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Intramolecular and Intermolecular Crosslinks

Crosslinking renders the collagen fibers stable and provides them with a degree of tensile strength and visco-elasticity adequate to perform their structural role. The degree of crosslinking, the number and density of the fibers in a particular tissue, and the orientation and diameter combine to provide this function.

Crosslinking begins with the oxidative deamination of the ε-carbon of lysine or hydroxylysine to yield the corresponding semialdehydes, and is mediated by the enzyme lysyl oxidase. Enzymatic activity is inhibited by β-aminoproprionitrile, chelating agents, isonicotinic acid hydrazide, and other carbonyl reagents. Lysyl oxidase exhibits particular affinity for the lysines and hydroxylysines present in the nonhelical extensions of collagen, but can, at a slower pace, also alter residues located in the helical region of the molecule.

In general, lysine-derived crosslinks seem to predominate in soft connective tissues such as skin and tendon, whereas hydroxylysine-derived crosslinks are prevalent in the harder connective tissues such as bone, cartilage, and dentine, which are less prone to yield soluble collagens.

Several other crosslinks have been identified and their location established. These more complex polyfunctional crosslinks can contain histidine or can result in the formation of naturally fluorescent pyridinium ring structures.

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