Hydroxylation is any chemical process that introduces one or more hydroxyl groups (-OH) into a compound (or radical) thereby oxidizing it. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation in proteins The principal residue to be hydroxylated in proteins is Proline. The hydroxilation occurs at the mathrm atom, forming hydroxylysine (Hyl). These three reactions are catalyzed by very large, multi-subunit enzymes prolyl 4-hydroxylase, proly… (More on Hydroxylation)