Isopeptide Crosslinks—Their Occurrence and Importance in Protein Structure

Isopeptide links refer to bonds between the ϵ‐amino group of lysine and the side‐chain carboxyl groups of glutamic or aspartic acid. Covalent crosslinks of this kind, which undoubtedly occur in protein feedstuffs and reduce their nutritional value, also participate In the fibrinogen → fibrin transformation and play a part in stabilizing the structure of keratins. Isopeptide links also arise on heating of both fibrous and globular proteins, the amount formed depending upon the severity of heat treatment. The mode of formation and the significance of these crosslinks require further elucidation.