Here is a term paper on the ‘Transport of Nitrogen’. Find paragraphs, long and short term papers on the ‘Transport of Nitrogen’ especially written for school and college students.

Uptake of Amino Acid:

Many cells are capable of concentrating amino acids from the extra-cellular environment, but the processes of transport have not been explored in detail in most cases. Separate carriers are known to exist for some neutral amino acids, cationic amino acids, and anionic amino acids. For example, patients have been recognized with a hereditary defect causing them to excrete abnormal quantities of cystine, arginine, lysine, and ornithine.

The formation of stones from the slightly-soluble cystine is the usual presenting complaint, hence the condition is known as cystinuria, but the mutation affects a single carrier for all amino acids with two positively charged groups so as to prevent their reabsorption from the glomerular filtrate in the kidney, permitting their escape in the urine. Such mutant genes have a frequency greater than 1:200, at least in England, with 8 homozygotes found upon screening 142,000 newborn.

A Na+-coupled symport mechanism for the absorption of neutral amino acids is also present in the brush borders of the intestinal mucosa and the kidney tubules. The passage of Na+ down its concentration gradient causes simultaneous transport of amino acids against their concentration gradient in the same way that glucose is moved in these cells. (Some amino acids are transported into striated muscles by a Na+-dependent process that is stimulated by insulin.)

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There is strong evidence that a known set of reactions involving 5-glutamyl peptides is used for amino acid transport into many tissues, including the brush borders of the kidney and the small bowel, the brain, and red blood cells. These peptides, commonly known as g–glutamyl peptides because they involve the carboxyl group attached to the g–carbon atom, include glutathione (5 – glutamylcysteinylglycine). The tissue content of glutathione ranges as high as eight millimoles per kilogram. (Glutathione is also used as a reducing agent, but this function is not involved in amino acid transport.)

The amino acid to be transported is converted to a g–glutamyl derivative by moving the g–glutamyl group of glutathione or another g–glutamyl peptide onto it. The g–glutamyl transferase is believed to be exposed on the external side of the plasma membrane, with the products released on the internal side. In any event, the g–glutamyl amino acid is attacked by a cyclotransferase that releases the amino acid inside the cell and forms pyroglutamate (5-oxoproline) from the glutamyl group.

The starting materials can be synthesized from the other product of the amino acid transfer, creating a cycle. When glutathione is used as a glutamyl group donor, the other product is cysteinylglycine, which can be hydrolyzed to its constituent amino acids. The ring in pyroglutamate is opened to form glutamate by a reaction requiring ATP. Glutathione can then be rebuilt from glutamate, cysteine, and glycine by two ATP-requiring synthetases.

According to the mechanism given, the transport of one molecule of amino acid requires the expenditure of three high- energy phosphate bonds, but there are alternative possibilities that are less expensive involving g–glutamylcysteine itself as the glutamyl group donor. The significance of these pathways remains to be seen.

Transport between Tissues:

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The skeletal muscles, the intestines, and the liver are particularly important in disposing of excess amino acids, but much of the nitrogen is channeled into only a few compounds for transport between these tissues. For example, the skeletal muscles export nitrogen mainly in the form of glutamine and alanine.

Glutamine is neutral, non-toxic, and crosses plasma membranes readily. It is synthesized as a device for storing and transporting ammonium ions, as well as for incorporation into proteins, and this function is especially important in the brain and the striated muscles. Muscles, with their great mass, are more significant quantitatively.

Transport of Amino Acids

Glutamine synthetize catalyzes the combination of ammonium ions and glutamate at the expense of ATP:

We will see how the nitrogen from many amino acids can appear in glutamate and in ammonium ions, which can then be combined in glutamine for transport from the muscles.

Most of the glutamine is extracted from the blood by the small intestine, which contains a glutaminase catalyzing the hydrolysis to glutamate and NH4+. Some of the ammonium ions leave the small bowel as such, but some are converted to citrulline for export. Both the ammonium ions and the citrulline go directly from the bowel to the liver through the portal blood and are used to make urea.

The nitrogen from amino acids in the muscles or the small bowel appears first in glutamate, and then in alanine by transamination. Alanine is the amino acid with the highest concentration in blood. The liver takes up alanine and removes the nitrogen for urea synthesis through the same chain of transamination.

Amino Acid Catabolism:

Metabolism of the amino group of amino acids can often be looked at separately from that of the carbon skeleton.

General Catabolic Pathways of Amino Acids

Some eight of the 20 amino acids commonly found in proteins are essential; that is to say, their carbon skeletons cannot be synthesized in the animal body.

The first stage in the catabolism of almost all amino acids is the formation of the corresponding oxo-acid:

The last stage is the formation of urea, NH2 • CO • NH2, and of CO2 and water.

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An L-amino-acid oxidase, directly catalysing reaction (1) above, occurs in animal tissues. The enzyme, however, is present in very low concentration even in tissues, such as liver or kidney, in which amino acid catabolism is vigorous. Most of the amino groups must thus be removed by another route.

Curiously, D-amino-acid oxidase is present in high concentration in kidney in particular. D-Amino acids occur infrequently in some peptides of bacterial origin but are not quantitatively important. In addition, some amino acids, e.g. methionine, racemize are quite readily. It is probable that the D-amino acid oxidase is a detoxifying enzyme.