Peptide bond

A peptide bond is the chemical bond that holds amino acids together in peptides and proteins. It forms in a dehydration, or condensation, reaction: the carboxyl group of one amino acid joins the amino group of another, and a molecule of water is given off. The resulting carbon-nitrogen link is a type of amide bond. In cells, ribosomes make peptide bonds during translation, adding one amino acid at a time.

Because electrons are shared across the carbonyl oxygen, carbon and nitrogen, the peptide bond has partial double-bond character. This makes it rigid and flat, so the atoms around it lie in a single plane, and it almost always takes the trans arrangement. The chain can rotate only at the bonds on either side of each alpha carbon, which limits how it can fold and helps produce regular shapes such as the alpha helix.

A chain of linked amino acids has a free amino group at one end, the N-terminus, and a free carboxyl group at the other, the C-terminus; sequences are written from N to C. Peptide bonds are broken by hydrolysis, which digestive enzymes such as pepsin and trypsin speed up.