Dopamine and serotonin

Section 2 of 4

Dopamine and serotonin are two of the brain's neurotransmitters. Your body builds each one from an amino acid in just two steps.

Tyrosine is one of the 20 or so amino acids your proteins are built from. Its side group is a -⁠CH₂- joined to a benzene ring that carries an -⁠OH group. Two enzymes turn it into dopamine. The first adds a second -⁠OH group to the ring, on the corner next to the first, and makes levodopa, the Parkinson's medicine. The second takes the -⁠COOH group off as carbon dioxide, CO₂, the step that turns levodopa into dopamine.

Tyrosine becomes dopamine Two skeletal formulas with an arrow between them. Left: tyrosine, a benzene ring with an -OH group at the bottom and, going up from the top, a chain whose second carbon carries an -NH2 group and a -COOH group. The -COOH group is drawn in amber. Right: dopamine, the same ring with a second -OH group, in amber, on the corner next to the first, and a chain of two carbons ending in an -NH2 group. The -COOH group is gone. OHNH₂OOHOHOHNH₂ tyrosine dopamine
Tyrosine becomes dopamine, C₈H₁₁NO₂. The parts that change are amber: the -⁠COOH group leaves as CO₂, and a second -⁠OH group joins the ring. The 3D model shows dopamine.

Tryptophan is the amino acid with two joined rings in its side group: a benzene ring and a five-membered ring with an N–⁠H group. The same two kinds of step turn it into serotonin. One enzyme adds an -⁠OH group to the benzene ring, and the enzyme that makes dopamine takes the -⁠COOH group off as CO₂.

Tryptophan becomes serotonin Two skeletal formulas with an arrow between them. Left: tryptophan, a benzene ring joined to a five-membered ring with an N-H group, and going up from the five-membered ring a chain whose second carbon carries an -NH2 group and a -COOH group. The -COOH group is drawn in amber. Right: serotonin, the same two rings with an -OH group, in amber, on the benzene ring, and a chain of two carbons ending in an -NH2 group. The -COOH group is gone. NHNH₂OOHNHHONH₂ tryptophan serotonin
Tryptophan becomes serotonin, C₁₀H₁₂N₂O, by the same two kinds of step: the -⁠COOH group (amber) leaves as CO₂, and an -⁠OH group (amber) joins the benzene ring. The 3D model shows serotonin.

Both messengers end in the same way: an -⁠NH₂ group on a chain of two carbons. That amine end picks up an H⁺ and pairs with the -⁠COO⁻ group in the receptor's pocket.

What dopamine does

Dopamine nerve cells deep in the brain help control movement: these are the cells that slowly die in Parkinson's disease. Other dopamine nerve cells are linked with motivation. They are most active when something turns out better than expected, and this helps the brain learn which actions are worth repeating.

What serotonin does

About 90% of your body's serotonin is made in the gut, where it helps the muscles of the gut wall squeeze food along. Platelets, the tiny cell pieces in blood that help it clot, carry serotonin and release it at a wound. In the brain, serotonin nerve cells reach almost every part, and serotonin plays a part in mood, sleep and appetite.

Serotonin has at least 14 kinds of receptor and dopamine has 5, each on its own set of cells. Moods and feelings come from many messengers and many brain cells working together.

Quick check

Your body turns the amino acid tyrosine into dopamine in two steps. What do the two steps do?