glycogenic amino acid uniquely found in collagen (q.v.), the chief structural protein of mammalian skin and connective tissue, and in some similar structural plant proteins. The hydroxyl group of hydroxylysine forms a chemical bond with sugars, attaching galactose monosaccharides and glucosyl-galactose disaccharides to the protein, thus contributing to collagen's unusual toughness and resiliency. Collagen is insoluable, but heat denaturation (boiling in water, dilute acids, or alkalies) yields digestible, soluble gelatin.