Collagen cross-linking reactions

Collagen cross-links. Besides amide bonds between amino acids in the same a chain, bonds between amino acid side chains of different a chains can form "cross-links". These bonds originate from enzymatically-oxidized side chains of lysine and hydroxylysine residues. The oxidized residues react with other lysine and hydroxylysine residues, forming difunctional products. Reactions of such products with oxidized lysine or hydroxylysine yield trifunctional cross-links (Reiser et al., 1992). 

On secretion from the cell, the tropoelastin is aligned with the microfibrils, and lysyl oxidase initiates the reactions that cross-link elastin molecules, using lysine residues within the hydrophilic alternating domains in the proteins. This cross-linking reaction is the same as that which occurs in collagen. In this reaction, 2, 3, or 4 lysine residues are cross- linked to form a stable stracture. The net result of the cross-linking is the generation of a fibrous mesh that encircles the cells. 

Natural polyphenols like flavonoids or tannins possess a well-known protective effect towards oxidation, which can be used against pathologies in which free radicals are incriminated, like can-cer. " In cosmetic formulations, these antioxidant molecules are useful to protect the constitutive elements of skin, like collagen or elastin, against degradations or cross-linking reactions responsible for a decrease in elasticity and the appearance of wrinkles with ageing. ... 

Gelatins are denatured proteins produced by thermal or chemical treatment of collagen (15). The reversible gel-sol conversion of the gelatin solution makes it easy to produce smooth surfaces. Glutaraldehyde treatment of the gelatin in the gel state makes it insoluble in water due to cross-linking reactions (16). 

FIGURE 6.21 Collagen fibers are stabilized and strengthened by Lys-Lys cross-links. Aldehyde moieties formed by lysyl oxidase react in a spontaneous nonenzymatic aldol reaction. 

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