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Lectins are carbohydrate-binding proteins, macromolecules that are highly specific for sugar moieties. Lectins should neither be confused with glycoproteins (proteins containing sugar chains or residues), lecithins (fatty substances in animals and plants), nor leptin, (the regulator of appetite and hunger, metabolism, and behavior). Long before a deeper understanding of their numerous biological functions, the plant lectins, also known as phytohemagglutinins, were noted for their particular high specificity for foreign glycoconjugates (e.g. those of fungi, invertebrates and animals). and used in biomedicine for blood cell testing and in biochemistry for fractionation.
Lectins perform recognition on the cellular and molecular level and play numerous roles in biological recognition phenomena involving cells, carbohydrates, and proteins. Lectins also mediate attachment and binding of bacteria and virus, as well as mediate the first-line defence against invading mictoorganisms with MBL, the mannan-binding lectin in the Innate immune system. It is hypothesized that some hepatitis C viral glycoproteins attach to C-type lectins on the host cell surface (liver cells) for infection. Lectins may be disabled by specific mono- and oligosaccharides, which bind to ingested lectins from grains, legume, nightshade plants and dairy; binding can prevent their attachment to the carbohydrates within the cell membrane. Some lectins may be powerful toxins as for instance ricin, and others have been incorporated into genetically engineered crops to transfer traits such as resistance to pests and resistance to herbicides.
|Table of the major lectins |
|Lectin Symbol||Lectin name||Source||ligand motif|
|Mannose binding lectins|
|ConA||Concanavalin A||Canavalia ensiformis||α-D-mannosyl and α-D-glucosyl residues|
branched α-mannosidic structures (high α-mannose type, or hybrid type and biantennary complex type N-Glycans)
|LCH||Lentil lectin||Lens culinaris||Fucosylated core region of bi- and triantennary complex type N-Glycans|
|GNA||Snowdrop lectin||Galanthus nivalis||α 1-3 and α 1-6 linked high mannose structures|
|Galactose / N-acetylgalactosamine binding lectins|
|RCA||Ricin, Ricinus communis Agglutinin, RCA120||Ricinus communis||Galβ1-4GalNAcβ1-R|
|PNA||Peanut agglutinin||Arachis hypogaea||Galβ1-3GalNAcα1-Ser/Thr (T-Antigen)|
|AIL||Jacalin||Artocarpus integrifolia||(Sia)Galβ1-3GalNAcα1-Ser/Thr (T-Antigen)|
|VVL||Hairy vetch lectin||Vicia villosa||GalNAcα-Ser/Thr (Tn-Antigen)|
|N-acetylglucosamine binding lectins|
|WGA||Wheat Germ Agglutinin, WGA||Triticum vulgaris||GlcNAcβ1-4GlcNAcβ1-4GlcNAc, Neu5Ac (sialic acid)|
|N-acetylneuraminic acid binding lectins|
|SNA||Elderberry lectin||Sambucus nigra||Neu5Acα2-6Gal(NAc)-R|
|MAL||Maackia amurensis leukoagglutinin||Maackia amurensis||Neu5Ac/Gcα2,3Galβ1,4Glc(NAc)|
|MAH||Maackia amurensis hemoagglutinin||Maackia amurensis||Neu5Ac/Gcα2,3Galβ1,3(Neu5Acα2,6)GalNac|
|Fucose binding lectins|
|UEA||Ulex europaeus agglutinin||Ulex europaeus||Fucα1-2Gal-R|
|AAL||Aleuria aurantia lectin||Aleuria aurantia||Fucα1-2Galβ1-4(Fucα1-3/4)Galβ1-4GlcNAc,|
The name "lectin" is derived from the Latin word legere, meaning, among other things, "to select".(lek'tin)
Although they were first discovered more than 100 years ago in plants, lectins are now known to be present throughout nature. It is generally believed that the earliest description of a lectin was given by Peter Hermann Stillmark in his doctoral thesis presented in 1888 to the University of Dorpat. Stillmark isolated ricin, an extremely toxic hemagglutinin, from seeds of the castor plant (Ricinus communis). The first lectin to be purified on a large scale and available on a commercial basis was concanavalin A, which is now the most-used lectin for characterization and purification of sugar-containing molecules and cellular structures. The legume lectins are probably the most well-studied lectins.
Most lectins do not possess enzymatic activity and are not produced naturally by the immune system. Lectins occur ubiquitously in nature. They may bind to a soluble carbohydrate or to a carbohydrate moiety that is a part of a glycoprotein or glycolipid. They typically agglutinate certain animal cells and/or precipitate glycoconjugates.
Lectins serve many different biological functions in animals, from the regulation of cell adhesion to glycoprotein synthesis and the control of protein levels in the blood. They may also bind soluble extracellular and intercellular glycoproteins. Some lectins are found on the surface of mammalian liver cells that specifically recognize galactose residues. It is believed that these cell-surface receptors are responsible for the removal of certain glycoproteins from the circulatory system. Another lectin is a receptor that recognizes hydrolytic enzymes containing mannose-6-phosphate, and targets these proteins for delivery to the lysosomes. I-cell disease is one type of defect in this particular system. Lectins are also known to play important roles in the immune system by recognizing carbohydrates that are found exclusively on pathogens, or that are inaccessible on host cells. Examples are the lectin complement activation pathway and mannose-binding lectin.
The function of lectins in plants (legume lectin) is still uncertain. Once thought to be necessary for rhizobia binding, this proposed function was ruled out through lectin-knockout transgene studies.
The large concentration of lectins in plant seeds decreases with growth, and suggests a role in plant germination and perhaps in the seed's survival itself. The binding of glycoproteins on the surface of parasitic cells is also believed to be a function. Several plant lectins have been found to recognise non-carbohydrate ligands that are primarily hydrophobic in nature, including adenine, auxins, cytokinin, and indole acetic acid, as well as water-soluble porphyrins. It has been suggested that these interactions may be physiologically relevant, since some of these molecules function as phytohormones. are another major family of protein ANCs, which are specific sugar-binding proteins exhibiting reversible carbohydrate-binding activities.
Lectins are similar to antibodies in their ability to agglutinate red blood cells; however, lectins are not immune system products. The toxicity of lectins has been identified by consumption of food with high lectin content, which can lead to diarrhea, nausea, bloating, vomiting, even death (as from ricin). Many legume seeds have been proven to contain high lectin activity, termed hemagglutinating activity. Soybean is the most important grain legume crop, the seeds of which contain high activity of soybean lectins (soybean agglutinin or SBA). SBA is able to disrupt small intestinal metabolism and damage small intestinal villi via the ability of lectins to bind with brush border surfaces in the distal part of small intestine. Heat processing can reduce the toxicity of lectins, but low temperature or insufficient cooking may not completely eliminate their toxicity, as some plant lectins are resistant to heat. (It is believed that undercooking red kidney beans increases toxicity.) In addition, lectins can result in irritation and over-secretion of mucus in the intestines, causing impaired absorptive capacity of the intestinal wall.
Purified lectins are important in a clinical setting because they are used for blood typing. Some of the glycolipids and glycoproteins on an individual's red blood cells can be identified by lectins.
A lectin (BanLec) from bananas inhibits HIV-1 in vitro. Achylectins, isolated from Tachypleus tridentatus, show specific agglutinating activity against human A-type erythrocytes. Anti-B agglutinins such as anti-BCJ and anti-BLD separated from Charybdis japonica and Lymantria dispar, respectively, are of value both in routine blood grouping and research.
Lectins from legume plants, such as PHA or concanavalin A, have been widely used as model systems to understand the molecular basis of how proteins recognize carbohydrates, because they are relatively easy to obtain and have a wide variety of sugar specificities. The many crystal structures of legume lectins have led to a detailed insight of the atomic interactions between carbohydrates and proteins.
In general, proteins may be characterized with respect to glycoforms and carbohydrate structure by means of affinity chromatography, blotting, affinity electrophoresis and affinity immunoelectrophoreis with lectins as well as in microarrays, as in evanescent-field fluorescence-assisted lectin microarray.
One example of the powerful biological attributes of lectins is the biochemical warfare agent ricin. The protein ricin is isolated from seeds of the castor oil plant and comprises two protein domains. Abrin from the jequirity pea is similar:
Foods with high concentrations of lectins, such as beans, cereal grains, seeds, nuts, and potatoes, may be harmful if consumed in excess in uncooked or improperly cooked form. Adverse effects may include nutritional deficiencies, and immune (allergic) reactions. Possibly, most effects of lectins are due to gastrointestinal distress through interaction of the lectins with the gut epithelial cells. A recent in vitro study has suggested that the mechanism of lectin damage may occur by interfering with the repair of already-damaged epithelial cells.
Lectin may cause leptin resistance, affecting its functions (signal have high levels of leptin and several effects gathering to protect from lipid overload), as indicated by studies on effects of single nucleotide polymorphisms on the function of leptin and the leptin receptor. Such leptin resistance may translate into diseases, notably it could be responsible for obesity in humans who have high levels of leptin.