cofactor
cofactor, a nonprotein component that is essential for the biological activity of an enzyme. Enzymes serve as catalysts in biochemical reactions, and many enzymes require cofactors to function properly. Hence, when the cofactor is removed from a complete enzyme (holoenzyme), the protein component (apoenzyme) no longer has catalytic activity.
A cofactor that is firmly bound to the apoenzyme and cannot be removed without denaturing the latter is referred to as a prosthetic group; most such groups contain an atom of metal, such as copper or iron. A cofactor that is bound loosely to the apoenzyme and can be readily separated from it is known as a coenzyme. Coenzymes take part in the catalyzed reaction, are modified during the reaction, and may require another enzyme-catalyzed reaction for restoration to their original state.
In general, cofactors participate directly in biochemical reactions. In particular, they assist in the transfer of atoms, electrons, or functional groups between molecules. They may also facilitate substrate binding, ensuring proper attachment to the enzyme’s active site. Some cofactors serve an additional role in providing structure, helping to support the three-dimensional shape of an enzyme.
Examples of enzymes that require cofactors are cytochrome c oxidase and carbonic anhydrase. Cytochrome c oxidase uses iron and copper as cofactors in the electron transport chain, which is the main energy-generating system of cells. Carbonic anhydrase, which plays an important role in respiration by influencing the transport of carbon dioxide in the blood, utilizes zinc ions to catalyze the reversible conversion of carbon dioxide into bicarbonate.
