Nitrogen is an essential element for all forms of life. It's abundantly used to incorporate the structure of nucleic acids and other cellular components besides proteins. Nitrogen metabolism in plants refers to the absorption and assimilation of nitrogenous compounds like nitrate (NO3-) and ammonium (NH4+), followed by the synthesis of amino acids, which are both precursor molecules for protein biosynthesis. The understanding of these processes is of key importance in plant biology and agriculture because of their direct effects on plant growth and productivity.
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Nitrate Assimilation
In general, the process by which plants convert nitrate uptake from the soil into their system involves its conversion into organic nitrogen compounds. Here are some of the steps and enzymes:
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Nitrate Uptake and Nitrate Uptake Pathways
Nitrate is brought into the cell mainly by specific nitrate transporters located in the plasma membrane of the root cells.
Reduction of Nitrate to Nitrite
Nitrate Reductase (NR)
After uptake by the plant, nitrate is reduced to nitrite (NO2-) within the cytoplasm catalyzed by the enzyme nitrate reductase. This process is driven by energy use in the form of NADH or NADPH.
Nitrite Reductase (NiR)
The nitrite produced is taken up in the chloroplasts (or plastids in non-photosynthetic tissues) and reduced to ammonium by nitrite reductase. There is also a reducing agent, although in photosynthetic tissues, principally ferredoxin.
Ammonium assimilation
This is accomplished by binding ammonium with the organic compounds, mostly combining the products into amino acids. The key enzymes are glutamine synthetase and glutamate synthase, sometimes collectively called the GS-GOGAT cycle.
Glutamine Synthetase (GS)
Role: This enzyme binds ammonium and glutamate to form glutamine in an ATP-dependent reaction.
Distribution: It occurs in the cytosol and the chloroplasts so that it can utilize any ammonium made available through nitrate reduction or any other metabolism pathway.
Glutamate Synthase (GOGAT)
Function: Glutamate synthase transfers the amide group from glutamine to 2-oxoglutarate, yielding two glutamate molecules. The reaction is central to keeping the level of cellular glutamate and is essential for many metabolic pathways.
Groups: Two forms of glutamate synthase: Fd – GOGAT in photosynthetic tissues and NADH – GOGAT in non-photosynthetic tissues.
Amino Acid Biosynthesis
Amino acids are the building blocks of proteins and play a vital role in plant metabolism. Biosynthesis involves several pathways and is specific for each type of amino acid. We will limit our discussion to the general pathways that lead to the synthesis of some amino acids.
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| Author: Mplanine, CC0, via Wikimedia Commons |
Glutamate and Glutamine
Central Role: Glutamate and glutamine are central to amino acid metabolism. Critical roles of glutamate Glutamate is a primary amino group donor involved in most transamination Groups of compounds or individual compounds for which glutamine serves as an amino group donor in biosynthetic reactions—two Key Amino Acids in the Aspartate Family are aspartate, lysine, methionine, threonine, and isoleucine.
Pathway of Biosynthesis
Aspartate derives from oxaloacetate through transamination. It, in turn, can be converted into some of the other amino acids of this family by a set of enzymatically catalyzed reactions.
AROMATIC AMINO ACIDS
KEY AMINO ACIDS: PHENYLALANINE, TYROSINE, TRYPTOPHAN
Shikimate Pathway: These are made by the Shikimate pathway in which Phosphoenolpyruvate and erythrose-4-phosphate are converted into chorismate, the common precursor for aromatic amino acids.
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Branched-Chain Amino Acids
Biosynthesis Pathway: These amino acids are synthesized from pyruvate and other intermediates through a list of enzymatic steps that share common pathways with fatty acid and carbohydrate metabolism.
Conclusion
Nitrogen metabolism incorporating nitrate and ammonium assimilation and amino acid biosynthesis is crucial for the development and growth of plants. Efficient conversion of inorganic nitrogen into organic molecules enables further building of proteins and other vital components in a plant. Understanding these biochemical pathways will provide insight into plant physiology and also help improve agricultural productivity through nitrogen use efficiency. Biotechnological and breeding advances directed toward the optimization of nitrogen metabolism could be developed to design practices in sustainable and industry-resilient agriculture that will tackle the current challenges relevant to food security and environmental sustainability.