Nitrogen Cycle

The process of Nitrogen being released from Alanine, oxidized by soil microbes, absorbed by a root, and reduced and assimilated into aspartic acid is known as the Nitrogen Cycle. This cycle is necessary because there is a shortage of nitrogen in the soil; therefore, most of the soil nitrogen is obtained from dead organic materials such as Amino Acids.

The first step in this process is nitrogen being released from the amino acid. This process is known as ammonification. Alanine is used as an organic source because it is found in humus, which is dead organic matter. The Alanine is decomposed into simpler compounds by soil-dwelling saprophytic bacteria and different fungi. They incorporate the nitrogen from amino acids and proteins and release the excess nitrogen in the form of ammonium ions.

The next part of this cycle is the oxidation of ammonium, also called nitrification. This step takes the nitrogen compound, in this case ammonia, and oxidizes it into nitrite and nitrate. This is a two step process first the ammonium is oxidized into nitrite and then the nitrite is oxidized into nitrate. Nitrite is toxic; therefore, nitrate is the form that almost all nitrogen is absorbed by most crop plants grown on dry land.

Step 1: 2NH4 (ammonium)+ 3O2 (Oxygen) „Ñ-2NO-2(Nitrite) + 4H+ (hydrogen) + 2H2O

Step 2: 2NO-2 (Nitrite) + O2 (oxygen) „Ñ- 2NO-3 (nitrate)

This process is detrimental because the plant must reduce it back to amino group (ammonium) before it can incorporate it into amino acids, which costs energy. It would be better if the plant could skip this step, however, bacteria also want ammonium and due to the fact that there are more bacteria the plant will not get enough ammonium through direct absorption of ammonium.

The next step is Nitrogen reduction. This process reduces nitrate back into the amino group (ammonium). This allows the plant to absorb the ammonium ions. This is also a two step process. The first step takes place in the cytosol called nitrate reductase. This reduces nitrate to nitrite. The nitrate reuctase enzyme is a large enzyme with multiple subunits. It contains FAD (flavin adenine dinucleotide), Mo (Molybdenum), and cytochrome 557, with a ferredoxin Fe4S4 center. Mo is bound to a cofactor containing a pteridine ring to form molybdopterin. The second step takes place in the plastids of plants.

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