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 [1].

An illustration of the co-factor’s role in the enzyme-substrate-complex. The co-factor is represented in light orange and it is highlighted with green. Two substrates are represented in orange and purple and the enzyme is represented in blue. The yellow co-factor binds to the enzyme at the binding site which is highlighted in red. Together with the substrates and the enzyme, all four elements create the enzyme-substrate-complex. This leads to a reaction and leaves the enzyme with the product in a changed state.

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.


  1. Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2008). Principles of Biochemistry (5th ed.). New York, NY: W.H. Freeman and Company. ISBN 978-0-7167-7108-1.



Theory overview