The existence of enzymes containing tungsten was long a matter of debate. The extreme rarity of this element makes it unlikely that it plays a large role in biological systems. Nevertheless, a number of tungsten enzymes from bacteria from extreme environments (e.g. hot springs) have been found and structurally characterized.
In many cases, they are functionally and structurally analogous to the corresponding molybdenum compounds.
The example shown here is an aldehyde ferredoxin oxyreductase. This is a tetrameric enzyme. Its active site contains a tungsten atom chelated by thiolate ligands and bound to an additional oxo ligand. Thus, its structure corresponds to the reduced form of a typical molybdopterin.