The vanadium-containing nitrogenase of Azotobacter

Area of Science:

• Biochemistry
• Microbiology
• Bioinorganic Chemistry

Background:

• Biological nitrogen fixation is crucial for life, primarily mediated by molybdenum-dependent nitrogenases.
• Vanadium's role in biological nitrogen fixation was hypothesized but not fully understood.
• Azotobacter species are known for their nitrogen-fixing capabilities.

Purpose of the Study:

• To investigate the existence and characteristics of a vanadium-dependent nitrogenase system in Azotobacter species.
• To compare the properties and requirements of vanadium nitrogenase (V-nitrogenase) with the well-characterized molybdenum nitrogenase (Mo-nitrogenase).
• To elucidate the composition and function of the vanadium-containing cofactor (FeVaco).

Main Methods:

• Genetic analysis to identify nitrogenase genes.
• Biochemical characterization of V-nitrogenase components (VFe protein and Fe protein).
• Vanadium K-edge X-ray absorption spectroscopy to determine the coordination environment of vanadium.
• Enzyme assays using extracted cofactors to assess nitrogenase activity.

Main Results:

• Azotobacter chroococcum and Azotobacter vinelandii possess a distinct V-nitrogenase system, replacing the Mo-dependent component with a vanadoprotein.
• V-nitrogenase expression is regulated by molybdenum availability, occurring under Mo-deficiency.
• The VFe protein contains vanadium, iron, and acid-labile sulfide; its cofactor (FeVaco) can partially restore activity to Mo-nitrogenase mutants.

Conclusions:

• A functional V-nitrogenase system exists in Azotobacter, representing an alternative to Mo-nitrogenase for biological nitrogen fixation.
• The FeVaco cofactor is integral to V-nitrogenase activity and likely forms part of its active site.
• This discovery expands our understanding of the diversity and adaptability of nitrogen fixation pathways in microorganisms.