Lectins areAnimal cellandplant cellCan synthesize and secrete proteins that can combine with sugarCell recognitionAnd adhesion reaction, mainly promoting adhesion between cells.Lectins have more than one sugar binding site, so they can participate in cell recognition and adhesion, and connect different cells.
Lectin refers to a kind of,invertebrateAnd purified from higher animalsglycoproteinOr sugar binding protein, because it can agglutinatered blood cell(containing blood group substances), so it is called agglutinin.See Table 6-1 for common types.Commonly usedPlant agglutinin(Phytoagglutin, PHA), usually named after the extracted plant, such asConcanavalin A（Conconvalina,ConA）、Wheat germ element（Wheat germ agglutinin, WGA）、Peanut agglutinin(Peanut agglutinin, PNA) andSoybean agglutinin(Soybean agglutinin, SBA). Lectins are their general names.Lectins are not products of source or involved in immune reaction. Lectins have some "affinity" characteristics and can beImmunocytochemistryMethod.Therefore, Ponder (1983) proposed that it should be called "lectin histochemistry" rather than "lectin immunohistochemistry".[1]
Although it is believed that the function of lectin in plants is to combinecell surfaceHowever, its function in animals also includes binding soluble extracellular or intracellular glycoproteins.For example, a lectin was found inMammalsOn the surface of animal liver cells, it can be specifically recognizedlactoseResidue。People believe thatAcceptorIt is responsible for removing specific glycoproteins from the circulatory system.Another example isMannose 6-phosphateThe receptor can recognize the hydrolytic enzyme containing this residue, then calibrate these proteins and send them tolysosome 。They provide many different biological functions - from the regulation of cell attachment, to glycoprotein synthesis, and the concentration of proteins in the blood.Lectins can also be identified bypathogenyFound in or unable to enter the host cellimmune systemPlay an important role in.
Plant agglutinin
Plant agglutininIt is a kind of non enzyme protein from plants that can agglutinate cells and precipitate monosaccharide or polysaccharide complexes and is not immune source.Because of itsmonosaccharideOr the specific binding ability of the sugar composition, so thatsignal transduction、immune reaction, plant defense and many other signal processes play an important role.At the same time, plant lectin has cell agglutinationantiviral, antifungal and inductionApoptosisorautophagyAnd other capabilities, solife sciencesIt has good research value and application prospect in medicine and agriculture.
Lectin refers to sugar from non immune sourcesBinding proteinorglycoproteinAnd should have the ability to make cells agglutinate or precipitate glycoconjugates.This definition includes three points: (1) Lectins are proteins orglycoprotein；(2) Lectins must have specific glycosyl binding properties, but exclude glycosyl specificantibody；(3) Because the regulation can make cellsagglutinationOr the precipitation characteristics of glycoconjugates, so the lectin molecule must have two or more sugar binding sites, so that some with sugar binding ability but only one sugar binding siteenzyme、Transporter、hormone、toxinEtc. are excluded.Lectins can recognizeglycoproteinandGlycolipidsMedium complexcarbohydrateStructure, and also recognize the glycosyl groups on the surface of cell membrane. Therefore, lectins can be used as a useful tool to study the structure of cell membrane.In addition, agglutinin can combine withfluorescein, enzymeBiotin、ferritinAnd colloidal gold without affecting itsbiological activityTherefore, lectins can also be usedImmunocytochemistryAnd the diagnosis and evaluation of tumor.Lectin protein contains at least oneDomainBased on this, lectins can be divided into four main categories: merolectins, hololectins, superlectins and chimerolectins.
The purified lectin is very important for clinical application because it can be used to identifyblood type。Some exist in humansred blood cellOnGlycolipidAnd glycoprotein can be identified by lectin.A lectin from Dolichos biforus, which can recognize A1 blood group after identification.From the plant Ulex europaeus/gorse/The agglutinin of Ulex pumila) can identify H blood group after identificationantigenAnd comes from the plant Vicia gramineae (Vicia gramineaeGramineae）The agglutinin can recognize the antigen of N blood group.[1]
The true function of lectin in plants remains to be studied, and whether it only has the function of cell attachment remains to be questioned.Lectins inseedIt shows a large amount of expression (usually purified from seeds) and decreases with plant growth, which shows thatsproutingOr the seed plays an important role in self survival.
Lectins are regarded asimmune systemAnd they still play an important role here today - lectinscomplementActivation pathway, mannose binding lectin, S, P, E lectin, etc.
"Advanced" lectins such asnatural killer cell Receptors have lower specificity for simple sugars, while for vagueoligosaccharideStructure display heightAffinity。
Lectins in legumes have been widely used asModel organismTo understand how proteins recognize the molecular basis of sugars, because they are relatively easy to obtain and have a wide range of sugars.The crystal structure of many lectins also reveals theatomeffect.
One example of the biological power of lectins is biological warfare agentsRicin。Ricin is a toxin produced by twoFunctional domainIt is composed of protein purified from castor bean.One is that one can combinecell surfaceOnGalactoseThe second functional domain is a lectin that can make ricin enter cellsRibosomal RNAInBaseExcited N-glycosidase, which will inhibit protein synthesis, and then lead to cell death.
characteristic
Labeling Technique of Lectins
Lectin has many characteristics. Here we only briefly mention its relationship withImmunocytochemistryMethods apply certain properties related to.As we know, the biofilm contains a certain amount of sugars, mainlyglycoproteinAnd glycolipids.The biggest characteristic of lectins is that they can recognize the complexcarbohydrateStructure, that is, the carbon lipid compound determinant cluster on the cell membrane surface.A lectin has specificity for a certainGlycosylThe ability to combine specificity, such asConcanavalinAndα—D-glucopyranosylmannose（α—D - Manopyranosy);Maltin combines with N-acetylglucosamine;Bean agglutinin binds to N-acetyllactamine (see Table 6-1 in this chapter).Therefore, lectin can be used as a probe to study specific glycosyl groups on cell membrane.On the other hand, lectins have multivalent binding ability, and can interact withfluorescein、Biotin, enzymecolloidal goldandferritinSo as to show the binding site at the light and/or electron microscope level.[1]
Nature and application
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application
It is generally believed that specific glycosyl groups on cell membrane can be used to distinguish cell types and reflect changes in cell differentiation, maturation and tumor cell sex change.Only in some special cases, the performance of its cell binding lectin can be estimated in advance, such as the specificity of physostigmine to blood group A substance, and the specificity of vitexin to blood group O substance 2-L-fucose. However, in most cases, the type of carbohydrate determinant recognized by the lectin, the nature and function of the molecule carrying the determinant,It is discovered completely by experimental experience.
classification
Lectins can be classified according to sugar specificity, molecular structure, binding sites and their functions.Animal lectins are divided intoc-type lectin , S-type lectin, P-type lectin, I-type lectin and Pentaxins.C-type lectin is Ca2+dependent;S-type lectin is specific recognitionβ-Lectin of galactoside bond;P-type lectin is a lectin that specifically recognizes mannose 6-phosphate;I-type lectin is similarimmunoglobulinAgglutinin;Pentraxins are lectins with five subunits.
nature
So farinvertebrateLectins found in the body areglycoprotein, sugarcovalent bondThe form is incorporated into agglutinin, and the types of sugar mainly include mannoseaminoglucose、Galactose, xylose is rare,Arabinose。The type of sugar in animal lectins is different from that in plant and microbial lectins. The protein in lectins is mainly composed of aspartic acidserineandthreonineComposition, rareSulfur-containing amino acid。The metal ions related to the activity of some lectins are usually Ca2+And Mg2+This is necessary for many sugars to bind or agglutinate.An essential condition for many lectins (such as type C lectin) to agglutinate is the presence of Ca2+。In Limulin, C-expansion+calmodulin like physiological activity is required;In Anthocidaris cr assispina, it is Ca2+Influence on molecular configuration of lectin: Ca2+The effect of oyster lectin is through changing the protein configuration, rather than directly participating inligandcombination.Some people think that Ca2+Stable structure and enhancedhydrogen bondAnd drainageGroupInteraction.
AgglutininAgglutination reactionOften bemonosaccharideInhibited, some need someDisaccharide、TrisaccharideOr polysaccharide, the sensitivity of inhibition is different.Some typical specific lectins are easily correspondingBlood group substanceSome sugars in the medium are inhibited, for example, the specific agglutinin of type A blood antigen is inhibited by N-acetyl-D-galactose;Type O specific agglutinin is L -FucoseInhibited.The specificity of lectin binding saccharides varies.The binding range of a few lectins is quite narrow.Use protease, such asTrypsin、Streptomyces proteaseIsothermal treatment and treatment of lectin can improve the sensitivity of its agglutination activity. Some additives and metal ions also affect the activity of lectin.
research meaning
Lectins exist widely in animals and plants.The biggest characteristic of agglutinin is that it can recognizeglycoproteinAnd glycolipids, especially the complexcarbohydrateStructure, that is, the glycosyl group on the surface of cell membrane.One kind of lectin hasSpecificityThe ability of glycosyl specific binding.Therefore, lectin can be used as a probe to study the structure of cell membrane.Lectins ininvertebrateThere are many kinds in the bloodbiological activity, can choose to agglutinate various cells, and has specificity for tumor cellsAgglutinationIt is important for immune defenseHumoral factorone of.On the other hand, lectins have multivalent binding ability, and can interact withfluorescein, enzymeBiotin、ferritinAnd colloidal gold without affecting its biological activity, which can be used forImmunocytochemistryResearch work, exploringcell differentiationThe biological evolution process of proliferation and malignant transformation shows thatTumor associated antigenIt has certain value in the diagnosis and evaluation of tumor.In addition,Plant agglutininIt also plays an important role in plants, such as in the process of seed germination;As a plantEmbryonic cellMitogenic factor;The protection function in crop pest control.Lectin studyingSpecificityIt is helpful to understand life phenomena or pathological changes at molecular or atomic level.
