Enzymes are simple if they are made of only proteins (e.g. pepsin, amylase etc.) while conjugate enzymes have an additional non-protein cofactor which may be organic or inorganic. Loosely attached organic cofactor is coenzyme. It plays an accessory role in enzyme catalyzed processes often by acting as a donor or acceptor of a substance involved in the reaction. ATP and NAD are common coenzymes. Isozymes are closely related variants of enzymes. It has similar function as another enzyme but has a different set of amino acids. Holoenzyme is the intact enzyme cofactor complex. Apoenzyme is an enzyme without its cofactor. It is the protein molecule to which a coenzyme will bind to produce the holoenzyme.