Inorganic Nitrogen Assimilation

Nitrogen is a major component of proteins, nucleic acids, coenzymes, and other cell constituents.

Most bacteria and archaea assimilate nitrogen in the form of nitrate and ammonia.

Nitrate enters the cells and is sequentially reduced to nitrite by nitrate reductase, then to ammonia by nitrite reductase, in a process called assimilatory nitrate reduction.

Ammonia assimilation occurs through two main pathways, depending on ammonia levels.

At high ammonia levels, bacteria and fungi use the reductive amination pathway, where glutamate dehydrogenase converts α-ketoglutarate to glutamate.

At low ammonia levels, the glutamine synthetase-glutamate synthase or GS-GOGAT system is activated.

Here, glutamine synthetase, or GS, converts ammonia and glutamate into glutamine, which is then processed by glutamate synthase, or GOGAT, to produce two glutamate molecules.

Nitrogen assimilation differs from nitrogen fixation, a process unique to certain prokaryotes. In nitrogen fixation, nitrogenase enzymes convert atmospheric nitrogen into ammonia, making it biologically available.