Amino Acid

What is an amino acid?

Amino acids are organic compounds with amino and carboxyl groups. The amino acids commonly used to build proteins have different side chains, whose properties contribute to how a protein folds and functions.

Protein building blocks

The Molecular Biology of the Cell textbook describes the twenty common amino acids used in protein chains, linked by peptide bonds. “Amino acid” is a broader chemical category; not every member is used to build proteins.

In neuroscience

Some amino acids also have signaling roles. The Neuroscience textbook’s discussion of GABA and glycine describes these inhibitory transmitters and the conversion of glutamate to GABA. GABA (gamma-aminobutyric acid) is made from glutamate and functions as a neurotransmitter, but is not incorporated into proteins.

Glutamate supports most fast excitatory signaling in the brain: through receptors such as AMPA and NMDA, it can move a receiving neuron toward firing. GABA usually reduces the likelihood of firing in mature circuits. Excitation and inhibition describe effects on cells, not whether a chemical makes a person feel excited or calm. The effect also depends on the receiving cell: developmental work on GABA signaling shows why the cell’s chloride balance matters.

This is why an amino acid can matter to neuroscience without being one of the twenty common protein building blocks.

Essential and Nonessential Amino Acids

Nutritional classifications concern whether the body can supply enough of an amino acid.

• Essential amino acids must come from the diet.
• Nonessential amino acids can be made by the body; “nonessential” does not mean unimportant.
• Conditionally essential amino acids may need an outside supply in particular physiological states.

Tryptophan, Serotonin, and Melatonin

Tryptophan is an essential amino acid used in serotonin synthesis. In the pineal gland’s melatonin pathway, tryptophan is converted to serotonin, then to N-acetylserotonin, and finally to melatonin. The intermediate steps require enzymes; these are different molecules with different functions. That biochemical role does not establish that eating more of it reliably improves mood or sleep.