it in a tungsten-pterin complex, with molybdopterin. Thus, molybdopterin may complex with either molybdenum or tungsten. Tungsten-using enzymes typically reduce free carboxylic acids to aldehydes. The first tungsten-requiring enzyme to be discovered also requires selenium (though the precise form is unknown). In this case, the tungsten-selenium pair has been speculated to function analogously to the molybdenum-sulfur pairing of some molybdenum cofactor-requiring enzymes. Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molybdopterin and also non-protein-bound selenium (thus removing the possibility of selenium in selenocysteine or selenomethionine form), a tungsten-selenium molybdopterin complex has not been definitively described. Enzymes that use molybdopterin Enzymes that use molybdopterin as cofactor or prosthetic group are given below. Molybdopterin is a: Cofactor of: xanthine oxidase, DMSO reductase, sulfite oxidase, nitrate reductase, ethylbenzene dehydrogenase, glyceraldehyde-3-phosphate ferredoxin oxidoreductase, respiratory arsenate reductase, carbon monoxide dehydrogenase, aldehyde oxidase. Prosthetic group of: formate dehydrogenase, purine hydroxylase, thiosulfate reductase. See also Molybdenum cofactor deficiency, a genetic illness. MOCOS, molybdenum cofactor sulfurase MOCS1, MOCS2, MOCS3, GEPH References Organophosphates Cofactors Pteridines Thiolates Pyrans Molybdenum