast cells and is involved in immune response, stomach acid secretion, and CNS signaling?

Accepted Answer

Histamine

Question 15

Which enzyme and cofactor are required for the conversion of glutamate to GABA?

Accepted Answer

Glutamate decarboxylase (GAD); cofactor: PLP (Vitamin B6)

Question 16

Which of the following is a clinical marker for dopamine metabolism measured in urine?

Accepted Answer

Homovanillic acid (HVA)

Question 17

Choline can be synthesized from which amino acid, and which cofactors are required for its methylation?

Accepted Answer

Serine; cofactors: PLP and SAM

Question 18

Which enzyme is responsible for the conversion of arginine to nitric oxide (NO) and citrulline, and what are the required cofactors?

Accepted Answer

Nitric oxide synthase (NOS); cofactors: Fe-Heme, FAD, FMN, BH4

Question 19

Which neurotransmitter acts as a retrograde messenger in the nervous system, and what is its effect on synaptic transmission?

Accepted Answer

Nitric oxide (NO); it modulates presynaptic neurotransmitter release

Neurotransmitter Synthesis & Degradation: Biochemical Pathways and Clinical Relevance

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