Structural Biochemistry/Carbohydrates/Lectins

Introduction
Lectins are proteins that recognize and bind specific carbohydrates found on the surfaces of cells. They play a role in interactions and communication between cells typically for recognition. Carbohydrates on the surface of one cell bind to the binding sites of lectins on the surface of another cell. Binding results from numerous weak interactions which come together to form a strong attraction. A lectin usually contains two or more binding sites for carbohydrate units. In addition, the carbohydrate-binding specificity of a certain lectin is determined by the amino acid residues that bind the carbohydrate. Lectins are specific carbohydrate-binding proteins: - Enormous diversity of carbohydrates have biological significance: Different monosaccharides can be joined to one another through any several -OH groups. Extensive branching is possible. Many more different oligosaccharides can be formed from 4sugars than oligopeptides from 4 amino acids - Lectins promote interactions between cells: Lectin is to facilitate cell-cell contact Lectin and carbohydrates are linked by a number of weak non-covalent interactions C-type(calcium required): calcium ion on the protein acts a bridge between protein and sugar through direct interactions with sugar -OH groups Carbohydrates-binding specificity of a particular lectin is determined by the amino acid residues that bind the carbohydrates. - Influenza virus binds to Sialic acid residues: Influenza virus recognizes sialic acid residues linked to galactose residues that are present on cell-surface glycoproteins. These carbohydrates are bound to hemagglutinin, a viral protein (virus is engulfed by the cell and starts to replicate). Neuraminidases are enzymes that cleave the glycosidic bond to sialic acid residues of hemagglutin, releasing the virus to infect new cells and spreading the infection.

Lectin Binding
Lectins are capable of binding to many different types of carbohydrates. Because of this capability, the way that a lectin binds to carbohydrates, the materials necessary for binding, and the strength of the bond varies. Some of the various forms of binding are discussed below.

-Monosaccharides and disaccharides have shallow grooves to which lectins bind, making the affinity of the bond low. Because of the difficulty that lectins face when binding to these carbohydrates, a subsite multivalency (which is a spatial extension of the grooves) is necessary to achieve binding. This extension makes it so that the contact site on the carbohydrate is embedded into a more complex contact region. This type of binding works most efficiently with small lectins, as evidenced by the lectin, hevein, which is only 43 amino acids long. Rapid binding kinetics also facilitate the binding of lectins to carbohydrates. An example of this is the binding of sialyl Lewisx (a tetrasaccharide) to P-selectin. Rapid binding kinetics allows for spatial complementarity to be reached between a low-energy conformation of the carbohydrate and the prearranged binding site of the lectin.

-The shape of the binding sites in carbohydrates plays a factor in its bondage to lectins. An example of this is the case of galectin-1 binding to ganglioside GM1 (a pentasaccharide). Nuclear Magnetic Resonance and other molecular modeling techniques were used to analyze the bond between these two molecules. The images found showed that two branches of the carbohydrate are bonded to the lectin. The α2, 3-sialylgalactose linkage is able to adopt three different, low-energy conformers. One of these conformers is energetically favorable for the binding of galectin- to ganglioside GM1. This process is evidence that lectins prefer certain conformations (shapes) when deciding how to bind to a carbohydrate. This evidence shows that oligosaccharides have limited flexibility. This limited flexibility makes oligosaccharides very favorable ligands, seeing as they avoid entropic penalties.

-Core substitutions have been found to occur in N-glycans. These substitutions are added to specific positions on the carbohydrate during its course to being assembled. These substitutions have been found to prominently affect the properties of glycans. The glycan properties are so affected, that they do not even need to be in the presence of lectins in order to be noticed. These substitutions, resulting in changes of certain parts of the carbohydrate, act as molecular switches governing the shape of glycans.

-Branching also introduces molecular switches. This property is most exemplified in the glycoside cluster effect. Enhancing the numerical valency of a molecule results in an increase in affinity. The type of branching appears to have a significant effect on this increase in affinity.

Importance of Carbohydrates in Cell Communication
Carbohydrates contain abundant information as a result of the various composition and structures that are possible. These diverse compounds result from the many OH groups available for linkage, which further allow for extensive branching. Additionally, the substituent attached to the anomeric carbon can assume either an alpha or beta configuration. The presence of these various carbohydrates on cell surfaces allows for effective cell-to-cell communication.

Functions of Lectins
Lectins are known to be very widespread in nature. They can bind to soluble carbohydrates or carbohydrate functional groups that are a part of a glycoprotein or glycolipid. Lectins typically bind these carbohydrates with certain animal cells and sometimes results in glycoconjugate precipitation.

In animals, lectins regulate the cell adhesion to glycoprotein synthesis, control protein levels in blood, and bind soluble extracellular and intracellular glycoproteins. Also, in the immune system, lectins recognize carbohydrates found specifically on pathogens, or those that are not recognizable on host cells. Clinically, purified lectins can be used to identify glycolipids and glycoproteins on an individual's red blood cells for blood typing.

C-Type Lectins
C-Type lectins are those that require a calcium ion. The calcium ion helps bind the protein and carbohydrate by interacting with the OH groups found on the carbohydrate. Calcium can also form a linkage between the carbohydrate and glutamates in the lectin. Binding is further strengthened through hydrogen bonds that form between the lectin side chains and the OH groups of the carbohydrate. Carbohydrate recognition and binding is made possible by a homologous domain consisting of 120 amino acids. These amino acids determine the specificity of carbohydrate binding.

C Type lectins carry a wide range of functions such as cell to cell adhesion, immune response to foreign bodies and self-cell destruction. C Type lectins are categorized into various different subgroups specific to the different protein functional domains. These lectins are calcium ion dependent and share linear structural homology in their carbohydrate-recognition domains. Among Eukaryotes and the animal kingdom, this wide range of protein families including endocytic receptors, collectins, and selectins is found most abundantly. The differences in members of the family vary in the different kinds of carbohydrate complexes that are recognized with high polarity and affinity. C type lectins are involved with immune defense mechanisms and help protect an organism against tumorous cells.

P-Type Lectins
P-Type lectins contain a phosphate group. CD-MPR and CI-MPR are the only two members of the P-lectin family, cation-dependent and cation-independent. The main function of P-type lectins in eukaryotic cells involves delivering newly synthesized soluble acid hydrolyses to the lysosome. They do this by binding to mannose 6-phosphate residues found on the N-linked oligosaccharides of the hydrolyses.

MPRs (Mannose-6-phosphate receptors) were discovered when studies on mucolipidosis II, a lysosomal storage disorder, were conducted. Hickman and Neufeld found that fibroblasts from ML II patients were able to absorb lysosomal enzymes excreted by normal cells, whereas fibroblasts from normal patients were not able to absorb the lysosomal enzymes. Hickman and Neufeld hypothesized that the lysosomal enzymes had a recognition tag that allowed for receptor-mediated uptake and transport to lysosomes. These tags later became known as MPRs.

CI-MPR is about 300 kDA and exists as a dimer. The overall folding of CI-MPR is similar to that of CD-MPR, but unlike CD-MPR, CI-MPR is cation-independent. In addition, CI-MPR binds to proteins that have the MPR tag, IFG-II (a peptide hormone), and other non lysosomal hydrolases. The N-terminal three domains of CI-MPR exists as a monomer, and forms a tri-lobed disk that has significant contact with one another. This attribute of the tri-lobed disk is vital in maintaining the structure of its sugar binding site. Phosphorylated Glycan Microarray demonstrates that CI-MPR shows little disparity between glycans having one or two phosphomonoesters when it comes to binding. This is unlike CD-MPR, which has been shown to have affinity towards glycans with two phosphomonoesters. In addition, CI-MPR binds to ligands at the cell surface, unlike CD-MPR. Overall, all of the ligand binding sites of CI-MPR are located on the odd-numbered domains. Four signature residues in CD-MPR and domain 3 of CI-MPR are conserved, and have been found to react with Man-6-P in the same manner, suggesting that the Man-6-P binding pockets are similar. One difference that has been found is the fact that the pocket in CD-MPR contains Mn 2+, whereas the binding pocket in CI-MPR does not. This could be the reason why CI-MPR is cation-independent.

CD-MPR is a 46 kDA cation-dependent homodimer. Three disulfide linkages formed by six cysteine residues in the extracellular region of CD-MPR are key to the folding of the homodimer. Because the 15 contiguous domains of the extrasystolic region are similar in size and amino acid sequence when compared to each other, it is understood that CD-MPR and CI-MPR have similar tertiary structures. In fact, CD-MPR domains 1, 2, 3, 11, 12, 13 and 14 of CI-MPR have the same fold in the extrasystolic domain. The overall fold of the CD-MPR monomer consists of a flattened beta barrel consisting of two antiparallel beta sheets, one with four strands, and the other with five strands. The CD-MPR dimer consists of two five stranded antiparallel beta sheets. E133, Y143, Q66, and R111 have been found to be essential in Man-6-P binding via mutagenesis studies of CD-MPR. CD-MPR’s binding and unbinding mechanism is similar to that of the oxy-to-deoxy transition of hemoglobin. The overall movement has been described as to be a “scissoring and twisting” motion in between the two subunits of the dimer interface. These two subunits are connected via a salt bridge. Absence of this salt bridge results in a weaker bind with lysosomal enzymes, signaling the importance of ionic interactions between the two subunits in binding.

Selectins
Selectins are a type of C-Type lectins that play a role in the immune system. Selectins consist of L, E, and P forms that bind to carbohydrates found on lymph-node vessels, endothelium, and activated blood platelets. They behave analogously to C type lectins in that both have a high affinity for calcium binding and are responsible for immune responses. Selectins are sugar binding polymers that are adhesive among other cells which causes it to be highly effective in targeting an inflammatory response for a localized region. Selectins target only specific kinds of binding sites, but thus allows it to be effective in conjunction with leukocyte cascading to minimize invasively targeting an infected region.

Examples of Lectins
Embryos are attached to the endometrium of the uterus through L-Selectin. This activates a signal to allow for implantation.

E. coli are able to reside in the gastrointestinal tract by lectins that recognize carbohydrates in the intestines.

The influenza virus contains hemagglutinin which recognizes sialic acid residues on the glycoproteins located on the surface of the host cell. This allows the virus to attach and gain entry into the host cell.