Nitrogen Metabolism

Introduction

All living organisms are composed of carbon, hydrogen, oxygen, nitrogen and many other chemical elements. All these elements contribute to finally organizing various biomolecules present in a cell. Nitrogen is next to carbon in importance in living organisms. Nitrogen is an essential constituent of amino acids, proteins, enzymes, vitamins, alkaloids, and some growth hormones in a living cell. Therefore, studying nitrogen metabolism is essential because the entire life process depends on these nitrogen containing molecules. Polymeric nitrogen containing compounds-proteins and nucleic acids define an organism’s significant attributes, such as function and structure. The nitrogen metabolism describes the biosynthesis of amino acids in plants and animals.

Nitrogen

• Nitrogen is the most abundant element in the atmosphere. It constitutes 78% concentration in the air

• Nitrogen is present in the molecular form of N2. There is a triple bond between two N-atoms

• Molecular Nitrogen or diatomic nitrogen (N2) is highly stable. Because of this stability, molecular Nitrogen is not very reactive in the atmosphere under normal conditions

• Due to a triple bond between nitrogen atoms, Nitrogen from the atmosphere cannot be taken directly by the plants

• Nitrogen has a shallow boiling point (-195.8 °C) which is even lower than oxygen

• Nitrogen is universally present in all organisms in different forms. It is a component of proteins, chlorophyll, enzymes, nucleic acids, hormones 

• Plants take Nitrogen in the form of nitrate (NO3- ) and ammonium ions (NH4+)

Nitrogen Metabolism

• Nitrogen metabolism refers to the biosynthesis of amino acids in plants and the animal body

• The metabolism of Nitrogen includes several processes-such as nitrogen fixation, nitrogen cycle, biosynthesis and breakdown of amino acids

• The degradation of amino acids in microorganisms and plants is a rare event but significant in animals and humans

Nitrogen cycle

• The nitrogen cycle is a biogeochemical cycle in which the Nitrogen gas is converted into organic substances and then back to nature

• Five steps in the nitrogen cycle involve ammonification, nitrification, nitrate assimilation, denitrification and nitrogen fixation

• The primary step that involves nitrogen mobilization is nitrogen fixation

Ammonification

• Ammonification is a process in which organic Nitrogen is converted into ammonium ions by the microbes present in the soil
• The primary organic nitrogen sources in the soil are dead and decaying plant and animal remains and animal excreta
• The bacteria involved in ammonia furcation are Bacillus ramosus, Bacillus Vulgaris, certain soil fungi, and actinomycetes

Nitrification

• Nitrification is when the ammonia is oxidized to nitrate and nitrite

• Ammonia is converted into nitrite by Nitrosomonas bacteria, and nitrite is converted into nitrate by Nitrobacter bacteria

Nitrate assimilation 

• Plants cannot observe the nitrates present in the soil; hence the nitrates are first reduced to nitrite bio enzyme nitrate reductase, and then the nitrite is converted into ammonia by a series of steps

• The process of reduction is known as nitrate assimilation

Denitrification 

• Denitrification is how nitrate and nitrite are converted into ammonia, nitrogen gas and nitrous oxide

• After denitrification, the gaseous Nitrogen is released into the atmosphere, leading to the completion of the nitrogen cycle

Nitrogen Fixation

• The conversion of molecular Nitrogen into beneficial Nitrogen compounds, such as ammonia, is called nitrogen fixation

• It is a reductive process

• This nitrogen fixation takes place by two different methods – abiological or physical and biological

Physical nitrogen fixation 

• Physical nitrogen fixation is a type of nitrogen fixation in which Nitrogen is reduced to ammonia without involving any living cell

• It is also known as abiological nitrogen fixation

• It can be of two types: industrial or natural

• In industrial nitrogen fixation, synthetic ammonia is produced by passing a mixture of Nitrogen and hydrogen to a bed of catalyst at a very high temperature and pressure. Iron oxides are used as a catalyst in this process

• In natural nitrogen fixation, Nitrogen is fixed during electrical discharges in the atmosphere that occur during lightning storms, when Nitrogen in the atmosphere can combine with oxygen to form oxides of Nitrogen. These nitrogen oxides may be hydrated and precipitate down to the earth as a combined form of Nitrogen as nitrate and nitrite

• The nitrate and nitrite dissolved in water are directly observed by the roots of plants

Biological nitrogen fixation 

• The conversion of atmospheric nitrogen into nitrogenous compounds involving living organisms is referred to as biological nitrogen fixation

• Biological nitrogen fixation cannot occur through  abiological or physical means

• It involves microorganisms that either live in close symbiotic association with other plants or are free-living (non-symbiotic)

• In biological nitrogen fixation, the Nitrogen is reduced into ammonia in a natural catalyst- nitrogenase

• The natural catalyst nitrogenase is found naturally in certain types of bacteria, such as a symbiotic rhizobium and Frankia, the free-living Azospirillum and Azotobacter and Blue-green algae (BGA).

Free-living soil bacteria which fix Nitrogen are as follows:

• Aerobic bacteria- Azotobacter, Beijerinckia, Derxia 
• Anaerobic photosynthetic bacteria- Clostridium, Rhodopseudomonas, Rhodospirillum
• Chemosynthetic bacteria- Desulfovibrio 
• Free-living fungi- Pullularia and Yeasts
• Heterocystous Cyanobacteria- Nostoc, Anabaena, Calothrix

Amino acids Metabolism

• Proteins are broken down into their constituent amino acids in digestion inside the stomach

• There are altogether twenty standard amino acids involved in the process of translation

• The primary site for the degradation of amino acids is in the liver, but a few amino acids are also degraded in muscles and kidneys

• At least one nitrogen atom is present in all amino acids. This one nitrogen atom forms their a-amino group. But several amino acids contain additional nitrogen atoms in their side chains

• Ammonium (NH4+) is the most reduced form of inorganic combined Nitrogen. This ammonium now becomes the primary source of amino acids, the building blocks of enzymes and proteins. Amino acids have two important chemical groups. (i) amino group (NH4+   ) and, (ii) carboxyl group (- COOH)

• The ammonium produced is the major source of the amino group. However, the carboxyl group must be provided by other organic molecules synthesized by the plants

• There are two significant reactions for amino acid biosynthesis in plants: The reductive amination reaction and the Transamination reaction

Conclusion

Thus, we can say that Nitrogen is an important element in the atmosphere, which is an important component of our body. Along with Nitrogen, carbon, hydrogen, oxygen and many other chemical elements are also composed in our body. Due to a triple bond between nitrogen atoms, Nitrogen from the atmosphere cannot be directly taken by the plants. The free Nitrogen from the atmosphere is converted into valuable compounds of Nitrogen so that plants can absorb them. And the process of conversion of atmospheric Nitrogen into beneficial compounds of Nitrogen is known as nitrogen fixation. Nitrogen fixation is an important step in the nitrogen cycle. 

Five steps in the nitrogen cycle involve ammonification, nitrification, nitrate assimilation, denitrification and nitrogen fixation. Ammonification is a process in which organic Nitrogen is converted into ammonium ions by the microbes present in the soil. The main sources of organic nitrogen in the soil are dead and decaying plant and animal remains and animal excreta. Nitrification is when the molar is oxidized to nitrate and nitrite. Ammonia is converted into nitrite by Nitrosomonas bacteria, and nitrite is converted into nitrate by Nitrobacter bacteria. 

Plants cannot observe the nitrates present in the soil; hence the nitrates are first reduced to nitrite bio enzyme nitrate reductase, and then the nitrite is converted into ammonia by a series of steps. The process of reduction is known as nitrate assimilation. Denitrification is how nitrate and nitrite are converted into ammonia, nitrogen gas and nitrous oxide. After denitrification, the gaseous Nitrogen is released into the atmosphere, leading to the completion of the nitrogen cycle. 

The conversion of molecular Nitrogen into beneficial Nitrogen compounds, such as ammonia, is called nitrogen fixation. It is a reductive process. This nitrogen fixation takes place by two different methods – biological or physical and biological.