What Is a Peptide? Amino Acids, Bonds and Sequences

Amino acids are the building blocks

Twenty standard amino acids are encoded by the genetic code. Each has a central carbon atom bearing an amino group, a carboxyl group and a distinctive side chain. The side chain gives each amino acid its character: some are charged, some attract water and some repel it.

The peptide bond

When two amino acids join, the carboxyl group of one reacts with the amino group of the other and a molecule of water is released. The new link is a peptide bond, which is a kind of amide bond. Each amino acid within a chain is called a residue.

A chain has a direction

One end of a chain has a free amino group (the N-terminus) and the other a free carboxyl group (the C-terminus). By convention, sequences are written from the N-terminus to the C-terminus. The sequence of Semax, Met-Glu-His-Phe-Pro-Gly-Pro, begins with methionine at its N-terminus and ends with proline.

How long is a peptide?

There is no single cut-off. Short chains are often called oligopeptides and longer ones polypeptides, and molecules of roughly fifty residues or more are commonly called proteins, but the boundary is a convention. Some compounds studied as peptides have only three residues, such as GHK or glutathione, and others have a few dozen.

Peptides and proteins

Most proteins fold into stable three-dimensional shapes. Short peptides usually have much less fixed structure, and many are flexible in solution.

Natural and synthetic peptides

Some peptides are made by cells. Most research peptides are made by chemical synthesis, which can include features cells do not use, such as mirror-image (D) amino acids or chemically modified ends. See How Peptides Are Made.

Why the sequence matters

A peptide’s properties depend on which amino acids it contains and in what order. The same amino acids in a different order make a different molecule. That is why a sequence, a molecular weight and a CAS number together identify a compound. See How to Read a Peptide Sequence.