Amino acid

An amino acid is a small organic molecule built around a central carbon atom, with an amino group (-NH2) on one side, a carboxyl group (-COOH) on the other, and a variable side chain that gives each amino acid its distinct chemical identity. There are 20 standard amino acids used to build peptides and proteins in the body.

How amino acids link together

Amino acids join through a peptide bond: the carboxyl group of one amino acid reacts with the amino group of the next, releasing a water molecule and forming a covalent link. Repeat that reaction down a chain and the result is a peptide — the specific sequence of amino acids used determines the peptide’s shape and what it can bind to.

Why the side chain matters

The side chain is what separates one amino acid from another. Some side chains are charged, some are hydrophobic, some can form additional bonds — and that variation is what lets a folded chain of amino acids act as a receptor agonist, fitting a specific binding site the way a key fits a lock. Swap out even one amino acid in the sequence and the resulting molecule may no longer fit the same receptor at all.

Where amino acids come from

The body obtains amino acids either by synthesizing them internally or by breaking down dietary protein during digestion. Once free, they’re reused as raw material — assembled into new peptides and proteins as needed, based on the sequence encoded by DNA.