A molecule that helps an enzyme catalyze a reaction but is not itself a protein is called a

• A. Substrate
• B. Product
• C. Cofactor
• D. Catalyst

Answer: (C)

Cofactors are non-protein helpers that enable enzymes to catalyze reactions. They can be inorganic ions like magnesium or iron, or organic molecules called coenzymes (often derived from vitamins, such as NAD+ or FAD). These partners bind to the enzyme and participate in the chemical steps of the reaction, helping to stabilize transition states or transfer electrons or groups. Without a cofactor, many enzymes remain inactive as apoenzymes; with the cofactor they form a functional holoenzyme that carries out catalysis. Substrates are the molecules that are transformed during the reaction, and products are the result— cofactors themselves are not consumed or produced by the reaction, they simply assist the enzyme.