Molybdenum atoms can be found in the center of many oxygen-transfer enzymes. The chemical properties of this element, which has several stable oxidation states at physiological pH and which forms stable monooxo and dioxo complexes, make it particularly suited to this role. Typical molybdenum based oxygen-transfer enzymes are xanthin oxidase, sulfite oxidase, nitrate reductase, formiate dehydrogenase, CO dehydrogenase, dimethylsulfoxide reductase and biotinsulfoxide reductase.
The example shown here is trimethylamine-N-oxide reductase. It catalyzes the reaction:
(CH3)3NO + 2 e- + 2 H+ → (CH3)3N + H2O
The active site as shown is the oxidized form of the enzyme. The molybdenum atom is chelated by four thiolate groups.The coordination sphere of the metal is completed by two cis-oxo ligands. During reduction, one of these oxygen atoms is transferred to the substrate.