Nitrite reductase Nitrogen assimilation

All plants depend on nitrate reductase to accomplish the seemingly trivial reaction of nitrate reduction to nitrite, often the first step of nitrogen assimilation into compounds required for growth (5, 22). Many bacteria use molybdenum or tungsten enzymes in anaerobic respiration where the terminal electron acceptor is a reducible molecule other than oxygen, such as nitrate (2, 50), polysulfide (51), trimethylamine oxide (33, 52) or dimethyl sulfoxide (DMSO) (2, 29, 30). 

Iron-containing proteins are essential for photosynthetic and respiratory electron transport. Chlorophyll synthesis is also dependent on iron, and chlorophyll content is diminished under iron limitation. Requirements for iron are also influenced by the nitrogen substrate used to support growth. Both nitrate and nitrite reductases contain iron in their catalytical centres. Iron efficiency models (Raven, 1988) and culture studies (Maldonado Price, 1999) show that cells using nitrate require 60-70% more cellular iron to support a given growth rate than those assimilating ammonia. 

Nitrogen-fixing plants are also capable of reducing NOg" in the roots and/or leaves of the plant. In soybeans, there is evidence to suggest that reduction may occur in both roots and leaves (26). There has been diflSculty, however, in detecting nitrite reductase activity in soybean root or nodule tissue (61). Using NOs", we have demonstrated that both tissues actively assimilate and incorporate into organic com-... 

Nitrate reductases are found in a wide range of eukaryotes and prokaryotes and have a crucial role in nitrogen assimilation and dissimilation (see Chapter 8.14). These enzymes catalyze the reaction shown in Equation (5) for the assimilatory nitrate reductases, this is followed by the reduction of nitrite to ammonia. Dissimilatory nitrate reductases [142 147] catalyze the reduction of nitrate to nitrite for respiration, to generate a transmembrane potential gradient.The assimilatory nitrate reductases have a molybdenum center similar to that of sulfite oxidase (see... 

In the global environment, nitrogen in the form nitrate or nitrite is assimilated (immobilized) into biomass in the form of NH3. This reduction is catalyzed by two assimilatory enzymes (nitrite reductase and nitrate reductase) and can be carried out by plants, fungi, and prokaryotes. This process is likely to dominate when reduced nitrogen is in low supply (e.g., during aerobic conditions). 

Figure 3. Model showing possible fates for lipid and carbohydrate carbon during nitrogen assimilation in P. tricomutum. Ruxes shown do not represent actual stoichiometries. (1) glutamine synthetase (2) nitrite reductase (3) nitrate reductase (4) malate glycolysis (5) cytosolic malate dehydrogenase (6) anaplerotic carbon flux (7) gluconeogenesis (8) glycolysis (9) carnitine acyltransferase (10) isocitrate lyase.

Certain bacteria can utilize nitrate nitrogen as the sole nitrogen source for the synthesis of all nitrogen containing compounds of the cell (Payne, 1973). This nitrate assimilation can occur under both aerobic and anaerobic conditions. In other instances (Payne, 1973) nitrate serves as a terminal hydrogen acceptor under anaerobic conditions and this process is called nitrate respiration. In both cases the product of nitrate reduction is nitrite. The nitrate reductases from bacteria have been differentiated by Pichinoty and Piechaud (1968) into nitrate reductase A which is membrane bound and can reduce chlorate in addition to nitrate as a substrate and nitrate reductase B which is... 

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