- A nonproteinaceous organic substance that usually contains a vitamin or mineral and combines with a specific protein, the apoenzyme, to form an active enzyme system.
- a molecule that is essential for the activity of some enzymes; it may exist free in solution within a living organism, but functions by binding to an enzyme to assist in catalyzing a reaction. The molecule itself may be temporarily changed during the reaction, but is ultimately restored to its original form. Many vitamins function as <ex>coenzymes</ex>.
- Any small <xref>molecule</xref> that is necessary for the <xref>functioning</xref> of an <xref>enzyme</xref>.
- a small molecule (not a protein but sometimes a vitamin) essential for the activity of some enzymes