**3.4 Collagen maturation**

Inter-and intra-molecular covalent cross-links are formed between and within tropocollagen (collagen triple-helix) units giving strength to collagen fibres. Intramolecular cross-links form between adjacent lysine groups and within individual triple-helix units and intermolecular cross-links occur between two triple-helix units comprising of two hydroxylysine groups and a lysine group.

The enzyme Lysyl oxidase converts the NH3 <sup>+</sup> group on the lysine and hydroxylysine sidechains to an aldehyde that then undergoes a condensation reaction forming an adol cross-link with other converted lysine sidechains. In each tropocollagen unit, four groups can contribute in the intermolecular cross-linking; lysines near the amino and carboxyl ends in the non-helical regions and hydroxylysines in the helical region. A hydroxyl-pyridinium cross-link is formed between one lysine and two hydroxylysine between residues near the amino-acid

#### **Figure 5.** *Collagen fibre showing the striations where tropocollagen is densely packed (light sections) [23] (used with permission).*

### *Collagen: From Waste to Gold DOI: http://dx.doi.org/10.5772/intechopen.94266*

end of one tropocollagen unit and the residues near the carboxyl-end of an adjacent tropocollagen unit. The enzyme Lysyl oxidase is small enough to fit between the 400-Å gaps between the triple-helix molecules to initiate the intermolecular cross-linking.

Collagen maturity or the amount of cross-linking increases drastically with age of the tissue and depends on the type and function of the tissue where collagen is found.
