"Serotonin is a pretty wild molecule for many reasons. First of all, it is formed form the amino acid L-tryptophan, which is one of the 20 standard amino acids required for life as we know it. Interestingly, tryptophan is also one of the few "essential" amino acids for humans, meaning that we don't make it ourselves, and have to get it from the diet. But don't worry, you've usually got plenty. The only way anyone could really suffer "tryptophan depletion" is if you're in a lab and they give you tons of other amino acids, or if you're starving. And if you're starving, you've obviously got bigger problems.
To make serotonin, start out with some L-tryptophan. This gets broken down in cells by an enzyme known as tryptophan hydroxylase to 5-hydroxytryptophan, this then gets broken down using the enzyme amino acid decarboxylase to 5-hydroxytryptamine, or 5-HT. Then the 5-HT is ready to be stored in vesicles in preparation for..."
Go to Neurotopia for more.