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Lectins arecarbohydrate-bindingproteins that are highly specific for sugargroups that are part of other molecules, so causeagglutination of particular cells or precipitation ofglycoconjugates andpolysaccharides. Lectins have a role in recognition at the cellular and molecular level and play numerous roles in biological recognition phenomena involving cells, carbohydrates, and proteins.[1][2] Lectins also mediate attachment and binding ofbacteria,viruses, and fungi to their intended targets.
Lectins are found in many foods. Some foods, such as beans and grains, need to be cooked, fermented or sprouted to reduce lectin content. Some lectins are beneficial, such asCLEC11A, which promotes bone growth, while others may be powerfultoxins such asricin.[3]
Lectins may be disabled by specificmono- andoligosaccharides, which bind to ingested lectins from grains, legumes,nightshade plants, and dairy; binding can prevent their attachment to the carbohydrates within the cell membrane. The selectivity of lectins means that they are useful for analyzingblood type, and they have been researched for potential use ingenetically engineered crops to transfer pest resistance.
| Table of the major plant lectins[4] | |||||
|---|---|---|---|---|---|
| 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 communisagglutinin, RCA120 | Ricinus communis | Galβ1-4GalNAcβ1-R | ||
| PNA | Peanut agglutinin | Arachis hypogaea | Galβ1-3GalNAcα1-Ser/Thr (T-Antigen) | ||
| AIL | Jacalin | Artocarpus integrifolius | (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 | 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, R2-GlcNAcβ1-4(Fucα1-6)GlcNAc-R1 | ||
William C. Boyd alone and then together with Elizabeth Shapleigh[5] introduced the term "lectin" in 1954 from the Latin wordlectus, "chosen" (from the verblegere, to choose or pick out).[6]
Lectins maybind to a soluble carbohydrate or to a carbohydratemoiety that is a part of aglycoprotein orglycolipid. They typicallyagglutinate certain animal cells and/or precipitateglycoconjugates. Most lectins do not possessenzymatic activity.
Lectins have these functions in animals:
The function of lectins in plants (legume lectin) is still uncertain. Once thought to be necessary forrhizobia binding, this proposed function was ruled out through lectin-knockouttransgene studies.[9]
The large concentration of lectins in plant seeds decreases with growth, and suggests a role in plantgermination and perhaps in the seed's survival itself. The binding of glycoproteins on the surface of parasitic cells also is believed to be a function. Several plant lectins have been found to recognize noncarbohydrateligands that are primarilyhydrophobic in nature, includingadenine,auxins,cytokinin, andindole acetic acid, as well as water-solubleporphyrins. These interactions may be physiologically relevant, since some of these molecules function asphytohormones.[10]
Lectin receptor kinases (LecRKs) are believed to recognize damage associated molecular patterns (DAMPs), which are created or released from herbivore attack.[citation needed] InArabidopsis, legume-type LecRKs Clade 1 has 11 LecRK proteins. LecRK-1.8 has been reported to recognize extracellularNAD molecules and LecRK-1.9 has been reported to recognize extracellularATP molecules.[citation needed]
Extraction of proteins and lectins can be extracted via similar processes, also with their analysis, and discovery. For examplecottonseed contains compounds of interest within the studies of extraction and purification of proteins[11]
Somehepatitis C viral glycoproteins may attach toC-type lectins on the host cell surface (liver cells) to initiate infection.[12] To avoid clearance from the body by theinnate immune system, pathogens (e.g.,virus particles andbacteria that infect human cells) often express surface lectins known asadhesins andhemagglutinins that bind to tissue-specificglycans on host cell-surface glycoproteins andglycolipids.[13] Multiple viruses, includinginfluenza and several viruses in theParamyxoviridae family, use this mechanism to bind and gain entry to target cells.[14]
Purified lectins are important in a clinical setting because they are used forblood typing.[15] Some of the glycolipids and glycoproteins on an individual's red blood cells can be identified by lectins.
Non blood-group antigens can be identified by lectins:
In neuroscience, theanterograde labeling method is used to trace the path ofefferentaxons withPHA-L, a lectin from thekidney bean.[16]
A lectin (BanLec) from bananas inhibitsHIV-1in vitro.[17] Achylectins, isolated fromTachypleus tridentatus, show specific agglutinating activity against human A-type erythrocytes. Anti-B agglutinins such as anti-BCJ and anti-BLD separated fromCharybdis japonica andLymantria dispar, respectively, are of value both in routine blood grouping and research.[18]
Lectins from legume plants, such asPHA orconcanavalin A, have been used widely 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 manycrystal structures of legume lectins have led to a detailed insight of the atomic interactions between carbohydrates and proteins.
Legume seed lectins have been studied for their insecticidal potential and have shown harmful effects for the development of pest.[19]
Concanavalin A and other commercially available lectins have been used widely inaffinity chromatography for purifying glycoproteins.[20]
In general, proteins may be characterized with respect toglycoforms and carbohydrate structure by means ofaffinity chromatography,blotting,affinity electrophoresis, andaffinity immunoelectrophoreis with lectins, as well as inmicroarrays, as inevanescent-field fluorescence-assisted lectin microarray.[21]
One example of the powerful biological attributes of lectins is the biochemical warfare agent ricin. The protein ricin is isolated from seeds of thecastor oil plant and comprises twoprotein domains.Abrin from thejequirity pea is similar:
Lectins are widespread in nature, and many foods contain the proteins. Some lectins can be harmful if poorly cooked or consumed in great quantities. They are most potent when raw as boiling, stewing or soaking in water for several hours can render most lectins inactive. Cooking raw beans at low heat, though, such as in aslow cooker, will not remove all the lectins.[22]
Some studies have found that lectins may interfere with absorption of some minerals, such ascalcium,iron,phosphorus, andzinc. The binding of lectins to cells in the digestive tract may disrupt the breakdown and absorption of some nutrients, and as they bind to cells for long periods of time, some theories hold that they may play a role in certaininflammatory conditions such asrheumatoid arthritis and type 1diabetes, but research supporting claims of long-term health effects in humans is limited and most existing studies have focused on developing countries where malnutrition may be a factor, or dietary choices are otherwise limited.[22]
The first writer to advocate a lectin-free diet was Peter J. D'Adamo, aNaturopath best known for promoting theBlood type diet. He argued that lectins may damage a person's blood type by interfering with digestion, food metabolism, hormones, insulin production—and so should be avoided.[23] D'Adamo provided no scientific evidence nor published data for his claims, and his diet has been criticized for making inaccurate statements about biochemistry.[23][24]
Steven Gundry proposed a lectin-free diet in his bookThe Plant Paradox (2017). It excludes a large range of commonplace foods includingwhole grains, legumes, and most fruit, as well as thenightshade vegetables: tomatoes, potatoes, eggplant, bell peppers, and chili peppers.[25][26] Gundry's claims about lectins are consideredpseudoscience. His book cites studies that have nothing to do with lectins, and some that show—contrary to his own recommendations—that avoiding the whole grainswheat,barley, andrye will allow increase of harmful bacteria while diminishing helpful bacteria.[27][28][29]
Lectins are one of many toxic constituents of many raw plants that are inactivated by proper processing and preparation (e.g., cooking with heat, fermentation).[30] For example, rawkidney beans naturally contain toxic levels of lectin (e.g.phytohaemagglutinin). Adverse effects may includenutritional deficiencies, andimmune (allergic) reactions.[31]
Lectins are considered a major family of proteinantinutrients, which are specific sugar-binding proteins exhibiting reversible carbohydrate-binding activities.[32] Lectins are similar toantibodies in their ability to agglutinate red blood cells.[33]
Many legume seeds have been proven to contain high lectin activity, termedhemagglutination.[34]Soybean is the most important grain legume crop in this category. Its seeds contain high activity of soybean lectins (soybean agglutinin or SBA).
Long before a deeper understanding of their numerous biological functions, the plant lectins, also known asphytohemagglutinins, were noted for their particularly high specificity for foreignglycoconjugates (e.g., those offungi and animals)[35] and used in biomedicine for blood cell testing and in biochemistry forfractionation.[citation needed]
Although they were first discovered more than 100 years ago in plants, now lectins are known to be present throughout nature. The earliest description of a lectin is believed to have been given byPeter Hermann Stillmark in his doctoral thesis presented in 1888 to theUniversity 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 wasconcanavalin A, which is now the most-used lectin for characterization and purification of sugar-containing molecules and cellular structures.[36] Thelegume lectins are probably the most well-studied lectins.