| Lectins - plant and animal carbohydrate-binding proteins
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| Lectins are defined as proteins that do not have enzymatic activity but that reversibly bind monosaccharides and oligosaccharides with high specificity. They can be present either in the soluble fraction of cells or in the membranes. Lectins were first discovered in the seeds of various plants because extracts of these seeds caused the agglutination of specific types of red blood cells (hemagglutination reaction). This activity was later shown to result from the interaction of the lectin with a glycan on the surfaces of the red blood cells. Some of the well-known plant lectins are Concanavalin A which recognizes mannose and specifically binds high-mannose types of oligosaccharides, phytohemagglutinin which recognizes galactose and GalNAc and binds to complex types of glycans such as those on brush border membranes, and peanut lectin which also recognizes galactose and binds to complex oligosaccharides.
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| The function of plant lectins are unknown, but some of them such as ricin are highly toxic to animal cells as well as to insects (see box). In fact, one function that has been
proposed for plant lectins is that they provide a defense for the plant against predators or against bacterial or fungal pathogens. Plant lectins have been very useful to researchers in many areas of biology since they provide valuable tools for the isolation and/or detection of specific carbohydrate structures as well as glycoproteins (via their specific carbohydrate structure) or glycolipids. They have been used for blood typing because of their ability to distinguish carbohydrate determinants in human blood cells, and for histochemical studies in lectin-blotting assays, as well as for chromatographic separations of various structures.
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| Carbohydrate-binding proteins are also present in animal cells, as well as in the microbial world. In animal cells, they were first noticed for their role in reaggregation of dissociated sponge (algae) cells, and this phenomenon was proposed to involve cell-cell recognition. Sometime later, lectins were proposed to be involved in homing of cells such as leukocytes to different internal organs (see box). The first direct evidence for lectins in mammalian cells was the isolation of a receptor protein from hepatocytes that bound circulating blood glycoproteins that had either lost or had the terminal sialic acids removed from their N-linked oligosaccharides. This 'lectin' was referred to as the asialoglycoprotein receptor and was shown to be a galactose-binding lectin.
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| Animal lectins were originally classified according to the carbohydrate structures that they recognized. However, now, with the potential for molecular cloning of proteins, they can be classified on the basis of amino acid sequence homologies and their relation by evolutionary classification. Thus the family of C-type lectins include some 20 members that share a common lectin sequence motif and require Ca++ for binding but have a variable carbohydrate recognition domain. On the other hand the S-type lectin family all recognize β-galactoside structures and do not require Ca++ for binding. Other families of lectins include P-types, I-types, and so on.
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Several of the animal lectins have been crystallized, permitting study of the molecular interactions of protein and carbohydrate at the level of atomic resolution. Several principles have evolved concerning the N and O-glycans and their lectins. First, the binding sites are generally of low affinity and occur in shallow indentations at the protein surface. A second point is that selectivity or specificity is achieved by a combination of hydrogen bonding, van der Waals forces and hydrophobic interactions between the sugars and the side chains on the amino acids . Finally, the region of contact between the carbohydrate and the protein usually involves only one to three monosaccharide residues. Although the binding sites are generally of low affinity, they are quite specific in terms of carbohydrate structures recognized.
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| An important example of carbohydrate-dependent cell-cell interactions occurs during inflammation. Injury to vascular endothelial cells elicits an inflammatory response that causes the release of cytokines (proteins affecting cell migration) from the injured tissue. The cytokines attract leukocytes to the site of the injury or infection to remove the invading organisms or damaged tissue. These leukocytes must be able to exit from the blood flow and attach to the injured tissue. They are able to do this because they have a tetrasaccharide, known as sialyl Lewis-X antigen, as a component of a membrane glycoprotein or glycolipid. The sialyl Lewis-X antigen is recognized by a lectin, E-selectin, that is present on the surface of the endothelial cells. The interaction between E-selectin and the sialyl Lewis-X antigen enables the leukocytes to adhere to the vascular wall even under the shear forces of the circulation. Figure 25.14 presents a model demonstrating the chemistry of this important interaction. Selectins mediate the initial adhesive step, which is described as tethering, then a 'rolling' of leukocytes along the endothelial cell surface. In fact, leukocytes also contain a selectin, l-selectin, that probably interacts with a saccharide structure on the endothelial cells. These weak binding interactions enable leukocytes to penetrate the interstitial layer and clean up the site of injury. (See also Chapter 36.) |
| While adherence of leukocytes to endothelial cells is important in fighting infection, it can be dangerous. In coronary disease, the leukocytes contribute to the development of atherosclerotic plaque, leading to ischemia (see also Chapter 17). Because of the significance of this interaction, many laboratories are seeking to develop novel chemicals, known as glycomimetics, that mimic the sialyl Lewis-X structure. Administration of these drugs to patients who have suffered a heart attack should theoretically block the selectin sites, inhibiting the binding of leukocytes to the vascular wall and diminishing the probability of further ischemia. |
| The S-type lectins, recently termed galectins, represent a family of proteins that bind β-galactoside terminal glycoconjugates, and within the family they share structural homology
in their carbohydrate recognition domains (CRD). Galectins are widely distributed throughout the animal kingdom and play a key role in many cell-cell interactions. Some galectins can induce apoptosis or programmed cell death; others induce metabolic changes such as cellular activation and mitosis.
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