So far in this series we have looked at how enzymes work, what can affect the rate at which they work, and how their action can be inhibited. Today we are looking at coenzymes and cofactors: non-proteins that are needed in order for some enzymes to be able to catalyse their reaction.
Coenzymes
Coenzymes are a specific type of cofactor. They are non-protein organic molecules. The majority are carrier molecules, transferring chemical groups (sometimes electrons) from one enzyme to another. They bind to enzymes with the substrate and get changed during the reaction, but they are continually recycled. A great example of this is the coenzyme called NAD used in respiration. It is reduced to NADH during glycolysis, the link reaction, and the Krebs cycle. Then it is oxidised back to NAD during oxidative phosphorylation. Many coenzymes are derived from vitamins – see, vitamins are important!
Cofactors
Many cofactors are ions or inorganic molecules which help the substrate to bind to the active site. Remember that chloride ions are cofactors for the enzyme amylase. Sometimes cofactors can be an integral part of the enzyme and are tightly bound to it with strong covalent bonds. If this is the case, they are referred to as prosthetic groups. Zinc ions are a prosthetic group of the enzyme carbonic anhydrase, which you may have come across in red blood cells. These ions are essential for the reaction to occur, but are not actually changed during the reaction.
Summary
- Coenzymes and cofactors are non-proteins which are necessary for some enzymes to catalyse a reaction.
- Coenzymes are organic molecules which often transfer electrons. They are changed in the reaction, are continually recycled, and are not permanently bound to an enzyme.
- Cofactors are often ions. They are not changed in the reaction. Specific examples are chloride ions and zinc ions.
- A cofactor which is tightly bound to an enzyme is called a prosthetic group.
