Chromogenic assessment of the three molybdo-selenoprotein formate dehydrogenases in Escherichia coli

Escherichia coli synthesizes three selenocysteine-dependent formate dehydrogenases (Fdh) that also have a molybdenum cofactor. Fdh-H couples formate oxidation with proton reduction in the formate hydrogenlyase (FHL) complex. The activity of Fdh-H in solution can be measured with artificial redox dyes but, unlike Fdh-O and Fdh-N, it has never been observed by chromogenic activity staining after non-denaturing polyacrylamide gel electrophoresis (PAGE). Here, we demonstrate that Fdh-H activity is present in extracts of cells from stationary phase cultures and forms a single, fast-migrating species. The activity is oxygen labile during electrophoresis explaining why it has not been previously observed as a discreet activity band. The appearance of Fdh-H activity was dependent on an active selenocysteine incorporation system, but was independent of the [NiFe]-hydrogenases (Hyd), 1, 2 or 3. We also identified new active complexes of Fdh-N and Fdh-O during fermentative growth. The findings of this study indicate that Fdh-H does not form a strong complex with other Fdh or Hyd enzymes, which is in line with it being able to deliver electrons to more than one redox-active enzyme complex.