An alternative path for the evolution of biological nitrogen fixation

Area of Science:

• Biochemistry
• Evolutionary Biology
• Geochemistry

Background:

• Nitrogenase catalyzes essential nitrogen fixation, converting dinitrogen gas to ammonia.
• Modern nitrogenase commonly uses molybdenum (Mo), but vanadium (V) and iron (Fe) variants exist.
• Early Earth's Mo-depleted oceans fueled theories of Mo-independent nitrogenase origins.

Purpose of the Study:

• To investigate the evolutionary path of nitrogenase, particularly the origins of Mo-dependent and Mo-independent forms.
• To determine if Mo-independent nitrogenases were ancestral or evolved later.

Main Methods:

• Phylogenetic analysis of nitrogenase proteins.
• Structure-based examinations of nitrogenase and related proteins.
• Inferences on the ancestral nitrogenase structure and metal-binding capabilities.

Main Results:

• Phylogenetic and structural data contradict the hypothesis of early Mo-independent nitrogenase.
• Evidence suggests Mo-dependent nitrogenase emerged first within methanogenic archaea.
• Alternative V- and Fe-dependent nitrogenases likely evolved later, possibly from Mo-dependent ancestors.

Conclusions:

• The ancestral nitrogenase possessed an open cavity for metal cluster binding, enabling reactivity.
• Nitrogenase evolution was driven by fixed nitrogen availability and environmental metal concentrations.
• The refinement to Mo-nitrogenase occurred when molybdenum became sufficiently bioavailable on early Earth.