noun, plural: isopeptide bonds
An isopeptide bond is the type of peptide bond that forms between the between the carboxyl group and an amino group of joining amino acids where at least one of them is part of the side chain. It may also form between the gamma-carboxamide group of glutamine and the primary amine of certain amino acids.
It is distinct from the eupeptide bond (also referred to simply as peptide bond). A eupeptide bond, in contrast, is an amide type of chemical bond that forms between the carbon 1 of alpha-amino acid and nitrogen 2 of another amino acid.
The isopeptide bond is less common compared with eupeptide bonds. The eupeptide bonds are the more common type.
An example of a peptide with an isopeptide bond is the glutathione. The glutathione has an isopeptide bond between the side chain of a glutamate residue and the amino group of a cysteine residue.
Ubiquitin is also another example. It is a protein that forms a complex with another protein by forming an isopeptide bond between its C-terminal of the glycine residue and the lysine side chain of the substrate protein.
The peptides containing isopeptide bonds are often associated with biosignaling and biostructural functions.