Metabolism of Nitrogen

Which are the elements that are most prevalent in living organisms? They are carbon, hydrogen and oxygen. Did you know that nitrogen is also a prevalent element in living organisms? Important components such as proteins, hormones, amino acids and vitamins, all contain nitrogen! Let’s learn more about nitrogen metabolism.

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Nitrogen Metabolism

The soil has limited amounts of nitrogen. Plants and microbes compete for this nitrogen. Therefore, it is a limiting nutrient for both agricultural and natural ecosystems. Let’s understand how this available nitrogen is cycled in the ecosystem a concept known as nitrogen metabolism.

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Nitrogen Cycle

The main part of nitrogen metabolism is the Nitrogen Cycle. A molecule of nitrogen is made of two nitrogen atoms held together by a very strong triple covalent bond (N ≡ N). There are three main pools of nitrogen – atmosphere, soil and biomass. Nitrogen cycles between these pools in the following manner:

Atmospheric Pool

The process of converting atmospheric nitrogen (N₂) to ammonia (NH₃) is ‘Nitrogen fixation‘. Atmospheric nitrogen is fixed in three ways – biological, industrial and electrical.

• ‘Biological nitrogen fixation’ involves living organisms that reduce nitrogen to ammonia.
• ‘Industrial nitrogen fixation’ involves industrial combustions, automobile exhausts, forest fires and power-generating stations as sources of nitrogen.
• ‘Electrical nitrogen fixation’ is when forces of nature such as lightning and ultraviolet radiation provide energy to convert nitrogen to nitrogen oxides.

Soil Pool

The above processes fix atmospheric nitrogen into the soil. This nitrogen is then taken up by plants and animals, consequently.

Biomass Pool

When plants and animals die, the organic nitrogen within them decomposes to ammonia. This process is ‘Ammonification‘ and it returns nitrogen back to the soil. Some of this ammonia evaporates and re-enters the atmosphere while a major part of it is converted by soil bacteria into nitrate as follows:

(i) First, ammonia is oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.

2NH₃ + 3O₂ → 2NO₂^– + 2H⁺ + 2H₂O

(ii) Then, nitrite is further oxidised to nitrate by Nitrobacter.

2NO₂^– + O₂ → 2NO₃^–

These reactions are called  ‘Nitrification‘ and the nitrifying bacteria are ‘Chemoautotrophs‘. Plants absorb the nitrate thus formed and transport it to the leaves where it is reduced to ammonia. This ammonia forms the amine group of amino acids. Nitrates in the soil are also reduced to nitrogen during ‘Denitrification‘ by Pseudomonas and Thiobacillus. In this way, nitrogen keeps cycling in the ecosystem.

Biological Nitrogen Fixation

Only a few prokaryotes can use the atmospheric nitrogen as N₂ and reduce it to ammonia. This reduction of nitrogen to ammonia by living organisms is ‘biological nitrogen fixation’. The enzyme it needs for this reaction – nitrogenase is present exclusively in prokaryotes and these microbes are called N₂ – fixers. These N₂ – fixers can be symbiotic or free-living. Some examples of free-living N₂ – fixers are Azotobacter, Bacillus, Anabaena, Nostoc etc.

Symbiotic Biological Nitrogen Fixation

The most popular example in this category is the symbiotic relationship between Rhizobium and the roots of legumes such as sweet pea, garden pea, lentils. The association is visible as nodules (small outgrowths) on the roots. Another example is the microbe Frankia that also produces nitrogen-fixing nodules on the roots of non-leguminous plants.

Did you know that if you cut through a nodule, its central portion is red or pink? What gives it this colour? It is the presence of leguminous haemoglobin or leg-haemoglobin. Let’s learn a little more about these nodules.

Nodule Formation

Another form of nitrogen metabolism is nodule formation. Nodule formation involves several interactions between the roots of the host plant and Rhizobium. They are:

• The Rhizobia multiply, colonise and attach themselves to the epidermal and root-hair cells of legumes.
• The root-hair curls allowing the bacteria to invade, create an infection thread and reach the cortex of the root. Here the bacteria initiate nodule formation.
• The infection thread then releases the bacteria into the cells. This leads to the differentiation of special nitrogen-fixing cells. In this way, the nodule establishes a direct connection for exchange of nutrients with the host.
• The nodule contains the enzyme nitrogenase which converts atmospheric nitrogen to the first stable product of nitrogen fixation – ammonia. The reaction is:

N₂ + 8e^– + 8H⁺ + 16ATP —→ 2NH₃ + H₂ + 16ADP + 16P_i

The energy required for the above reaction (8ATP for each NH₃ produced) comes from the respiration of host cells. Also, the enzyme nitrogenase is very sensitive to molecular oxygen and therefore needs anaerobic conditions. The oxygen-scavenger called leg-haemoglobin protects nitrogenase from oxygen in the nodules.

Interestingly, this microbe lives as aerobic, free-living organism where nitrogenase is not functional. But during nitrogen-fixation, they become anaerobic, thus protecting the enzyme from oxygen.

Fate of Ammonia

What happens to the ammonia generated after nitrogen-fixation? It is protonated to form ammonium ion (NH₄⁺) at physiological pH. Although plants can accumulate nitrate and NH₄⁺ ions, NH₄⁺ ions are toxic to them. Thus, it is in turn, used to synthesize amino acids in plants as follows:

(i) Reductive amination: Here, ammonia reacts with α-ketoglutaric acid to form glutamic acid in the presence of the enzyme – glutamate dehydrogenase.

α-ketoglutaric acid + NH₄⁺ + NADPH → glutamate + H₂O + NADP

(ii) Transamination: Here, the amino group of one amino acid is transferred to the keto group of a keto acid in the presence of the enzyme – transaminase. Asparagine and glutamine – the two most important amides in plants arise from two amino acids – aspartic acid and glutamic acid, respectively. Another NH₂^– radicle replaces the hydroxyl group of the acid to give an amide. The xylem then transports these amides that contain more nitrogen to different parts of the plant.

Solved Example For You

Question: What is the process of the reduction of nitrates in the soil to nitrogen?

1. Nitrification
2. Nitrogen fixation
3. Ammonification
4. Denitrification

Solution: The answer is “D”. Denitrification results in the conversion of nitrates in the soil to nitrogen.