Co-factors

Some enzymes are complete and active as protein alone, but many are not. In a large number of cases an enzyme needs an extra, non-protein partner in order to become catalytically active. This attached non-protein helper is called a co-factor. When a co-factor is involved, the protein portion of the enzyme on its own is referred to as the apoenzyme, and it is inactive by itself.

Co-factors come in three kinds.

• Prosthetic groups. These are organic co-factors, and what sets them apart is that they are bound very tightly to the apoenzyme. A good example is the haem group found in the enzymes peroxidase and catalase, which break down hydrogen peroxide into water and oxygen. In these enzymes the haem is firmly built into the enzyme and forms part of its active site.
• Co-enzymes. These are also organic molecules, but their attachment to the apoenzyme is only loose and temporary, coming together mainly during the actual moment of catalysis. A single co-enzyme can serve many different enzymes in many different reactions. The working chemical parts of a good number of co-enzymes are vitamins; for instance the co-enzymes nicotinamide adenine dinucleotide (NAD) and NADP both contain the vitamin niacin.
• Metal ions. Several enzymes require a metal ion to function. The metal forms coordination bonds with side chains at the active site and, at the same time, forms one or more coordination bonds with the substrate, thereby helping to hold and orient the reactants. An example is zinc, which acts as a co-factor for the protein-digesting enzyme carboxypeptidase. …