What is a peptide?

A peptide is a short chain of amino acids joined together by peptide bonds.

Amino acids are small molecules, and they're the building blocks of every protein in your body. There are 20 main amino acids, each with a one-letter code. When two amino acids join, they form a peptide bond and give off a molecule of water. Keep adding amino acids and you get a chain. That chain is a peptide.

Peptides by size

Name	Amino acids	Example
Dipeptide	2	Glycylglycine, the first peptide ever made in a lab (1901)
Tripeptide	3	Glutathione (glutamate, cysteine, glycine)
Oligopeptide	A few, up to about 20	Oxytocin (9)
Polypeptide	Many, in one long chain	Insulin's B-chain (30)
Protein	Usually 50 or more, folded into a shape	Ribonuclease A (124)

The line between a long peptide and a small protein isn't exact. Chemists generally call molecules of about 50 or more amino acids proteins, and shorter ones peptides.

Where the word comes from

German chemist Emil Fischer introduced the name "peptide" in 1902, from the Greek pepsis, meaning digestion. A year earlier, he and Ernest Fourneau had made the first synthetic dipeptide, glycylglycine.

Peptides are everywhere in nature

Your body makes thousands of peptides. Some are hormones that carry signals, such as insulin and oxytocin. Others are found in venoms, antibiotics made by bacteria, and plants. See peptides in the body for well-known examples.