The primary precursor in the biosynthesis of serotonin is tryptophan. Serotonin is a neurotransmitter derived from the essential amino acid tryptophan through a two-step enzymatic process. First, tryptophan is hydroxylated by the enzyme tryptophan hydroxylase to form 5-hydroxytryptophan (5-HTP). Next, 5-HTP is decarboxylated by aromatic L-amino acid decarboxylase to produce serotonin (5-hydroxytryptamine or 5-HT).

In contrast, tyrosine and phenylalanine are mainly involved in the biosynthesis of catecholamines such as dopamine, norepinephrine, and epinephrine. Histidine serves as the precursor for histamine, another biologically important amine, but not serotonin.

Thus, understanding the distinct pathways of amino acid metabolism is crucial for grasping neurotransmitter biosynthesis and related physiological functions.

Reference: Lehninger Principles of Biochemistry, 7th Edition, Chapter 21: Signal Transduction and Neurotransmitter Biosynthesis