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7.6: Nitrogen Fixation

The process of nitrogen fixation is important for life on earth, because atmospheric nitrogen is ultimately the source of amines in proteins and DNA. The enzyme playing an important role in this process is called nitrogenase and it is found in certain types of anaerobic bacteria called diazotrophs. Symbiotic relationships between some plants (legumes, for example) and the nitrogen-fixing bacteria provide the plants with access to reduced nitrogen. The overall reduction reaction catalyzed by nitrogenase is

\[\text{N}_2 + 6\text{H}^+ + 6\text{e}^- \rightarrow 2\text{NH}_3\]

In these reactions, the hydrolysis of 16 ATP is required. The ammonia can be assimilated into glutamate and other molecules. Enzymes performing nitrogenase catalysis are very susceptible to oxygen and must be kept free of it. It is for this reason that most nitrogen-fixing bacteria are anaerobic. Movement of amines through biological systems occurs largely by the process of transmination, discussed below in amino acid metabolism.