Hydroxyproline confers stability upon collagen probably through intramolecular hydrogen bonds that may involve bridging water molecules. Prolyl hydroxylase requires ascorbic acid to maintain its enzymatic activity. In fact, the ascorbic acid is not used as a coenzyme but is necessary for the continued activity of prolyl hydroxylase, which can be also called a dioxygenase (1). The atom that becomes attached to C-4 of proline comes from the molecular oxygen. The other oxygen atom is transferred to a-ketoglutarate which is converted into succinate. A tightly enzyme-bound Fe2+ ion is required for the reaction to occur. An intermediate Fe2+ complex is formed which inactivates the enzyme. The prolyl hydroxylase is then regenerated with the assistance of ascorbate which becomes oxidized. It is clear that vitamin C serves here as an antioxidant. In addition, vitamin C is used up in the process so the ascorbic acid must be in sufficient supply for collagen synthesis to occur normally. In vitamin C deficiency, the collagen synthesized cannot form fibers properly because proline residues are not hydroxylated. This leads to skin lesions, blood vessels fragility and poor wound healing. Ascorbic acid is also an electron donor for several other enzymes. These are the processes that require the participation of ascorbic acid: carnitine synthesis the formation of norepinephrine from dopamine catabolism of the amino acid tyrosine To learn more about the enzymes that require ascorbic acid for catalytic activity see Ref. 2. References 1. Myllyla et al.(1984) J.Biol.Chem. 259, 5403-5405. Ascorbate is consumed stoichiometrically in the uncoupled reactions catalyzed by prolyl 4-hydroxylase and lysyl hydroxylase. 2. Levine, M., Rumsey, S.C. et al. Vitamin C in: Biochemical and Physiological Aspects of Human Nutrition. pp.541-567, Stipanuk, MH, editor, W.B. Saunders Publishers, Philadelphia, 2000.