Biological Fixation of Nitrogen

Nitrogen fixation is a distinctive property possessed by a select group of organisms, because of the presence of the enzyme nitrogenase in them. The process of nitrogen fixation is primarily confined to microbial cells like bacteria and cyanobacteria.

Nitrogen fixation requires

• the molecular nitrogen
• a strong reducing power to reduce nitrogen like reduced FAD (Flavin adenine dinucleotide) and reduced NAD (Nicotinamide Adenine Dinucleotide)
• a source of energy (ATP) to transfer hydrogen atoms from NADH₂ or FADH₂ to dinitrogen
• enzyme nitrogenase
• compound for trapping the ammonia formed since it is toxic to cells

The reducing agent (NADH₂ and FADH₂) and ATP are provided by photosynthesis and respiration.

The overall biochemical process involves stepwise reduction of nitrogen to ammonia. The enzyme nitrogenase is a Mo-Fe containing protein and binds with molecule of nitrogen (N₂) at its binding site. This molecule of nitrogen is then acted upon by hydrogen (from the reduced coenzymes) and reduced in a step wise manner.

It first produces diamide (N₂H₂) then hydrazime (N₂H₄) and finally ammonia (2NH₃). NH₃ is not liberated by the nitrogen fixers. It is toxic to the cells and therefore these fixers combine NH₃ with organic acids in the cell and form amino acids.