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Protein folding in the endoplasmic reticulum Protein synthesis
Although many lectins are destroyed by normal cooking (which is why grains and beans are edible), many are not. Relative resistance to lectins was pan of the classic description of wheat germ agglutinin (WGA) made by Aub in 1963. WGA as Freed points out is in fact one of the more heat sensitive lectins, being destroyed after 15 minutes at 75 degrees C, whereas other wheat lectins in gluten and gliandin resist autoclaving at 110 degrees C for 30 minutes. Gibbons and Dankers noted that in over 100 food plants found to contain active lectins, seven were autoclave resistant (apple, carrot, wheat bran, canned corn, pumpkin seeds, banana and wheat flour). Nachbar and Oppenheim also noted high levels of lectin activity in dry roasted peanuts, , , and Kellogg's . The banana agglutinin was actually enhanced by heating, and was inhibitable by n-acetyl glucosamine (NAG) and N-acetylgalactosamine (blood group A antigen) glycoproteins. Phytohemagglutinins from kidney beans can resist mild cooking and retain lectin activity even at 90 degrees C for 3 hours. Pre-soaking the beans however resulted in complete loss of lectin activity. Several investigators noted year-to-year and batch-to-batch variations in the lectin content of foods, so the occasional lectin is likely to occur even with foods normally considered safe.
Lectins are apparently most widely distributed in plants, where they were found in almost 1000 plants of some 3000 examined in recent years. They are particularly abundant in legumes and they account for between 1.5 and 3 percent of the total protein content of soy and jack beans. The second most common source of lectins are seafood.
Protein synthesis inhibitor - Revolvy
The binding of lectins to sugar is quite weak. It does not form a covalent bond, but is reversible, like enzyme-substrate or antigen-antibody reactions. Lectin-sugar reactions actually share many factors in common with antigen-antibody reactions, especially precipitation, which has prompted several investigators to suggest that lectins are plant antibodies. However this has been tempered by several major differences between the two. Antibodies are made by higher organisms which have specific immunologic organs. Lectins are present as constituent proteins. Second, antibodies are all structurally similar to one another, whereas lectins are structurally diverse; examination of the amino acid sequence, molecular size and other molecular properties show that lectins have little in common other than they are all proteins. For example soybean agglutinin is a glycoprotein with no di-sulphide bond; its molecular weight is 120,000, It consists of four subunits and has two binding sites. Wheat germ agglutinin is not a glycoprotein and is rich in di-sulphide bonds with a molecular weight of 36,000, It has two identical subunits and four binding sites for sugars.
Lectins which are especially rich in di-sulphide bonds such as WGA are very resistant to proteolytic enzymes, detergents, urea, alkalis and acids. Foodstuffs are naturally rich in fiber Important cause of allergies. Dietary lectins also stimulate mast cells which can degranulate and release stored histamine, leading several researchers to ascribe a role for dietary lectins in the genesis of food allergy. However it is not generally known why some individuals become sensitized to food in their diets. In an attempt to clarify this, coeliac disease has been extensively studied, since patients with this disease usually normalize when placed on a gluten free diet. Researchers reported that the mucous membranes of coeliac patients showed sugar residues which were capable of binding to the lectins in wheat germ, which resulted In a cytoxic reaction. Rats treated with Concavallin-A or wheat germ lectin developed a gut membrane that was paradoxically impermeable to small molecules, but very permeable to large, highly allergenic molecules, a situation which is mimicked in food allergies and coeliac disease.
would have protein synthesis inhibited if ..
Mammalian mannose-binding proteins (MBP) ' were first isolated from the serum of rabbits (1), and subsequently were found in the liver and serum ofhumans (2-5) and rodents (5-8). Analysis of the encoded human MBP, like its two rat homologues (9), reveals that the protein is divided into three domains: a cysteine-rich NH2-terminal domain which stabilizes the a helix of the second collagen-like domain, and a third COOH-terminal carbohydrate-binding region (10). MBP may have a role in host defense; this is suggested first by its ability to bind high mannose glycans, which are present in the cell walls of many pathogens, including some Gramnegative bacteria (11-13), mycobacteria (14), yeasts and fungi, certain parasites (15), and envelope glycoproteins of certain viruses, such as the human immunodeficiency virus (HIV) (16, 17). Second, the synthesis of human MBP appears to be stress induced, as it is an acute-phase protein (10). Third, human MBP is a member of a family of homologous lectin-like proteins (18) that includes proteins found in the coelomic fluid of sea urchins (19) and in the hemolymph of Sacraphaga perigintaa (20), which may have a role in the host defense of these organisms.
Nachbar tested 88 common food items and reported erythrocyte agglutination activity in 38. Many foods showed agglutinating activity so substantial that the extracts could be diluted several fold. Crude extracts of various foods tomato, lettuce, cucumber, wheat bran and whole wheat, sesame and sunflower seeds, vanilla yogurt, coconut, banana and baby food banana, carrot, onion, apple, alfalfa and soya protein have also been found to bind, and in some instances precipitate the components of human saliva, including cellular debris and bacteria. This may have some significance in the development of caries. Interestingly, avocadoe lectin inhibited the sucrose dependent adherance of S. mutans to plaque pellicle. Approximately 1 to 5% of the ingested dietary lectins are absorbed into the blood stream. Here they can clump and bind to red and black blood cells, destroying them. It has been proposed that much of the low grade anemias seen In the third world may be resulting from destruction of red blood cells by lectin rich grain and bean diets.
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lectin 1 could inhibit protein synthesis?
Until recently, it was not recognized that nature could employ sugars for the synthesis of highly specific compounds that can act as carriers of biologic information. Monosaccharides can serve as "letters" in a vocabulary of biologic specificity, where the words are formed by variations In the nature of the sugars present, the type of linkage, and the presence or absence of branch points. The first proof that sugars could serve as specificity determinants came from the discovery that influenza virus could agglutinate red cells only In the presence of the membrane bound sialic acids. It these were removed, the virus no longer binds to the cell. Removal of sialic acid exposes the terminal underlying galactose unit and results in the rapid clearance of the treated cells from the bloodstream. Sugars on cell surfaces also seem to determine the distribution of the circulating cells within the body. Radioactively treated rat lymphocytes will migrate to the spleen when re-injected into the animal. However if the sugar fucose is removed from the surface of the cells before reintroduction, the cells migrated to the liver instead, as if "the fucose served as a ZIP code- directing the calls where to go." It was not until 1953 that Morgan and Watkins demonstrated that the specificity of the ABO blood group-system was determined by sugars. For example, the difference between blood types A and B lies in a simple sugar unit that sticks out from the end of a carbohydrate chain of a glycoprotein or glycolipid. In blood A the determinant is acetylgalactosamine and in group B it is galactose.
Ricin inhibition of in vivo protein synthesis in ..
The first lectin to be purified was concanavallin-A, isolated from the jack bean. In 1936 Sumner and Howell noted that the addition of Con-A to a solution of glycogen caused the sugar to precipitate, and that the agglutination of red cells by this lectin was inhibited by cane sugar. They suggested that the hemagglutination by Con-A might be the consequence of a reaction between the protein with carbohydrates on the surface of the red cells. In other words lectins bind sugars, and they agglutinate cells by means of this binding. For example the agglutination of red cells by Con-A specifically inhibited by the sugars mannose or glucose, Indicating that Con-A binds mannose and glucose on the cell surface. It was soon discovered that lectins not only agglutinate red blood cells, but also other kinds of cells including lymphocytes, spermatozoa, bacteria and fungii.
Unfolded protein response - Wikipedia
Since blastogenesis can also occur in suppressor -T cell populations, it is quite feasible that significant suppression of graft versus host responses in tissue transplants can be accomplished by the use of lectins. Significant studies are now under way at Stanford University showing that lectins can be used exclusively to maintain transplants in animals for up to two years. Lentil lectin induces striking transplant tolerance In both mice and humans. Peanut agglutinin has been used to isolate suppressor T-cells in vivo, these having been first Induced by Con-A. Tomato lectin has been shown to inhibit the transformation of peripheral lymphocytes challenged by recall antigens, and actually suppressed spontaneous DNA synthesis. The inhibition of lymphocyte transformation was not stopped by exogenously added Interleukin 1 and/or Interleukin 2, even at extremely high concentrations. This could be significant as the average American diet results in the ingestion of at least 200 mg. of tomato lectin annually, with vegetarians probably ingesting a far greater amount.
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