Collagen

• Ascorbic acid is crucial for the proper synthesis of collagen, a fibrous protein found in bone, tendons, cartilage and other connective tissue. Fibroblasts make collagen by synthesizing a precursor form called procollagen. Procollagen is primarily made up of a simple three-amino-acid sequence repeated many times over. The second and third amino acids of this repeating triplet are prolines. The third proline in the triplet sequence must be modified before the procollagen can be secreted from the cell.
  
  

Hydroxylation

• The enzyme prolyl 4-hydroxylase modifies the third proline by adding a hydroxyl group (an -OH group) to carbon 4 in the proline (the carbon at the tip of the five-membered ring). This hydroxyl group forces the resulting proline derivative, 4-hydroxylproline, to assume only one of two possible conformations, which is very important for the proper structure and function of the protein. During an accompanying reaction catalyzed by the same enzyme, the enzyme's iron atom becomes oxidized (loses electrons) to a +2 oxidation state, deactivating the enzyme. Vitamin C acts as the reducing agent to reactivate the enzyme. In other words, vitamin C is an essential cofactor for prolyl 4-hydroxylase, without which it can't function properly.
  
  

Norepinephrine

• Vitamin C is also an important enzyme cofactor and reducing agent during biosynthesis of a neurotransmitter called norepinephrine. During the first stage of the reaction, the amino acid L-tyrosine is converted to L-DOPA through hydroxylation. L-DOPA is in turn modified to yield dopamine. At this stage, an enzyme called dopamine beta-hydroxylase adds a hydroxyl group to the side chain on the dopamine. Just as in collagen synthesis, vitamin C acts as a reducing agent during this last step, providing the necessary electrons.
  
  

Carnitine

• Carnitine is an important small molecule your cells use to help transport fatty acids into the mitochondria, the powerhouses of your cells, to break them down for energy. During carnitine biosynthesis, an enzyme called N6-trimethyl lysine dioxygenase (TMLD) hydroxylates the 3-carbon position of a molecule called N6-trimethyllysine (TML). TMLD does not function in the absence of vitamin C unless other reducing agents like 3-mercaptoethanol are supplied; consequently, in this reaction, just as in the others, vitamin C acts as a reducing agent to keep iron in the TMLD in a lower oxidation state (i.e., with more electrons).