Vanadium K-edge X-ray absorption spectrum of the VFe protein of the vanadium nitrogenase of Azotobacter chroococcum

Abstract

The free-living, nitrogen-fixing bacterium Azotobacter chroococcum has recently been shown to contain two nitrogenase systems¹. One of these is the well-characterized molybdenum-containing enzyme^2,3 and the other, which is encoded by different genes, contains vanadium. Both enzyme systems consist of two proteins: an iron protein and a molybdenum- or vanadium-containing protein which also contains iron. The vanadium-containing nitrogenase protein, Ac1^*, has a relative molecular mass ~-210,000 and contains 2 vanadium atoms, 23 iron atoms, and 20 acid-labile sulphide ions per mole⁴. X-ray absorption spectroscopy has been extensively used to probe the environment of molybdenum in the MoFe protein⁵. The molybdenum is closely associated with iron and sulphur in an iron-molybdenum cofactor, FeMoco, which is probably the N₂-binding site of the enzyme⁶. We report here vanadium X-ray absorption data of the VFe protein, the first such report for any vanadium-containing protein, and compare them with those from a recently synthesized VFeS cluster compound⁷. The results suggest that the vanadium in this protein is present in a vanadium-containing cofactor analogous to FeMoco. We conclude that substitution of vanadium for molybdenum probably accounts for the different substrate specificities of the two nitrogenases.