A cofactor is one or more non-protein components required by an enzyme in order to carry out its normal catalytic function. Some cofactors are metal ions, such as ions of iron, zinc, or copper, or simple organic molecules. When the cofactor consists of a complex molecule, it is known as a prosthetic group. An example is heme in hemoglobin. Prosthetic groups are always tightly bound to their parent enzyme. Cofactors that are not tightly bound, yet are essential to the enzyme's normal activity, are referred to as coenzymes.