Nitrogen Cycle: Proteolysis & Ammonification


Nitrogen Cycle: Proteolysis & Ammonification

Several biochemical steps involved in the nitrogen cycle are:

1. Proteolysis
2. Ammonification
3. Nitrification
4. Nitrate reduction and
5. Denitrification.

1. Proteolysis:
Plants use the ammonia produced by symbiotic and non-symbiotic Nitrogen fixation to make their amino acids & eventually plant proteins. Animals eat the plants and convert plant proteins into animal proteins. Upon death, plant and animals undergo microbial decay in the soil and the nitrogen contained in their proteins is released. Thus, the process of enzymatic breakdown of proteins by the microorganisms with the help of proteolysis enzymes is known as “proteolysis".

The breakdown of proteins is completed in two stages. In first stage proteins are converted into peptides or polypeptides by enzyme "proteinases" and in the second stage polypeptides / peptides are further broken down into amino acids by the enzyme "peptidases".

Proteins ————————> Peptides ————————> Amino Acids
                       Proteinases                                Peptidases   

The amino acids produced may be utilized by other microorganisms for the synthesis of cellular components, absorbed by the plants through mycorrhiza or may be de animated to yield ammonia.

The most active microorganisms responsible for elaborating the proteolytic enzymes (Proteinases and Peptidases) are Pseudomonas, Bacillus, Proteus, Clostridium Histolyticum, Micrococcus, Alternaria, Penicillium etc.
2. Ammonification (Ammo acid degradation):
Amino acids released during proteolysis undergo deamination in which nitrogen containing amino (-NH2) group is removed. Thus, process of deamination which leads to the production of ammonia is termed as "ammonification". The process of ammonification is mediated by several soil microorganisms. Ammonification usually occurs under aerobic conditions (known as oxidative deamination) with the liberation of ammonia (NH3) or ammonium ions (NH4) which are either released to the atmosphere or utilized by plants ( paddy) and microorganisms or still under favorable soil conditions oxidized to form nitrites and then to nitrates.

The processes of ammonification are commonly brought about by Clostridium sp, Micrococcus sp, Proteus sp. etc. and it is represented as follows.

CH3 CHNH2 COOH + 1/2 O2 —————–> C H3COCOOH      +      NH3
   Alanine                                    deaminase         Pyruvic acid               ammonia

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