Activation of nit-1 nitrate reductase by W-formate dehydrogenase.

Formate dehydrogenase ( FDH ) from Clostridium thermoaceticum is a known tungsten enzyme. FDH was tested for the presence of nitrogenase-type cofactor and nitrate reductase-type cofactor by the Azotobacter vinelandii UW-45 and Neurospora crassa nit-1 reconstitution assays, respectively. Tungsten formate dehydrogenase (W- FDH ), containing only a small Mo impurity, activated the nit-1 nitrate reductase extracts when molybdate was also added, but not when tungstate was added. These results show W- FDH contains the cofactor common to all known Mo-enzymes except nitrogenase. The difference between the redox chemistries of W- FDH and W-substituted sulfite oxidase appears to relate to differences in tungsten ligation other than that donated by the cofactor or to variations in the protein environment surrounding the tungsten active site.