FIGURE 5.

Mechanism of nitrogen assimilation and integrated pathways with different enzymes involved in channelizing nitrate toward amino acids and proteins. Nitrate and nitrite are the primary nitrogen source for cereal crops. Nitrate is converted to nitrite by regulatory enzyme: nitrate reductase (NR, highlighted in creamish color) (1.7.7.2). Nitrite is further reduced to ammonia by nitrite reductase (1.7.7.1, highlighted in creamish color). Ammonia is channelized for amino acid synthesis primarily by action of glutamate dehydrogenase (1.4.1.2, highlighted in creamish color¶and 1.4.1.3, highlighted in purple color). Glutamate is further converted to glutamine by glutamine synthetase (6.3.1.2, highlighted in orange color; 1.4.7.1, highlighted in green color and 1.4.1.14, highlighted in red color). Glutamine and glutamate are the primary amino acids routed for protein synthesis. Partial fraction of ammonia is involved in arginine metabolism using carbamoyl phosphate as C-skeleton. In addition to the primary nitrogen metabolism, secondary nitrogen sources such as nitric oxide, nitrous oxide, nitroalkane, nitrile, hydrazine, and formamide also contribute in nitrogen metabolism. Cyanoamino acid metabolism releases formamide which converts to formate for further used in methane and glyoxylate metabolism.