A prosthetic group is a non-protein (non-amino acid) component of a conjugated protein. The prosthetic group may be organic (such as a vitamin, sugar, or lipid) or inorganic (such as a metal ion). Prosthetic groups usually bond covalently to their protein. They often play an important role in the function of many proteins such as enzymes. A good example of a prosthetic group is the heme group in hemoglobin. A protein without its prosthetic group is called an apoprotein, while a protein combined with its prosthetic group is called a holoprotein.
Prosthetic groups are also sometimes called coenzymes or cofactors because they are essential to the function of a protein. In enzymes, prosthetic groups are involved in the active site in some way. Further examples of organic prosthetic groups are vitamin derivatives: thiamine (vitamin B1), thiamine pyrophosphate, pyridoxine (Vitamin B6), pyridoxal-phosphate. This is another reason why vitamins are so important in our diet. Inorganic prosthetic groups are usually transition metal ions such as iron (in heme groups, for example in cytochrome c oxidase and hemoglobin), zinc (for example in carbonic anhydrase), magnesium (for example in some kinases), and molybdenum (for example in nitrate reductase).