Isopeptides

Isopeptides are a class of peptides that differ from standard peptides in the way their amino acid residues are linked. In typical peptide bonds, the carboxyl group of one amino acid reacts with the amino group of another, forming an amide linkage. Isopeptide bonds, however, can be formed through different chemical linkages, most commonly involving the side chain functional groups of amino acids. For instance, a peptide bond might form between the epsilon-amino group of lysine and the alpha-carboxyl group of another amino acid, or between the gamma-carboxyl group of glutamic acid and the amino group of another amino acid.

This alternative mode of linkage leads to branched peptide structures or cyclic peptides with different connectivity compared to linear peptides. Isopeptide bonds play crucial roles in various biological processes. A well-known example is ubiquitination, where the protein ubiquitin is attached to target proteins via an isopeptide bond, typically between the C-terminus of ubiquitin and a lysine residue on the target protein. This linkage is central to protein degradation pathways and cellular signaling. Another significant example is the formation of the bacterial cell wall, where cross-linking of peptidoglycan chains involves isopeptide bonds. The presence of isopeptide bonds can affect a peptide's stability, conformation, and biological activity. Understanding isopeptides is important for comprehending complex protein modification and cellular functions.