coenzyme

Meaning: A coenzyme is a small organic molecule, not a protein, that an enzyme needs in order to work. It usually carries electrons, hydrogen atoms or a chemical group from one reaction to another. Many are made from vitamins: NAD⁺ from niacin, FAD from riboflavin, coenzyme A from pantothenic acid. Coenzymes such as NAD⁺ act like co-substrates: reduced in one reaction, reoxidized in another, and so recycled. An enzyme without its cofactor is an apoenzyme; with it, a holoenzyme. Cofactors include metal ions (Zn²⁺, Mg²⁺) and coenzymes. A prosthetic group is a cofactor bound very tightly or covalently.

On the MCAT: Metabolism passages ask you to name NAD⁺ or FAD as the electron carrier in glycolysis, the citric acid cycle or the electron transport chain. A vitamin deficiency lowers the coenzyme supply, so the rate falls and extra substrate cannot fully restore it. In dialysis experiments, the small coenzyme passes through the membrane while the protein stays behind, so activity is lost and then returns when the coenzyme is added back.

Common confusion: A coenzyme is not an enzyme and does not catalyze by itself. It is the organic kind of cofactor, so a metal ion is a cofactor but not a coenzyme.