Cofactors

in biochemistry, substances necessary for the catalytic activity of a particular enzyme. Cofactors are essential constituents of most enzyme systems and are classified as fol-lows: prosthetic groups, which are strongly bonded with the protein carrier apoenzyme; coenzymes, which are relatively easy to separate from apoenzymes; and metal ions (metallic coenzymes). No clearly defined boundary exists between the terms “cofactor,” “coenzyme,” and “prosthetic group.”