| dc.description.abstract | The journey of structural studies on lectins, starting with ConA in the 70s, has crossed many milestones. Lectins, multivalent carbohydrate-binding proteins of non-immune origin, specifically bind diverse sugar structures. They have received considerable attention in recent times on
account of the realization of the importance of protein-sugar interactions, especially at the cell surface, in biological recognition. They occur in plants, animals, fungi, bacteria and viruses. Plant lectins constitute about 40% of the lectins of known structure. They can be classified into five
structural groups, each characterized by a specific fold. Among them, legume lectins constitute the most extensively investigated group. Basic Winged bean lectin (WBAI) is a glycosylated, homodimeric, legume lectin
with Mr 58000. The structure of WBAI complexed with methyl-a-galactose,
determined earlier in this laboratory, provided information about the oligomeric state and the carbohydrate specificity of the lectin in terms of lectin-monosaccharide interactions. The present work was initiated to understand the carbohydrate specificity of the lectin, especially at the
oligosaccharide level, with special reference to its blood group specificity.
The hanging drop method was used for crystallizing WBAI and its complexes. Intensity data were collected on Mar Research imaging plates mounted on Rigaku RU-200 or ULTRAX-18 X-ray generators. The data
were processed using DENZO and SCALEPACK of HKL suite of programs. The structure factors from the processed data were calculated
using TRUNCATE of CCP4 suite of programs. The molecular replacement program AMoRe was used for structure solution. Structure refinement was carried out using the CNS software package. Model building was done using the molecular graphics program O. INSIGHT II, ALIGN, CONTACT and PROCHECK of CCP4 were used for the analysis and validation of the refined structures.
WBAI exhibits differential affinity for different monosaccharide derivatives of galactose. In order to elucidate the structural basis for this differential affinity, the crystal structures of the complexes of basic winged bean lectin with galactose, 2-methoxygalactose, N-acetylgalactosamine and methyl-a-N-acetylgalactosamine have been determined. Lectin-sugar interactions involve four hydrogen bonds and a stacking interaction in all of them. In addition, a N-H O hydrogen bond involving the hydroxyl group substituted at C2 exists in the galactose and 2-methoxygalactose complexes. The additional hydrophobic interaction, involving the methyl group, in the
latter leads to the higher affinity of the methyl derivative. In the lectin - N-
acetylgalactosamine complex the N-H O hydrogen bond is lost, but a compensatory hydrogen bond involving the oxygen atom of the acetamido group is formed. In addition, the CH3 moiety of the acetamido group is
involved in hydrophobic interactions. Consequently, the 2-methyl and the
acetamido derivatives of galactose have nearly the same affinity for the lectin. The methyl group, a-linked to the galactose, takes part in additional hydrophobic interactions. Therefore, methyl-a- N-acetylgalactosamine has higher affinity than N-acetylgalactosamine to the lectin. The structures of basic winged bean lectin-sugar complexes provide a framework for
examining the relative affinity of galactose and galactosamine for the lectins that bind to them. The complexes also lead to a structural explanation for the blood group specificity of basic winged bean lectin, in terms of its monosaccharide specificity. The Tn-determinant (GalNAc-a-O-Ser/Thr) is a human specific tumor associated carbohydrate antigen. Having epithelial origin, it is
expressed in many carcinogenic tumors including breast, prostate, lung and
pancreatic cancers. The crystal structure of WBAI in complex with GalNAc-a-O-Ser (Tn-antigen) has been elucidated, in view of its relevance to diagnosis and prognosis of various human cancers. The Gal moiety occupies the primary binding site and makes interactions similar to those found in
other Gal/GalNAc specific legume lectins. The nitrogen and oxygen atoms of the acetamido group of the sugar make two hydrogen bonds with the protein atoms whereas its methyl group is stabilized by hydrophobic interactions. A water bridge formed between the terminal oxygen atoms of
the serine residue of the Tn-antigen and the side chain oxygen atom of Asn128 of the lectin increase the affinity of the lectin for Tn-antigen compared to that for GalNAc. A comparison with the available structures
reveals that while the interactions of the glyconic part of the antigen are
conserved, the mode of stabilization of the serine residue differs and depends on the nature of the protein residues in its vicinity. The structure provides a qualitative explanation for the thermodynamic parameters of the formation of the complex of the lectin with Tn-antigen. Modelling studies indicate the possibility of an additional hydrogen bond with the lectin when the antigen is part of a glycoprotein.
WBAI binds A-blood group substance with higher affinity and B-blood group substance with lesser affinity. It does not bind the O substance. The crystal structures of the lectin, complexed with A -reactive and B -
reactive di and tri saccharides, have been determined. In addition, the complexes of the lectin with fucosylated A- and B-trisaccharides and with a variant of the A-trisaccharide have been modelled. These structures and models provide valuable insights into the structural basis of blood group specificities. All the four carbohydrate binding loops of the lectin contribute to the primary combining site while the loop of variable length contributes to the secondary binding site. In a significant advance to the current
understanding, the interactions at the secondary binding site also contribute
substantially, albeit in a subtle manner, to determine the blood group specificity. Compared to the interactions of the B- trisaccharide with the lectin, the third sugar residue of the A -reactive trisaccharide forms an additional hydrogen bond with a lysine residue in the variable loop. In the former, the formation of such a hydrogen bond is prevented by a shift in the
orientation of the third sugar resulting from an internal hydrogen bond in it.
The formation of this bond is also facilitated by an interaction dependent
change in the rotamer conformation of the lysyl residue of the variable loop. Thus, the difference in the interactions at the secondary site is generated by coordinated movements in the ligand as well as the protein. A comparison of the crystal structure and the model of the complex involving the variant of the A-trisaccharide results in the delineation of the relative contributions of the interactions at the primary and the secondary sites in determining blood group specificity.
At the disaccharide level, WBAI exhibits higher affinity for á1-3 linked Gal/GalNAc containing oligosaccharides, compared to that of other á linked oligosaccharides. With an objective of understanding the preferential binding of WBAI for á 1-3 linked Gal/GalNAc containing oligosaccharides, crystal structure of the complexes of the lectin with Galá1-4Gal, Galá1-4GalâEt and Galá1-6Gal have been determined. The reducing sugar of the
disaccharides with linkages other than á1-3 binds to the lectin through a water bridge whereas the same sugar moiety with á 1-3 linkage makes direct interactions with the loop L4 of the protein. The modelling study on the complex of the lectin with Galá1-2Gal further upholds this observation.
Different structures involving WBAI, reported earlier and presented here, were used to investigate the plasticity of the lectin. The front curved â-sheet, which nestles the metal binding region and on which the carbohydrate binding loops are perched, is relatively rigid. On the contrary, the flat back â-sheet, involved in the quaternary association in legume lectins, is flexible. This flexibility is probably necessary to account for the variation in quaternary structure.
With the results presented in this thesis, 14 crystal structures of WBAI, in the free form and in complex with different sugars, have been reported, all from this laboratory. It is now, perhaps, appropriate to examine the new information and insights gained from these investigations, on the
structure and function of the lectin. Earlier X-ray studies of WBAI contributed substantially in establishing that legume lectins are a family of proteins in which small alterations in essentially the same tertiary structure lead to large alterations in quaternary association. Structural studies on WBAI, particularly those reported here, also contributed to the elucidation
of the nuances of carbohydrate recognition by lectins. A comparative study of the available structures also revealed the flexible and rigid regions of the protein.
The study of the influence of covalently linked sugars on the structure of Erythrina corallodendron lectin (ECorL), a homolog of WBAI, is the content of appendix of the thesis. The three-dimensional structure of
the recombinant form of Erythrina Corallodendron lectin(rECorL) complexed
with lactose, has been elucidated by X-ray crystallography. Comparison of this
non-glycosylated structure with that of the native glycosylated lectin reveals that the tertiary and quaternary structures are identical in the two forms, with local changes observed at one of the glycosylation sites(Asn17). These changes take place in such a way that hydrogen bonds with the neighbouring protein molecules in rECorL compensate those made by the glycan with the protein in ECorl. contrary to an earlier report, this study demonstrates that the glycan attached to the lectin does not influence the oligomeric state of the lectin. Identical interactions between the lectin and the non-covalently bound lactose in the two forms indicate, in line with earlier reports, that glycosylation does not affect the carbohydrate specificity of the lectin. The present study, the first of its kind involving a glycosylated protein with a well defined glycan and the corresponding deglycosylated form, provides insights into the structural aspects of protein glycosylation. | en |
