when reacted with molybdenum or tungsten, are a class of cofactors found in most molybdenum (Mo) and all tungsten (W) enzymes. The nomenclature for this biomolecule is potentially confusing: molybdopterin contains no molybdenum. When it is complexed with molybdenum (molybdate), it is usually called molybdenum cofactor.
Molydopterin consists of a pyranopterin, a complex heterocycle featuring a pyran fused to a pterin ring. In addition, the pyran ring features two thiolates, which serve as ligands in molybdo- and tungstoenzymes. In some cases, the alkyl phosphate group is replaced by an alkyl diphosphate nucleotide. Enzymes that contain the molybdopterin cofactor include xanthine oxidase, DMSO reductase, sulfite oxidase, and nitrate reductase. The only Mo-containing enzyme that does not feature this cofactor is nitrogenase.