Nitrogen and nitrogen cycle play an important role in biological process for many microorganisms as catalyzing different reactions. For example, nitrate reduction is used for conversion into ammonia and denitrification, where denitrification is an important cellular respiration process. Nitrogenase enzyme in prokaryotes can fix the atmospheric nitrogen by catalyzing nitrogen fixation (i.e. reduction of nitrogen to ammonia). Nitrate can be introduced into the cytoplasm through a nitrate:nitrite antiporter NarK or a nitrate/nitrite transporter NarU. Nitrate is then reduced by a nitrate reductase resulting in the release of water, an acceptor, and a nitrite. Nitrite can also be introduced into the cytoplasm through a nitrate:nitrite antiporter NarK. Nitrite can be reduced by an NADPH-dependent nitrite reductase resulting in water, NAD, and ammonia. Nitrite can interact with a hydrogen ion and ferrocytochrome c through a cytochrome c-552 ferricytochrome resulting in the release of ferricytochrome c, water, and ammonia. Another process by which ammonia is produced is by a reversible reaction of hydroxylamine with a reduced acceptor through a hydroxylamine reductase. This results in an acceptor, water, and ammonia. Water and carbon dioxide react through a carbonate dehydratase resulting in carbamic acid. This compound reacts spontaneously with hydrogen ion resulting in the release of carbon dioxide and ammonia. Carbon dioxide can interact with water through a carbonic anhydrase resulting in hydrogen carbonate. This compound interacts with cyanate and hydrogen ion through a cyanate hydratase resulting in a carbamic acid. Ammonia can be metabolized by reacting with L-glutamine and ATP-driven glutamine synthetase resulting in ADP, phosphate, and L-glutamine. The latter compound reacts with oxoglutaric acid and hydrogen ion through an NADPH-dependent glutamate synthase resulting in the release of NADP and L-glutamic acid. L-Glutamic acid reacts with water through an NADP-specific glutamate dehydrogenase resulting in the release of oxoglutaric acid, NADPH, hydrogen ion, and ammonia.

Nitrogen Metabolism References