Hydroxylysine (Hyl)

5-Hydroxylysine is an amino acid with the molecular formula C₆H₁₄N₂O₃. It is a hydroxy derivative of lysine. It is most widely known as a component of collagen.It is biosynthesized from lysine via oxidation by the enzyme lysyl hydroxylase.

Hydroxylysine is found only in animal proteins and mostly in collagens. The hydroxyl groups of hydroxylysine residues have two important functions: they serve as attachment sites for carbohydrates and they play a crucial role in stabilizing intra- and intermolecular crosslinks. Hydroxylysine also exists in collagen-like domains of several other proteins that are not defined as collagens. The content of hydroxylysine in the triple-helical domains of collagen peptide chains varies from 6 residues per 1,000 amino acids. Only 17% of the lysyl residues are hydroxylated in collagen type III, whereas almost 90% are hydroxylated in collagen type IV and about 80% in collagen type VI.