Isopeptide bond

An isopeptide bond is a type of amide bond formed between a carboxyl group of one amino acid and an amino group of another. An isopeptide bond is the linkage between the side chain amino or carboxyl group of one amino acid to the α-carboxyl, α-amino group, or the side chain of another amino acid. In a typical peptide bond, also known as eupeptide bond, the amide bond always forms between the α-carboxyl group of one amino acid and the α-amino group of the second amino acid. Isopeptide bonds are rarer than regular peptide bonds.^[1] Isopeptide bonds lead to branching in the primary sequence of a protein. Proteins formed from normal peptide bonds typically have a linear primary sequence.

Amide bonds, and thus isopeptide bonds, are stabilized by resonance (electron delocalization) between the carbonyl oxygen, the carbonyl carbon, and the nitrogen atom. The bond strength of an isopeptide bond is similar to that of a peptide due to the similar bonding type. The bond strength of a peptide bond is around 300 kJ/mol, or about 70 kcal/mol.^[2]

Amino acids such as lysine, glutamic acid, glutamine, aspartic acid, and asparagine can form isopeptide bonds because they all contain an amino or carboxyl group on their side chain. For example, the formation of an isopeptide bond between the sidechains of lysine and glutamine is as follows:

• Gln−(C=O)NH₂ + Lys-NH₃⁺ → Gln−(C=O)NH−Lys + NH₄⁺

The ε-amino group of lysine can also react with the α-carboxyl group of any other amino acid as in the following reaction:

• Ile-(C=O)O^- + Lys-NH₃⁺ → Ile-(C=O)NH-Lys + H₂O

Isopeptide bond formation is typically enzyme-catalyzed.^[3] The reaction between lysine and glutamine, as shown above, is catalyzed by a transglutaminase. Another example of enzyme-catalyzed isopeptide bond formation is the formation of the glutathione molecule. Glutathione, a tripeptide, contains a normal peptide bond (between cysteine and glycine) and an isopeptide bond (between glutamate and cysteine). The formation of the isopeptide bond between the γ-carboxyl group of glutamate and the α-amino group of cysteine is catalyzed by the enzyme γ-glutamylcysteine synthetase.^[3] The isopeptide bond is formed instead of a eupeptide bond because intracellular peptidases^[3] are unable to recognize this linkage and therefore do not hydrolyze the bond. An isopeptide bond can form spontaneously as observed in the maturation of the bacteriophage HK97 capsid.^[4] In this case, the ε-amino group of lysine autocatalytically reacts with the side chain carboxamide group of asparagine.^[4] Spontaneous isopeptide bond formation between lysine and asparagine also occurs in Gram-positive bacterial pili.^[5]