Hydroxylysine is closely related to lysine and is important in the formation of collagen. It is also found in gelatin and in the digestive enzymes trypsin and chymotrypsin.

Hydroxyproline is an important component of collagen, which makes up the white, fibrous connective tissue and is part of the skin, bones, and cartilage. Hydroxyproline is converted from proline by hydroxylation only after proline gets into the amino acid chains that form body proteins.