Molybdopterin

Molybdopterins, when reacted with molybdenum or tungsten in the form of molybdate or tungstate, are a class of cofactors found in most molybdenum (Mo) and all tungsten (W) enzymes. Synonyms for molydopterin are: MPT and pyranopterin-dithiolate.

The nomenclature for this biomolecule can be confusing: molybdopterin per se contains no molybdenum; rather, this is the name of the ligand that will eventually bind the active metal. After molydopterin is eventually complexed with molybdate, the complete ligand 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 molybdenum-containing enzymes that do not feature molydopterins are the nitrogenases (enzymes that fix nitrogen). These contain an iron-sulfur center of a very different type, which also usually contains molybdenum. However, if molybdenum is present, it is directly bonded to other metal atoms.

Biosynthesis
The biosynthesis of molybdopterin begins with guanosine triphosphate. Two enzymatic reactions convert this triphosphate to the cyclic phosphate of pyranopterin. One of these enzymes utilizes radical SAMs, often associated with C—X bond-forming reactions. This intermediate pyranopterin is then converted to the molybdopterin via the action of three further enzymes. In this conversion, the enedithiolate is formed, although the substituents on sulfur remain unknown. Sulfur is conveyed from cysteinyl persulfide in a manner reminiscent of the biosynthesis of iron-sulfur proteins. The monophosphate is adenylated (coupled to ADP) in a step that activates the cofactor toward binding Mo or W. These metals are imported as their oxyanions, molybdate, and tungstate. Finally, Mo or W is inserted to give the molybdopterin cofactor. In some enzymes, such as xanthine oxidase, the metal is bound to one molybdopterin, whereas, in other enzymes, e.g., DMSO reductase, the metal is bound to two molybdopterin cofactors.

Models for the active sites of enzymes molybdopterin-containing enzymes are based on a class of ligands known as dithiolenes.

Selenium-containing version, and ability to bind active tungsten
Tungsten has not been found to be necessary or used in eukaryotes, but it is an essential nutrient for some bacteria. For example, enzymes called oxidoreductases use tungsten in a similar manner as molybdenum by using it in a tungsten-pterin complex with molybdopterin. Thus, molybdopterin may complex with either molybdenum or tungsten in use by bacteria. Tungsten-using enzymes typically reduce free carboxylic acids to aldehydes. The first tungsten-requiring enzyme to be discovered also requires selenium, and in this case the tungsten-selenium pair may function analogously to the molybdenum-sulfur pairing of some molybdenum cofactor-requiring enzymes. One of the enzymes in the oxidoreductase family which sometimes employ tungsten (bacterial formate dehydrogenase H) is known to use a selenium-molybdenum version of molybdopterin. Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molydopterin and also non-protein bound selenium, a tungsten-selenium molybdopterin complex has not been definitively described.

Enzymes that use molydopterin as cofactor or prosthetic group
Cofactor of: xanthine oxidase, DMSO reductase, sulfite oxidase, nitrate reductase, ethylbenzene dehydrogenase, glyceraldehyde-3-phosphate ferredoxin oxidoreductase, respiratory arsenate reductase

Prosthetic Group of: formate dehydrogenase, purine hydroxylase, thiosulfate reductase

See