Lectins are carbohydrate-binding proteins that are highly specific for the glycosyl groups of other molecules. Lectins have a recognition at the cellular and molecular levels and play many roles in the biorecognition of cells, carbohydrates and proteins. Lectins also mediate the attachment and binding of bacteria and viruses to their intended targets. Lectins are ubiquitous in nature and found in many foods. Some foods (such as beans and grains) need to be cooked or fermented to reduce the lectin content. Some lectins are beneficial, such as CLEC11A, which promotes bone growth, while other lectins may be potent toxins, such as ricin. Specific monosaccharides and oligosaccharides may destroy lectins, which bind to lectins in grains, legumes, nightshades, and dairy products. Binding prevents them from attaching to carbohydrates in the cell membrane. The selectivity of lectins means that they can be used to analyze blood types and have been studied for transfer of pest resistance in transgenic crops.
Figure 1. Structure of lateral hemagglutinine.
Lectins are ubiquitous in nature. They can bind to a soluble sugar or a carbohydrate moiety of a glycoprotein or a portion of a glycolipid. They typically agglutinate certain animal cells and/or precipitated glycoconjugates. Most lectins do not have enzymatic activity.
Lectins have these functions in animals:
The function of lectins in plants (legume lectins) remains uncertain, as shown in Figure 2. Once thought to be essential for Rhizobium binding, the proposed function is ruled out by lectin knockout transgenic studies. High concentrations of lectins in plant seeds decrease with growth, suggesting their role in plant germination and seed survival. The binding of glycoproteins on the surface of parasitic cells is also considered a function. Several plant lectins have been found to be primarily hydrophobic, non-carbohydrate ligands including adenine, auxin, cytokinin and indole acetic acid, and water-soluble porphyrins. These interactions may be physiologically related because some of them act as plant hormones.
Figure 2. Structure of the monosaccharide binding site of lentil lectin.
Bacteria and viruses
Some hepatitis C virus glycoproteins may attach to the C-type lectin on the surface of the host cell (hepatocytes) to initiate infection. In order to prevent the innate immune system from clearing the virus from the body, pathogens (such as virus particles and bacteria that infect human cells) usually express surface lectins, called adhesins and hemagglutinins, which bind to tissue-specific glycans on host cell-surface glycoproteins and glycolipids.
As a biochemical tool
Concanavalin A and other commercially available lectins have been widely used in affinity chromatography to purify glycoproteins. Typically, affinity chromatography, blotting, affinity electrophoresis, and affinity immunoelectrophoresis with lectin, as well as in microarrays, such as in field-field fluorescence-assisted lectin microarrays, for glycoforms and carbohydrates Structure to characterize proteins.
In biochemical warfare
An example of the powerful biological properties of lectins is the biochemical warfare agent ricin. The ricin is isolated from the seeds of the castor oil plant and contains two protein domains. A similar protein of pea: one domain is a lectin that binds to the galactosyl residue on the cell surface and allows the protein to enter the cell. The second domain is an N-glycosidase that cleaves nucleobases from ribosomal RNA, thereby inhibiting protein synthesis and cell death.
1. Maverakis E.; et al. Glycans in the immune system and The Altered Glycan Theory of Autoimmunity. J Autoimmun. 2015,57 (6): 1-13.
2. Oldroyd, Giles E.D.; et al. Coordinating Nodule Morphogenesis with Rhizobial Infection in Legumes. Annual Review of Plant Biology. 2008, 59: 519-46.