Some enzymes require "helper-molecules" for catalysis to take place. These helper-molecules are called cofactors. Cofactors are non-protein molecules that bind to the enzyme and contribute to reactions in a number of different ways. Co-factors can either be inorganic ions, such as the Zn2+ ions required by ADH, or they can be more complex organic or metalloorganic molecules. If a co-factor is bound tightly (sometimes covalently) to the enzyme, it is termed a prosthetic group .
Figure 1: Illustrating the role of a co-factor. This co-factor binds the enzyme and contributes to the reaction, and leaves the enzyme with the product in a changed state. This could, for example, be a changed oxidation state, as is the case for NAD+, which then needs to be restored elsewhere before the co-factor can assist in another reaction.