Cross-links

The rigidity and strength of a collagen ?ber is imparted through covalent cross-links both among and within the tropocollagen molecules.  As there are little, if any Cys residues in the last mature collagen these covalent cross-links are not disul?de bonds as generally found in proteins, but rather are different cross-links formed among Lys and its aldehyde derivative allysine. Allysine residues are created from Lys by the action of the monooxygenase lysyl oxidase in Figure. This copper-containing enzyme needs the coenzyme pyridoxal phosphate, derived from vitamin B6, for activity. The aldehyde set on allysine then reacts spontaneously with either the side-chain amino group of Lys or with other allysine residues on other polypeptide chains to form covalent interchain bonds.

The importance of cross-linking to the usual functioning of collagen is described   through the disease lathyrism.   This  happens  in  humans   and  other animals  by  the  ingestion  of  sweet  pea  Lathyrus  odoratus  seeds  that contain the chemical β-aminopropionitrile. This composite irreversibly  inhibits lysyl  oxidase  thus  preventing  the cross-linking  of the tropocollagen  molecule,  resulting  in  serious  abnormalities  of  the  bones  joints  and  huge  blood vessels due to the fragile collagen. One collagen de?ciency infect the Ehlers Danlos syndrome type V is due to a de?ciency in lysyl oxidase and concludes in hyperextensibility and hypermobile joints of the skin.