What best defines a cofactor?

• A. Non-protein molecule that helps enzymes do reactions
• B. A protein that helps enzymes
• C. A type of substrate
• D. A lipid that modifies enzyme

Answer: (A)

Cofactors are non-protein helpers that enzymes need to carry out their reactions. They can be inorganic ions like Mg2+ or Zn2+, or small organic molecules called coenzymes (such as NAD+, FAD, or coenzyme A). These molecules aren’t part of the enzyme’s protein structure, but they participate in the chemical steps of catalysis, often by stabilizing charges, transferring electrons, or shuttling chemical groups between substrates. Some cofactors stay bound to the enzyme as prosthetic groups; others bind and release as the reaction proceeds.

So the best definition is a non-protein molecule that helps enzymes do reactions. The other descriptions describe different things: a protein helper would be a regulatory or accessory protein, not a cofactor; a substrate is what the enzyme acts upon; a lipid that modifies an enzyme isn’t the general idea of a cofactors.