Lectins are proteins of non-immune origin that recognize and reversibly bind to carbohydrates and glycoconjugates. They may be free or cell-bound components located extracellularly and occur widely in animals, plants, and microorganisms. The ability of lectins to agglutinate cells distinguishes them from other large molecules that can bind to carbohydrates. And, unlike anti-carbohydrate immunoglobulins that agglutinate cells, lectins are not products of an immune response. In general, lectins are oligomeric proteins with different structure and molecular size. They differ in primary, secondary, and tertiary structure, number and assembly of subunits, metal requirements, and the constitution of carbohydrate-binding sites. Carbohydrate recognition domain (CRD) is one of the molecular domains in the lectin polypeptide chain. The three-dimensional structure of CRD determines the specificity of lectins to carbohydrates. CRD interacts with monosaccharides or oligosaccharides through noncovalent bonds involving hydrogen bonds, van der Waals, and hydrophobic interactions with reversibility, high specificity, and no catalytic or immune activity.
According to the overall structures, lectins can be divided into merolectins, holoectins, chimerolectins and superlectins. Lectins can also be classified into five specificity groups according to the monosaccharides that bind to them with the highest affinity: mannose, galactose/N-acetylgalactosamine, N-acetylglucosamine, fucose, and N-acetylneuraminic acid. Specificity families of lectins have a conserved amino acid profile. These molecules exhibit a high similarity in their residues including those involved in binding to monosaccharides, most of which coordinate the metal ions essential for the integrity of subunits and the correct positioning of the residues. Some lectins specific for mannose also bind to glucose, but only in rare cases they react with galactose. Lectins of the same specificity group may differ markedly in affinity for oligosaccharides, and a few of them recognize only oligosaccharide derivatives of the above monosaccharides. The affinity of lectins to monosaccharides is generally weak with association constants in the millimolar range, whereas that to oligosaccharides is much higher, up to three orders of magnitude.
Lectins are invaluable tools for detection, isolation, and structural and functional investigation of complex carbohydrates and glycoconjugates in cells, tissue sections, and biological fluids for diagnosis, and pharmacological and therapeutic applications. In immobilized form, lectins are covalently bound to Sepharose for purification and isolation of glycoproteins, glycopeptides, and oligosaccharides by affinity chromatography. Lectins can also be derivatized with fluorescent dyes or enzymes for the detection of glycoconjugates in tissue sections and on cells, as well as on and in subcellular organelles. Thus, changes occurring on cell surfaces and in cells can be examined by lectin-based histochemical and cytochemical reagents. Amerigo Scientific offers a variety of unconjugated or conjugated lectins for harvesting valuable information about glycans and their involvement in various physiological and pathological processes.
.
