Nitrate and Ammonia Assimilation by Plants
All plants require nitrogen because it has a role to play in the general metabolism and plants which do not fix nitrogen, use other combined nitrogen sources such as nitrate and ammonia for carrying on metabolic activity. Nitrogen fixation is detained to selected microbes and plants.
Nitrate is absorbed by most plants and reduced to ammonia with the help of Nitrate reductase and nitrite reductase enzymes. In first step, conversion of nitrate to nitrite is catalyzed by nitrate reductase enzyme and this enzyme has other important constituents like FAD, cytochrome, NADPH or NADH and molybdenum.
The overall process of nitrate reduction takes place in the cytosol and is energy. Nitrate reductase enzymes studied in many plants and observed that is continuously synthesized, degraded and inducible means that increase in nitrate concentration in the cytosol induces more of nitrate reductase to be synthesized. In second step, nitrites are further converted into ammonia by the enzyme nitrite reductase. Reduction of nitrite into ammonia takes place chiefly in the leaves and reduced ferredoxin is the reducing agent in this conversion. Ammonia so formed is incorporated into Keto-acids to synthesize amino acids which are considered as initial products of nitrogen assimilation in plants.
Nitrite reductase is able to accept electrons from sources (NADH, NADPH or FADH2). The enzyme nitrite reductase, which catalyses the reduction of nitrite to ammonia, is located in the stroma. Reduced ferredoxin is the electron donor for this reaction. Ammonia so formed has to be utilized quickly by plants because accumulation of ammonia has a toxic effect. Some plants including algae leach out excess ammonia which can further are oxidized to nitrite and nitrate by microorganisms in the soil or water.
Amino acid synthesis by plants
Ammonia formation is achieved by plants either by nitrogen fixation or by reduction of nitrate to nitrite. Ammonium (NH4+) is the most reduced from of inorganic combined nitrogen. Amino acids are supposed to be initial products of nitrogen assimilation. Each amino acid consists of at least one carboxyl (-COOH) group and one or several amino (-NH2) groups. The R-group (side chain) is what makes each amino acid unique.
Ammonium so produced is the major source of amino group. However, the carboxyl group has to be provided by other organic molecule synthesized by the plants. Synthesis of amino acids takes place by two main methods, they are follows;
Reductive amination reaction
In this reaction, ammonia reacts with -ketoglutaric acid, which results in the formation of glutamic acid.
Enzyme responsible for this reaction is glutamate dehydrogenase. Similarly another amino acid called aspartic acid is produced by reductive amination of oxaloacetic acid.
It has been noted that reductive amination represents the major port of entry for ammonia into the metabolic stream in plants. This initiates synthesis of glutamic acid followed by other amino acids.
The process involves transfer of amino (-NH2) group from one amino acid to the keto group of keto acid. Glutamic acid is main from which other seventeen amino acids are formed through transamination. The enzyme responsible for this reaction is called transaminase.
There are two most important amides found in plants. They are asparagine and glutamine. They are formed from two amino acids called glutamic acid and aspartic acid. During this process hydroxyl (-OH) part of the acid is replaced by another (-NH2) radicle. The enzymes responsible for such reaction may be glutamine synthetase or asparagine synthetase.