A cofactor is a non-protein chemical compound that is essential for the biological activity of a protein, often an enzyme. Cofactors can be metal ions, like zinc or magnesium, or organic molecules known as coenzymes, such as NAD+ or coenzyme A. They assist in various biochemical reactions by stabilizing enzyme-substrate complexes or participating directly in the reaction. Cofactors are crucial for many metabolic processes, as they help enzymes catalyze reactions more efficiently. Without the appropriate cofactors, enzymes may be inactive or less effective, leading to disruptions in cellular functions and metabolic pathways.