(Solved): Brock biology of micro organisms 15th edition chapter 3 review questions 8. Name two common enzyme ...

Expert Answer

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8.
Two common enzymes that function to incorporate NH3 into the cell are glutamine synthetase (GS) and glutamate dehydrogenase (GDH).



Nitrogen is an essential element for synthesising macromolecules like amino acids, nucleotides, and proteins. However, nitrogen gas (N2) is highly abundant in the earth's atmosphere but unavailable for most organisms as they cannot assimilate it directly. Therefore, the biological conversion of atmospheric nitrogen to a chemically useful form (such as ammonia/NH3) is crucial for life.

Upon entering the cell, NH3 can be assimilated into various amino acids or utilized in the biosynthetic processes by two common enzymatic pathways: The GS-GOGAT cycle and the GDH-dependent pathway.

Glutamine synthetase (GS) is the central enzyme in the GS-GOGAT cycle present in most organisms, including bacteria, plants, and animals. GS catalyzes the ATP-dependent conversion of glutamate (an amino acid) into glutamine, which serves as the primary storage form of nitrogen in cells. The incorporation of NH3 into glutamate is highly regulated to prevent energy wastage, and GS plays a key role in the nitrogen regulation. Additionally, the glutamine synthesized by GS can be utilized in the biosynthesis of nucleotides, amino sugar production, and other metabolic processes.



The GS-GOGAT cycle is a metabolic pathway involved in the assimilation of ammonium (NH4+) into organic compounds like amino acids. The pathway is named after the two key enzymes involved in the reaction, glutamine synthetase (GS) and glutamate synthase or glutamate oxoglutarate aminotransferase (GOGAT).

In this pathway, GS catalyzes the ATP-dependent conversion of glutamate into glutamine by incorporating NH4+ into glutamate. The formed glutamine acts as a storage form of nitrogen in the cell and is used as a nitrogen donor in various biosynthetic processes such as the production of nucleotides and proteins.

The produced glutamate is further metabolized by GOGAT into two molecules of glutamine. There are two types of GOGAT enzymes, the Fd-GOGAT (ferredoxin-dependent) and NADH-GOGAT (NADPH-dependent) found in different organisms.

The GS-GOGAT cycle is the most common pathway for ammonium assimilation in various prokaryotes, algae, and plants. However, in some organisms, like cyanobacteria, the GOGAT cycle alone may be insufficient for nitrogen assimilation, and additional pathways such as the glutamate dehydrogenase pathway may be employed.

Glutamate dehydrogenase (GDH) is another enzyme involved in NH3 incorporation into the cell. GDH catalyzes the reversible interconversion of glutamate and a-ketoglutarate (an amino acid) in the presence of NADH or NADPH. This reaction can either liberate NH3 (transamination) or incorporate NH3 (deamination) depending on the metabolic demand. In animals, the expression of GDH is regulated by the amino acid levels in the blood and serves as a means of controlling the nitrogen balance.


In conclusion, both GS and GDH are essential enzymes involved in NH3 assimilation into the cell, serving as key control points for nitrogen metabolism and regulation.