Question

The catecholamines are produced in the brain and are synthesized from the amino acid

• A. Tyrosine
• B. Threonine
• C. Tryptophan
• D. Serine

Hint:

Remember the pathway sequence: Tyrosine $\rightarrow$ L-DOPA $\rightarrow$ Dopamine $\rightarrow$ Norepinephrine $\rightarrow$ Epinephrine.
The rate-limiting and most tightly regulated step in this entire biosynthetic pathway is the first step catalyzed by tyrosine hydroxylase.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Catecholamines are a class of biogenic amines acting as neurotransmitters and hormones in the central and peripheral nervous systems.
This group includes dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline).
They are synthesized in a sequential enzymatic pathway starting from a single precursor amino acid.

Step 2: Detailed Explanation:
The primary precursor for catecholamine biosynthesis is L-tyrosine, a non-essential amino acid derived either from the diet or synthesized from phenylalanine in the liver.
The metabolic pathway proceeds through the following specific enzymatic steps:
1. Hydroxylation: L-tyrosine is converted to L-dihydroxyphenylalanine (L-DOPA) by the rate-limiting enzyme tyrosine hydroxylase, which requires tetrahydrobiopterin ($BH_4$) as a cofactor.
2. Decarboxylation: L-DOPA is decarboxylated to form dopamine by the pyridoxal phosphate-dependent enzyme aromatic L-amino acid decarboxylase (DOPA decarboxylase).
3. Hydroxylation: In noradrenergic neurons, dopamine is transported into synaptic vesicles where it is converted to norepinephrine by the copper-dependent enzyme dopamine $\beta$-hydroxylase.
4. Methylation: In the adrenal medulla and specific brain stem neurons, norepinephrine is methylated to epinephrine by the enzyme phenylethanolamine N-methyltransferase (PNMT), using S-adenosylmethionine (SAM) as the methyl donor.
Other amino acids like tryptophan (precursor to serotonin), threonine, and serine are not precursors for catecholamines.

Step 3: Final Answer:
Therefore, the catecholamines are synthesized from the amino acid Tyrosine, corresponding to option (A).