Nitrogen Metabolism – Nitrogen Cycle, Biological Nitrogen Fixation

Introduction

Living things are essentially made up of carbon, nitrogen, oxygen, and other types of chemicals. Nitrogen (N2) is a part of amino acids, proteins, nucleic acids, vitamins, enzymes, alkaloids, DNAs and RNAs. Along with these, nitrogen is also present in some growth hormones. All these components play a significant role in controlling all the cellular activities in living organisms. The entire life process of all living things depends on the molecules that contain nitrogen. Hence, nitrogen metabolism is considered to be very important in living things. A living organism cannot survive without nitrogen. 

Nitrogen Cycle

Besides oxygen, carbon, and hydrogen, nitrogen is one of the most important compounds in living things. As nitrogen is a component of amino acids, proteins, hormones, chlorophylls, and vitamins, the plants compete with microbes for the nitrogen available in the soil. The nitrogen present in the soil is limited to some extent. Hence, it can be said that nitrogen is a limited component for both natural and agricultural ecosystems.

Nitrogen cycle plays a key role in the process of nitrogen metabolism. Nitrogen exists in the form of two nitrogen atoms joined by a very strong triple covalent bond. The process in which the nitrogen is converted into ammonia is called Nitrogen fixation. In short, the process of the nitrogen cycle is to convert nitrogen from one form to another. Nitrogen fixation, ammonification, nitrification and denitrification are some of the important processes in the nitrogen cycle.

The earth’s atmosphere has up to 78% of nitrogen. But, there is a scarcity of usable nitrogen in the ecosystem. Nitrogen is present in the environment in various chemical forms such as organic nitrogen, ammonium (NH4+), nitrite (NO2-), nitrate (NO3-), nitrous oxide (N2O), nitric oxide (NO), inorganic nitrogen gas (N2), etc. The process of the nitrogen cycle is carried out by microbes. The lightning and ultraviolet radiations convert nitrogen into nitrogen oxides naturally.

Biological Nitrogen Fixation

Very few organisms are capable of fixing nitrogen by themselves. Nitrogen is always present in the atmosphere as nitrogen gas or dinitrogen. The dinitrogen is a highly inert gas and cannot undergo any chemical reaction.

When the living things reduce nitrogen to ammonia, this process is called biological nitrogen fixation. Nitrogenase is an enzyme that is capable of nitrogen reduction. It is exclusively present in prokaryotes. These kinds of microbes are known as N2 fixers. 

N≡N NH3

The nitrogen-fixing microbes may be either free-living or symbiotic. For example, free-living nitrogen-fixing microbes are Azotobacter and Beijernickia, Rhodospirillum, and Bacillus. Along with these, cyanobacteria such as Anabaena and Nostoc are also free-living nitrogen-fixers.

Mechanism of Biological Nitrogen Fixation

Studying the mechanism of biological nitrogen fixation is essential in nitrogen metabolism. The process of biological nitrogen fixation requires molecular nitrogen, nitrogenase, a component to trap the formed ammonia, strong reducing power, and source of energy. All these components help in reducing nitrogen, such as FAD (Flavin adenine dinucleotide) and NAD (Nicotinamide Adenine Dinucleotide). These components also help in transferring hydrogen atoms from NADH2 or FADH2 to dinitrogen and trapping the produced ammonia.

The process of biological nitrogen fixation involves stepwise reduction of nitrogen to ammonia. The nitrogenase is a Mo-Fe-containing protein and is bound with the molecule of nitrogen (N2). Later, this nitrogen molecule acts on hydrogen (from the reduced coenzymes) and is step wisely reduced. At first, it produces diamide (N2H2) and then produces hydrazine (N2H4). At last, it produces ammonia (2NH3).

NH3 is considered a toxic component to the cells. Thus, the nitrogen fixers combine NH3 with organic acids that are present in the cell. As a result, amino acids are formed. The equation for biological nitrogen fixation can be derived as:

N2 + 16ATP + 8H+ + 8e- 2NH3 + 16ADP + 16Pi +

Assimilation

Plants can easily absorb nitrate and ammonium from the soil with the help of their root hairs. When the nitrate is absorbed, it is first reduced to nitrite ions and ammonium ions. This reduction process takes place to incorporate nitrogen into amino acids, nucleic acids, and chlorophyll.

Some nitrogen is assimilated in the form of ammonium ions directly from the nodules in plants having a symbiotic relationship with rhizobia. There is a more complex cycling of amino acids between Rhizobia bacteroides and plants. The bacteroids are provided to the amino acids by the plants. Hence, undergoing ammonia assimilation is not at all necessary here. This is why the bacteroids pass amino acids to the plant and form an interdependent relationship. Many animals, fungi, and many other heterotrophic organisms can obtain nitrogen by ingestion of amino acids, nucleotides, and other small organic molecules. Hence, all the other heterotrophs, along with any of the bacteria, are capable of using inorganic compounds such as ammonium as the sole N source. Such kinds of various N sources are very carefully regulated in all living organisms.

Ammonification

When the organic nitrogen present in the dead plants and animals is decomposed into ammonia, it is called ammonification. The ammonia gets volatilized and re-enters the atmospheric air. But the majority of ammonia is converted into nitrate by soil bacteria. The following equation describes the whole ammonification process in the following steps:

2NH3 + 3O2     2NO-2 + 2H2O

2NO-2 + O2    2NO-3

Conclusion

Nitrogen is an essential element in the entire life process of all living things. The atmospheric nitrogen cannot be used by the plants directly. Very few plants especially, the roots of legumes, can fix this atmospheric nitrogen into biologically usable forms. A very strong reducing agent and energy in ATP is necessary for nitrogen fixation. Nitrogen-fixing microbes such as Rhizobium and the enzyme nitrogenase are very important in the whole process. The ammonia produced after the nitrogen fixation is converted into amino acids as the amino group. Hence, nitrogen metabolism is very important in living organisms.