With help of pyridoxine, serotonin is synthesized from which of the following precursor amino acid?
**Core Concept**
Serotonin synthesis in the brain involves the conversion of a precursor amino acid into 5-hydroxytryptophan, which is then decarboxylated to form serotonin. Pyridoxine, also known as vitamin B6, serves as a cofactor for the enzyme tryptophan hydroxylase, which catalyzes the initial step of serotonin synthesis.
**Why the Correct Answer is Right**
The correct precursor amino acid is tryptophan. Tryptophan is converted into 5-hydroxytryptophan by the enzyme tryptophan hydroxylase, which requires pyridoxine as a cofactor. This reaction is the rate-limiting step in serotonin synthesis. The resulting 5-hydroxytryptophan is then decarboxylated by the enzyme aromatic L-amino acid decarboxylase to form serotonin.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a precursor amino acid for serotonin synthesis. While it's true that glycine is an inhibitory neurotransmitter, it's not directly involved in serotonin synthesis.
**Option B:** Phenylalanine is an amino acid that can be converted into tyrosine, which is then converted into dopamine and subsequently into norepinephrine and epinephrine. It's not directly involved in serotonin synthesis.
**Clinical Pearl / High-Yield Fact**
It's worth noting that serotonin synthesis can be affected by various factors, including pyridoxine deficiency, which can impair tryptophan hydroxylase activity. This can have implications for mood disorders, such as depression, which are often associated with serotonin dysfunction.
**Correct Answer: C. Tryptophan.**