Ascorbic Acid

*Acidum ascorbicum*

For medical students2 min readUpdated 2026-10-10

Ascorbic acid (widely known as vitamin C) is an essential bioactive compound functioning as a precursor vitamin in a wide range of biochemical processes. The key feature of this molecule is its ability to easily donate and accept electrons, making it an indispensable participant in metabolism.

SynonymVitamin C (precursor vitamin)

Reaction typeRedox reactions (neither phosphorylation nor carboxylation)

ProcessesAntioxidant defense and enzymatic hydroxylation

Reduced formAscorbic acid (contains an en-diol group)

Oxidized formDehydroascorbic acid (contains two keto groups)

Biological Function and Enzyme Classes

From a biochemical standpoint, the primary biological function of ascorbic acid lies in its direct participation in redox reactions.

Unlike other coenzymes and vitamins that may specialize in phosphorylation (transfer of a phosphate group), transamination (transfer of an amino group), or carboxylation (incorporation of carbon dioxide), vitamin C functions exclusively with the transfer of electrons and protons.

In the body, this substance serves two major directions:

Hydroxylation. Ascorbic acid acts as a cofactor for enzymes that introduce a hydroxyl group (-OH) into substrate molecules. This is critical for the maturation of many structural proteins.
Antioxidant function. Due to its ability to easily donate electrons, vitamin C neutralizes free radicals, preventing oxidative damage to cells and tissues.

Reaction Mechanism: Two Forms of One Molecule

The mechanism of ascorbic acid is based on a reversible redox reaction. The molecule exists in the body in two interconvertible forms that transition into one another depending on whether the system donates or accepts electrons.

Ascorbic acid is the reduced form. Its key structural feature is the presence of an en-diol group (two hydroxyl groups attached to carbon atoms linked by a double bond). This specific group makes the molecule an excellent electron donor.
Dehydroascorbic acid is the oxidized form. When ascorbic acid performs its antioxidant work or participates in hydroxylation, it loses hydrogen. As a result, the en-diol group transforms, forming two keto groups (carbon atoms double-bonded to oxygen, C=O).

Chemistry of Oxidation and Reduction

The transition scheme between the two states looks like a classic reversible reaction:

Ascorbic acid $\leftrightarrow$ Dehydroascorbic acid

During oxidation (reaction moving to the right), the ascorbic acid molecule gives up two protons ($2H^+$) and two electrons ($2e^-$).
During reduction (reaction moving to the left), dehydroascorbic acid takes back two protons ($2H^+$) and two electrons ($2e^-$), turning back into the active en-diol form.

This continuous cycle of detaching and attaching $2H^+$ and $2e^-$ ensures uninterrupted antioxidant defense and support for hydroxylation reactions in tissues.

Mnemonic

Remember the forms easily: Ascorbic acid has an EN-DIOL (two OH groups, "di-ol"), and upon oxidation, it loses hydrogen and becomes DEHYDROascorbic acid, retaining TWO KETO groups (C=O). The transition costs exactly $2H^+$ and $2e^-$.

Frequently asked questions

For the maturation of which specific protein is hydroxylation with vitamin C required?

Hydroxylation involving vitamin C is required for the maturation of collagen. Vitamin C acts as a cofactor for the hydroxylation of proline and lysine during the synthesis of this fibrillar protein, ensuring the formation of hydrogen bonds that stabilize the triple helix of the tropocollagen molecule.

Which specific enzymes use vitamin C as a cofactor?

Specific hydroxylases involved in collagen synthesis use vitamin C as a cofactor:

Prolyl hydroxylase for the hydroxylation of proline.
Lysyl hydroxylase for the hydroxylation of lysine.

What disease develops with a severe deficiency of ascorbic acid?

A severe deficiency of ascorbic acid leads to scurvy. This disease is characterized by increased fragility and weakness of blood vessels, hemorrhages under the skin and mucous membranes, bleeding gums, anemia, and damage to the periodontal and dental apparatus.

In which biochemical reactions does vitamin C participate?

Ascorbic acid participates exclusively in oxidation-reduction (redox) reactions. It does not take part in transamination, carboxylation, or phosphorylation reactions.

How does the reduced form structurally differ from the oxidized form?

The reduced form (ascorbic acid) contains an active en-diol group. The oxidized form (dehydroascorbic acid) lacks protons and contains two keto groups.

What does the molecule lose upon transitioning to dehydroascorbic acid?

Upon oxidation, the molecule strictly loses two protons ($2H^+$) and two electrons ($2e^-$). This process is reversible during reduction.

Go deeper

Mechanisms of enzymatic hydroxylation involving vitamin C.
Detailed antioxidant function: targets and pathways of radical neutralization.
Biochemical properties of the en-diol group.
Metabolism and regeneration pathways of dehydroascorbic acid within the cell.