ABSTRACT
The nitrogen cycle is the movement of nitrogen through the food chain from simple inorganic compounds, mainly ammonia, to complex organic compounds. Biological nitrogen fixation is carried out by the nitrogenase complex which consists of two proteins: a reductase, which provides electrons with high reducing power, and a nitrogenase, which uses these electrons to reduce N2 to NH3. The nitrogenase complex is extremely sensitive to inactivation by O2, so the enzyme must be protected from this reactive substance. The next step in the nitrogen cycle is the assimilation of inorganic nitrogen, in the form of ammonia, into organic nitrogen-containing compounds. All organisms assimilate ammonia via two main reactions catalyzed by glutamate dehydrogenase and glutamine synthetasegiving rise to the amino acids glutamate (Glu) and glutamine (Gln), respectively. Glutamate dehydrogenase catalyzes the reductive amination of the citric acid cycle intermediate a-ketoglutarate. Glutamine synthetase catalyzes the incorporation of ammonia into glutamine, deriving energy from the hydrolysis of adenosine triphosphate (ATP).
