A coenzyme (a.k.a. a prosthetic group) is an organic non-protein molecule that is a functional part of an enzyme. Coenzymes are generally defined as cofactors that are organic molecules. Many coenzymes are phosphorylated water-soluble vitamins. However, nonvitamins may also be coenzymes, such as heme coenzymes in catalytic hemoproteins. Coenzymes covalently or non-covalently bind to the inactive apoenzyme to form the active holoenzyme.