ABSTRACT
Structures of two crystal forms of the dimeric acidic winged bean agglutinin (WBAII) complexed with methyl-alpha-D-galactose have been determined at 3.0 A and 3.3 A resolution. The subunit structure and dimerisation of the lectin are similar to those of the basic lectin from winged bean (WBAI) and the lectin from Erythrina corallodendron (EcorL). The conformation of a loop and its orientation with respect to the rest of the molecule in WBAII are, however, different from those in all the other legume lectins of known structure. This difference appears to have been caused by the formation of two strategically placed salt bridges in the former. Modelling based on the crystal structures provides a rationale for the specificity of the lectin, which is very different from that of WBAI, for the H-antigenic determinant responsible for O blood group reactivity. It also leads to a qualitative explanation for the thermodynamic data on sugar-binding to the lectin, with special emphasis on the role of a tyrosyl residue in the variable loop in the sugar-binding region in generating the carbohydrate specificity of WBAII.
Subject(s)
Fabaceae/chemistry , Lectins/chemistry , Lectins/metabolism , Methylgalactosides/metabolism , Plants, Medicinal , Salts/metabolism , ABO Blood-Group System/immunology , Binding Sites , Crystallography, X-Ray , Dimerization , Glycosylation , Lectins/genetics , Lectins/immunology , Ligands , Magnoliopsida/chemistry , Methylgalactosides/chemistry , Methylgalactosides/immunology , Models, Molecular , Oligosaccharides/chemistry , Oligosaccharides/immunology , Oligosaccharides/metabolism , Plant Lectins , Protein Binding , Protein Conformation , Protein Subunits , Salts/chemistry , Static Electricity , Substrate Specificity , Tyrosine/genetics , Tyrosine/metabolismABSTRACT
Murine monoclonal IgA J539 binds to methyl beta-D-galactopyranoside. With nearly the same affinity, it binds to methyl 6-O-pivaloyl-beta-D-galactopyranoside (4) and to methyl 6-O-beta-D-gentiobiosyl-beta-D-galactopyranoside (7). These observations confirm that the combining area of J539 is of the surface-, and not of the cavity-type.
Subject(s)
Antibodies, Monoclonal/immunology , Antigens/immunology , Immunoglobulin A/metabolism , Immunoglobulin Fab Fragments/immunology , Methylgalactosides/immunology , Methylglycosides/immunology , Animals , Antibody Affinity , Antigen-Antibody Reactions , Binding Sites, Antibody , Chemical Phenomena , Chemistry , Ligands , MiceABSTRACT
Methyl 2-deoxy-2-fluoro-beta-D-glactopyranoside (2) and methyl 4-deoxy-4-fluoro-beta-D-glactopyranoside (7) have been prepared, and the possibility of their binding to (1 leads to 6)-beta-D-galactopyranan-specific immunoglobulin A J539 (Fab') has been investigated. Compound 2 does not show binding, whereas 7 does. It appears that the 2-hydroxyl group of methyl beta-D-galactopyranoside may take part in hydrogen bonding to the protein.
