Enzymes are protein catalysts that are responsible for lowering the energy barrier of many biological reactions. They function by reducing the activation energy of the reactions that they catalyse which allows the metabolic reaction to occur faster and at physiological temperatures. Enzymes are specific for one substrate due to the unique arrangement of their active sites. However many enzymes cannot function on their own, and instead require non-protein cofactors. Enzymes that require such cofactors are called apoenzymes. When the cofactor and the apoenzyme are combined a holoenzyme is produced, this holoenzyme is able to catalyse reactions.
Cofactors function by providing functional groups needed for the reaction or by slightly altering the structural conformation of the enzyme to which they are bound. This alteration allows substrates to bind more easily, making a reaction more probable. Cofactors can be prosthetic groups which are tightly bound to the enzyme, or the cofactors can be coenzymes which can be loosely bound. Coenzymes are usually released from the active site of the enzyme following a reaction.