Nitrogen Metabolism — Revision Notes

⚡ 30-Second Revision

Nitrogen Fixation: — $N_2 \rightarrow NH_3$. By prokaryotes (nitrogenase). Requires 16 ATP, anaerobic conditions. Examples: *Rhizobium* (symbiotic), *Azotobacter* (free-living aerobic), *Clostridium* (free-living anaerobic), *Nostoc* (cyanobacteria).
Leghemoglobin: — O2 scavenger in root nodules, protects nitrogenase.
Nitrification: — $NH_3 \rightarrow NO_2^- \rightarrow NO_3^-$. By *Nitrosomonas* ($NH_3 \rightarrow NO_2^-$) and *Nitrobacter* ($NO_2^- \rightarrow NO_3^-$). Aerobic.
Nitrate Assimilation (Plants): — $NO_3^- \rightarrow NO_2^-$ (Nitrate Reductase, cytoplasm, NADH/NADPH). $NO_2^- \rightarrow NH_3$ (Nitrite Reductase, plastids, reduced ferredoxin).
Ammonia Assimilation:

- Reductive Amination: $\alpha$-ketoglutaric acid + $NH_4^+ \rightarrow$ Glutamate (Glutamate Dehydrogenase, NADH/NADPH). - Transamination: Amino acid$_1$ + Keto acid$_2 \rightarrow$ Keto acid$_1$ + Amino acid$_2$ (Transaminases, pyridoxal phosphate).

Ammonification: — Organic N $\rightarrow NH_3$ (decomposers).
Denitrification: — $NO_3^- \rightarrow N_2$ (e.g., *Pseudomonas*, *Thiobacillus*). Anaerobic.

2-Minute Revision

Nitrogen metabolism is crucial for life, revolving around the Nitrogen Cycle. The cycle starts with Nitrogen Fixation, where atmospheric $N_2$ is converted to ammonia ($NH_3$) by nitrogenase-possessing prokaryotes like *Rhizobium* (symbiotic, protected by leghemoglobin) and free-living *Azotobacter*.

This is an energy-intensive, oxygen-sensitive process. Ammonia is then converted to nitrites ($NO_2^-$) by *Nitrosomonas* and then to nitrates ($NO_3^-$) by *Nitrobacter* in Nitrification. Plants absorb nitrates, reducing them back to ammonia via Nitrate Reductase (nitrate to nitrite) and Nitrite Reductase (nitrite to ammonia).

The resulting ammonia is assimilated into organic molecules, primarily glutamate, through Reductive Amination (with $\alpha$-ketoglutarate) or the GS-GOGAT pathway. Glutamate then donates its amino group to other keto acids via Transamination to form other amino acids.

Dead organic matter is broken down to ammonia in Ammonification. Finally, Denitrification returns nitrates to atmospheric $N_2$ by bacteria like *Pseudomonas* under anaerobic conditions, completing the cycle.

Key enzymes, bacteria, and conditions are high-yield for NEET.

5-Minute Revision

Nitrogen is a vital macronutrient, forming the backbone of proteins, nucleic acids, and ATP. Its metabolism is centered on the Nitrogen Cycle, a series of transformations making atmospheric nitrogen available to life.

The cycle begins with Nitrogen Fixation, the conversion of inert atmospheric $N_2$ into ammonia ($NH_3$). This is exclusively performed by prokaryotes using the nitrogenase enzyme complex, which is highly sensitive to oxygen and requires substantial ATP (16 ATP per $N_2$).

Examples include symbiotic bacteria like *Rhizobium* (in legume root nodules, where leghemoglobin scavenges oxygen to protect nitrogenase) and free-living bacteria like *Azotobacter* (aerobic), *Clostridium* (anaerobic), and cyanobacteria (*Nostoc*).

Next, Nitrification occurs in two aerobic steps: *Nitrosomonas* oxidizes $NH_3$ to $NO_2^-$, and *Nitrobacter* oxidizes $NO_2^-$ to $NO_3^-$. Plants primarily absorb $NO_3^-$. Inside plant cells, Nitrate Assimilation reduces $NO_3^-$ back to $NH_3$. This involves Nitrate Reductase (cytoplasm, $NO_3^- \rightarrow NO_2^-$, uses NADH/NADPH) and Nitrite Reductase (plastids, $NO_2^- \rightarrow NH_3$, uses reduced ferredoxin).

The resulting ammonia is toxic and must be quickly assimilated into organic molecules. The main pathways are:

1
Reductive Amination: — $NH_4^+$ combines with $\alpha$-ketoglutaric acid to form glutamate, catalyzed by Glutamate Dehydrogenase (GDH).
2
Transamination: — An amino group from glutamate is transferred to various keto acids (e.g., pyruvic acid, oxaloacetic acid) to form other amino acids (e.g., alanine, aspartate), catalyzed by transaminases (requiring pyridoxal phosphate). Glutamine and asparagine, important nitrogen transport forms, are also synthesized from glutamate and aspartate, respectively, by adding another amino group.

Organic nitrogen from dead organisms and waste is converted back to $NH_3$ by decomposers in Ammonification. Finally, under anaerobic conditions, Denitrification by bacteria like *Pseudomonas* and *Thiobacillus* reduces $NO_3^-$ back to gaseous $N_2$, returning it to the atmosphere and completing the cycle. For NEET, focus on the specific organisms, enzymes, substrates, products, and conditions for each step.

Prelims Revision Notes

Nitrogen is a critical component of proteins, nucleic acids, and ATP. Most organisms cannot use atmospheric $N_2$ directly due to its triple bond.

I. Nitrogen Fixation:

Conversion of $N_2$ to $NH_3$.
Exclusively by prokaryotes (bacteria, archaea).
Enzyme: Nitrogenase (MoFe protein and Fe protein).
Conditions: Highly sensitive to oxygen (anaerobic environment required), high ATP requirement (16 ATP per $N_2$).
Symbiotic Fixers:

* *Rhizobium* with legumes (e.g., pea, bean, clover). Forms root nodules. * Leghemoglobin: Pink pigment in nodules, acts as an oxygen scavenger to protect nitrogenase. * *Frankia* with non-legumes (e.g., *Alnus*).

Free-living Fixers:

* Aerobic: *Azotobacter*, *Beijerinckia*. * Anaerobic: *Rhodospirillum*, *Clostridium*. * Cyanobacteria: *Nostoc*, *Anabaena* (have heterocysts for anaerobic conditions).

II. Nitrification:

Oxidation of $NH_3$ to $NO_3^-$.
Two steps, by chemoautotrophic bacteria (aerobic process).

1. $NH_3 \rightarrow NO_2^-$ (nitrite): By *Nitrosomonas*, *Nitrococcus*. 2. $NO_2^- \rightarrow NO_3^-$ (nitrate): By *Nitrobacter*.

Plants absorb $NO_3^-$ (most common) or $NH_4^+$.

III. Nitrate Assimilation (in Plants):

Reduction of absorbed $NO_3^-$ to $NH_3$.
Two steps:

1. $NO_3^- \rightarrow NO_2^-$: By Nitrate Reductase (cytoplasm, uses NADH/NADPH). Inducible enzyme. 2. $NO_2^- \rightarrow NH_3$: By Nitrite Reductase (plastids - chloroplasts/roots, uses reduced ferredoxin). Nitrite is toxic, so this step is rapid.

IV. Ammonia Assimilation (in Plants):

Incorporation of $NH_3$ into organic molecules (amino acids).
$NH_3$ is toxic at high concentrations.
Main pathways:

1. Reductive Amination: $\alpha$-ketoglutaric acid + $NH_4^+ + NADPH \xrightarrow{\text{Glutamate Dehydrogenase}} \text{Glutamate} + H_2O + NADP^+$. 2. Transamination: Transfer of amino group from one amino acid (e.g., Glutamate) to a keto acid to form a new amino acid. Catalyzed by Transaminases (aminotransferases), requires pyridoxal phosphate.

Amide Formation: — Glutamine (from glutamate) and Asparagine (from aspartate) are formed by adding another amino group, requiring ATP. They are important for nitrogen transport.

V. Ammonification:

Decomposition of organic nitrogen (proteins, nucleic acids) from dead organisms/waste into $NH_3$.
By decomposers (bacteria, fungi).

VI. Denitrification:

Reduction of $NO_3^-$ back to gaseous $N_2$ or $N_2O$.
By bacteria like *Pseudomonas*, *Thiobacillus denitrificans*.
Occurs under anaerobic conditions.
Returns nitrogen to the atmosphere, completing the cycle.

Vyyuha Quick Recall

Nice Nitrogen Fixers Need Nice Atmosphere, And Don't Die!

Nice Nitrogen Fixers: Nitrogen Fixation (e.g., *Rhizobium*, *Azotobacter*)
Need Nice Atmosphere: Nitrification (*Nitrosomonas*, *Nitrobacter*)
And Don't Die: Ammonification, Denitrification (*Pseudomonas*)

This mnemonic helps recall the main processes and some key bacteria in the nitrogen cycle. For assimilation, remember 'G' for Glutamate (from reductive amination) and 'T' for Transamination (forming other amino acids).