Question

Compare amino acid metabolism in plants with that in mammals, in terms of where the amino acids come from, the main kinds of things plants do for amino acid metabolism, versus the main things mammals do.

Answer 1

The amino acids metabolism in plants

Amino acids (AAs) are abundantly found in the food plants and their individual concentration is of enormous importance in terms of nutrition. Therefore it is mandatory to explore the food plants for their nutritional importance. The ammonium produced by nitrogen fixation in the bacteroid is rapidly excreted to cytosol of infected cell of soybean nodules and then assimilated into glutamine and glutamic acid, by glutamine synthetase/glutamate synthase pathway. Most of the nitrogen is further assimilated into ureides, allantoin, and allantoic acid, via purine synthesis, and they are transported through xylem to the shoots. Nitrate absorbed in the roots is reduced by nitrate reductase and nitrite reductase to ammonia either in the roots or leaves. The ammonia is also assimilated by glutamine synthetase/glutamate synthase pathway, and mainly transported by asparagine, and not ureides. The nitrogen transported into leaves is readily utilized for protein synthesis, and then, some of them are decomposed and retransported to roots, apical shoots, and pods via phloem mainly in the form of asparagine.

Plants are photoautotrophs, and they can synthesize all organic compounds from inorganic materials such as carbon dioxide (CO2), water (H2O), and minerals using light energy. Amino acids are the key metabolites in nitrogen (N) metabolism of higher plants. First, the inorganic N, such as ammonium absorbed in the roots or produced from nitrate reduction, nitrogen fixation, and photorespiration, is initially assimilated into glutamine (Gln) and glutamate (Glu) by the glutamine synthetase (GS)/glutamate synthase (GOGAT) pathway. Second, amino acids are the essential components of proteins. Third, amino acids are used for long-distance transport of nitrogen among organs (roots, nodules, stems, leaves, pod, seeds, and apical buds) through xylem or phloem. Fourth, nonprotein amino acids may play a role in protecting plants from feeding damages by animals, insects, or infection by fungi. In this chapter, we would like to review the amino acid metabolism in soybean nodules, roots, leaves, pods, and seeds. In addition, we will introduce the amino acids transport via xylem and phloem among organs.

In plants, inorganic nitrogen (i.e. NO3– and NH4+) taken up by roots is incorporated into glutamine and glutamate (primary nitrogen assimilation), which is used to synthesize other amino acids and nitrogenous compounds by transamination. This process happens either in root or shoot tissue, depending on factors such as the molecular species of nitrogen taken up and the carbon/nitrogen balance of the plant. Once synthesized, amino acids are delivered to the so-called sink organs (developing roots and leaves, flowers, and seeds) that are largely dependent on reduced nitrogen supplied by the long-distance transport systems of the plant.

Amino acid transfer between organs through xylem and phloem is critical for optimizing nitrogen allocation in the plant to the growth conditions or developmental stage. In Ricinus, half of the amino acids delivered to the roots via the phloem are eventually relocated into the ascending xylem sap, indicating a significant amount of phloem-to-xylem transport. Amino acids delivered to developing fruits are predominantly supplied by the phloem, the content of which becomes enriched in amino acids as it moves towards fruits, revealing active xylem-to-phloem transfer. Distribution and recycling of amino acids through the xylem and phloem ensures optimal nitrogen allocation between organs. Amino acid transfer is also very active at the cellular level. Many amino acids are synthesized in the chloroplast and transported into the cytosol for protein synthesis and secondary metabolite production, or transported and stored in the vacuole. Distribution among the organelles is unequal: the highest amino acid content is found in the cytosol and lowest in the vacuole, suggesting directional amino acid transfer across the membrane rather than simple equilibrative diffusion.

Plants synthesize Amino Acids from the Primary elements, the Carbon and Oxygen obtained from air, Hydrogen from water in the soil, forming Carbon Hydrate by means of photosynthesis and combining it with the Nitrogen which the plants obtain from the soil, leading to synthesis of amino acids, by collateral metabolic pathways. Only L-Amino Acids are part of these Proteins and have metabolic activity

Soybean plants absorb inorganic N from the roots, and they can fix atmospheric N2 in the nodules associated with soil bacteria rhizobia. Nutrients and water flow via xylem and phloem in soybean plants. Soybean roots absorb water and nutrients in soil solution, and they are transported to the shoots via xylem vessels by the transpiration and root pressure. The fixed N in nodule is also transported to the shoots via xylem. On the other hand, photoassimilates (mainly sucrose), amino acids (Asn, etc.), and minerals (potassium, etc.) are transported from leaves to the apical buds, roots, nodules, and pods via the phloem by osmotic pressure or protoplasmic streaming.

Amonic acids performs various important roles in plants :

1. Inorganic nitrogen assimilation

2. Synthesis of proteins and nucleic acids in plants

3. Nitrogen transport and storage in plants

4. Amino acids for protecting the plants.

The amino acid metabolism in mammals

Ammonia production occurs in all tissues of the body during the metabolism of a variety of compounds. Ammonia is produced by the metabolism of amino acids and other compounds which contain nitrogen. Ammonia exists as ammonium ion (NH4+) at the physiological pH and is produced in body mainly by the process of transamination followed by deamination, from biogenic amines, from amino groups of nitrogenous base like purine and pyrimidine and in the intestine by intestinal bacterial flora through the action of urease on urea. Ammonia disposal takes place primarily by the hepatic formation of urea. The blood level of ammonia must remain very low because even slightly elevated concentrations (hyperammonemia) are toxic to the central nervous system (CNS). A metabolic mechanism exists by which nitrogen is moved from peripheral tissues to the liver for its ultimate disposal as urea, while at the same time maintaining low levels of circulating ammonia.

The amino acids take part in certain common reactions like transamination followed by deamination for production of ammonia. The amino group of amino acids is utilized for formation of urea which is an excretory product for protein metabolism. The amino acid is transaminated to produce a molecule of glutamate. Glutamate is the one amino acid that undergoes oxidative deamination to liberate free ammonia for the synthesis of urea. Once free ammonia is formed in peripheral tissues, it must be transferred to the liver for the conversation to urea. This is carried out by the ‘glucose-alanine cycle”. In the glucose-alanine cycle, alanine which is formed by the transamination of pyruvate gets transported in the blood to the liver, where it is transaminated by alanine transaminase to pyruvate. The non-toxic storage and transport form of ammonia in the liver is glutamine. Ammonia is loaded via glutamine synthetase by the reaction, NH3 + glutamate → glutamine. It occurs in nearly all tissues of the body. Ammonia is unloaded via glutaminase by a reaction, glutamine --> NH3 + glutamate. It specifically occurs in kidneys and intestine and in very low concentration in the liver. This reaction is induced by acidosis.

Animals get these amino acids by eating plants or animals that eat plants. This works because plants can make all twenty amino acids including the ten or so "essential" ones that most animals can't. In body there are two different types of amino acids: essential and non-essential. Non-essential amino acids are amino acids that body can create out of other chemicals found in body. Essential amino acids cannot be created, and therefore the only way to get them is through food. Here are the different amino acids

Non-essential:

· Alanine (synthesized from pyruvic acid)

· Arginine (synthesized from glutamic acid)

· Asparagine (synthesized from aspartic acid)

· Aspartic acid (synthesized from oxaloacetic acid)

· Cysteine (synthesized from homocysteine, which comes from methionine)

· Glutamic acid (synthesized from oxoglutaric acid)

· Glutamine (synthesized from glutamic acid)

· Glycine (synthesized from serine and threonine)

· Proline (synthesized from glutamic acid)

· Serine (synthesized from glucose)

· Tryosine (synthesized from phenylalanine)

Essential:

· Histidine

· Isoleucine

· Leucine

· Lysine

· Methionine

· Phenylalanine

· Threonine

· Tryptophan

· Valine

Protein in diets comes from both animal and vegetable sources. Most animal sources (meat, milk, eggs) provide what's called "complete protein", meaning that they contain all of the essential amino acids. Vegetable sources usually are low on or missing certain essential amino acids. For example, rice is low in isoleucine and lysine. However, different vegetable sources are deficient in different amino acids, and so by combining different foods you can get all of the essential amino acids throughout the course of the day. Some vegetable sources contain quite a bit of protein. Nuts, beans and soybeans are all high in protein. By combining them, you can get complete coverage of all essential amino acids.

To synthesize proteins within the body, animals need raw amino acids as building blocks. In feedstuffs, amino acids are present within the protein chains, which are then broken down during digestion by enzymes in the gastrointestinal tract. Once processed down to individual amino acids, the amino acids are absorbed into the bloodstream to be used to form new proteins. The digestive system breaks all proteins down into their amino acids so that they can enter the bloodstream. Cells then use the amino acids as building blocks to build enzymes and structural proteins.