Hydroxylation

Overview
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
Proline is the principal residue to be hydroxylated in proteins, which occurs at the $$\mathrm{C^{\gamma}}$$ atom, forming hydroxyproline (Hyp), an essential element of collagen, in turn a necessary element of connective tissue. Proline hydroxylation is also a vital component of hypoxia response via hypoxia inducible factors. In some cases, proline may be hydroxylated instead on its $$\mathrm{C^{\beta}}$$ atom. Lysine may also be hydroxylated on its $$\mathrm{C^{\delta}}$$ atom, forming hydroxylysine (Hyl).

These three reactions are catalyzed by very large, multi-subunit enzymes prolyl 4-hydroxylase, prolyl 3-hydroxylase and lysyl 5-hydroxylase, respectively. These reactions require iron (as well as molecular oxygen and α-ketoglutarate) to carry out the oxidation, and use ascorbic acid (vitamin C) to return the iron to its reduced state. Deprivation of ascorbate leads to deficiencies in proline hydroxylation, which leads to less stable collagen, which can manifest itself as the disease scurvy. Since vitamin C is rich in citrus fruits, British sailors were given limes to combat scurvy on long ocean voyages; hence, they were called "lymies".