Ammonia incorporation into amino acids

A heterogeneous group of microorganisms, including many bacterial, fungal, and algal species, are capable of assimilatory nitrate reduction, a process that reduces nitrate and nitrite to ammonia, which can be subsequently incorporated into amino acids. 

Nitrogen fixation is the term for a process that converts diatomic nitrogen gas (N2) in the atmosphere into ammonia (NH3). This is a very important biological process because it converts N2, which is very unreactive, into a form that can more readily be incorporated into amino acids and other molecules. Transition metals (such as vanadium or molybdenum) are found in the active sites of the enzymes that carry out this important reaction. 

High concentrations may cause significant respiratory failure with acute lung injury, non-cardiogenic pulmonary oedema and ARDS. Ammonia is converted to carbamoyl phosphate by the enzyme carbamoyl phosphate synthase, and then enters the urea cycle to be either incorporated into amino acids or excreted in the urine. 

Numerous plants and microorganisms can assimilate ammonium ions by incorporating them into amino acids and other nitrogen-containing biochemicals. Animals acquire all of their amino acids when they eat plants (or other animals). When plants or animals die (or release waste), the nitrogen is returned to the soil. The usual forms of nitrogen returned to the soil in animal wastes or in the output of the decomposers are ammonia, urea, or uric acid. 

Ammonium is often the most abundant form of inorganic nitrogen in the surface layers after phytoplankton blooms have removed the greater part of the nitrate and phosphate. In the assimilation processes of phytoplankton, ammonium is preferentially used for synthesizing proteins. When nitrate is incorporated it must first be reduced to ammonia before it can be transferred into amino acid compounds. However, there is no indication that growth rates are particularly increased by either form. 

The phototrophs that assimilate fixed nitrogen preferentially utilize ammonium over nitrate or nitrite. (Some phototrophs assimilate DON, such as urea, to meet their nitrogen needs, and, hence, are not strict autotrophs.) Ammonia is fevored over the more oxidized forms of DIN as less redox energy is required to incorporate its nitrogen into biomolecules, primarily amino acids and the nucleotide bases. 

The ammonia can then be utilised for amino acid synthesis in some or all of the microorganisms in the intestine, a process requiring the enzyme glutamate dehydrogenase to incorporate the ammonia into glutamate... 

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

A stimulating development of urea alcoholysis has been demonstrated very recently for better AE, in an innovative integrated process that incorporates fatty ester hydrolysis to co-amino-alkanoic acids [44], Within the scope of this chapter, the most interesting step of this process is the recycling of waste alcohol, formed by the hydrolysis step, for urea alcoholysis. Dialkyl carbonate is produced together with ammonia thereafter, the ammonia is engaged in the amination reaction to obtain the amino acids. The overall process avoids the storage of NH3 that is necessary for the amination route, and transforms a waste product-the alcohol-into the valuable dialkyl carbonate. 

Ammonia is toxic at high concentrations, even though ammonium ion, NH4+ is an intermediate in many reactions. For its utilization, ammonia must be incorporated into organic forms, transferred, and then incorporated into other compounds, for example, amino acids and nucleotides. The amino acids glutamine and glutamate and the compound carbamoyl phosphate are the key intermediates of nitrogen assimilation, leading to different classes of compounds. 

In addition to being incorporated into tissue proteins, amino acids, after losing their nitrogen atoms by deamination and/or transamination, may be catabolized to yield energy or to form glucose. Conversely, the nonessential amino acids may be synthesized from carbohydrate metabolism intermediates and ammonia or from essential amino acids. This section is devoted to the mechanisms of such metabolic processes and their interrelationships with carbohydrate and lipid metabolic pathways. 

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