Ammonium Assimilation PDF

## Chapter 12: Assimilation of Mineral Nutrients ### Conversion of Ammonium to Amino Acids Requires Two Enzymes - Glutamine synthetase (GS) combines ammonium with glutamate to form glutamine. - This requires one ATP and a divalent cation (Mg2+, Mn2+, or Co2+). - Plants have two classes of GS: one...

## Chapter 12: Assimilation of Mineral Nutrients ### Conversion of Ammonium to Amino Acids Requires Two Enzymes - Glutamine synthetase (GS) combines ammonium with glutamate to form glutamine. - This requires one ATP and a divalent cation (Mg2+, Mn2+, or Co2+). - Plants have two classes of GS: one in the cytosol and one in root plastids or shoot chloroplasts. - Cytosolic forms are expressed in germinating seeds or vascular bundles. - Plastid forms generate amide nitrogen for local consumption in roots and reassimilate photorespiratory NH4+ in shoots. - Elevated plastid levels of glutamine stimulate the activity of glutamate synthase (GOGAT). - GOGAT transfers the amide group of glutamine to 2-oxoglutarate, yielding two molecules of glutamate. - There are two types of GOGAT: - One accepts electrons from NADH and is located in plastids of nonphotosynthetic tissues. - The other accepts electrons from ferredoxin (Fd) and is found in chloroplasts. ### Ammonium Assimilation - Plant cells avoid ammonium toxicity by rapidly converting it to amino acids. - The primary pathway involves glutamine synthetase and glutamate synthase. ### Glutamate Dehydrogenase - Catalyzes a reversible reaction that synthesizes or deaminates glutamate. - An NADH-dependent form is found in mitochondria and chloroplasts. - It cannot substitute for the GS-GOGAT pathway for assimilation of ammonium. - Primarily deaminates glutamate. ### Transamination Reactions - Transfer nitrogen once assimilated into glutamine and glutamate. - Catalyzed by aminotransferases. - Aspartate aminotransferase transfers the amino group of glutamate to the carboxyl atom of aspartate. - Requires pyridoxal phosphate (vitamin B) as a cofactor. ### Asparagine and Glutamine Link Carbon and Nitrogen Metabolism - Asparagine serves as a protein precursor, but also as a key compound for nitrogen transport and storage. - Its high nitrogen-to-carbon ratio makes it stable and useful for transport. - Asparagine synthesis involves the transfer of the amide nitrogen from glutamine to aspartate, catalyzed by asparagine synthetase. ### Biological Nitrogen Fixation - Biological nitrogen fixation accounts for most of the fixation of atmospheric N2 into ammonium. - It represents the key entry point of molecular nitrogen into the biogeochemical cycle of nitrogen. - Described by the symbiotic relations between nitrogen-fixing organisms and higher plants. - Specialized structures form in roots when infected by nitrogen-fixing bacteria. ### Free-Living and Symbiotic Bacteria Fix Nitrogen - Some bacteria can convert atmospheric nitrogen into ammonium. - Most are free-living in the soil. - Some form symbiotic associations with higher plants. - These prokaryotes directly provide the host plant with fixed nitrogen in exchange for other nutrients and carbohydrates. ### Nitrogen Fixation Requires Anaerobic Conditions - Oxygen irreversibly inactivates the nitrogenase enzymes involved in nitrogen fixation. - Nitrogen fixation must be fixed under anaerobic conditions. *** The image being described is a diagram showing the chemical structures and metabolic pathways involved in ammonium assimilation. These pathways include glutamine synthetase, glutamate synthase, glutamate dehydrogenase, and aspartate aminotransferase. The image also shows the structures of ammonium, glutamine, glutamate, 2-oxoglutarate, aspartate, and asparagine.