In collagen synthesis, hydroxyproline is formed from

Collagen is initially synthesized as a larger precursor polypeptide, pro-collagen. Numerous prolyl and lysyl residues of pro-collagen are hydroxylated by prolyl hydroxylase and lysyl hydroxylase, enzymes that require ascorbic acid (vitamin C). Hydroxy-prolyl and hydroxy-lysyl residues provide additional hydrogen bonding capability that stabilizes the mature protein. In addition, glucosyl and galactosyltransferases attach glucosyl or galactosyl residues to the hydroxyl groups of specific hydroxy-lysyl residues.Ref: Harper 28th Ed. Page-41, Harper 27th Ed. Page-39