Mannose D-Glucose -binding Lectins

Physical characterization of con A revealed monodispersity in the ultracentrifuge at pH 2-5, and a two-peak pattern at pH 7, suggesting pH-dependent association of subunits.136,261 262 It is now known262 -264 

The single polypeptide-chain protomers (however, see later) are compactly folded, forming dome-shaped structures approximately 4.2 nm high x 4.0 nm wide x 3.9 nm thick. Each subunit contains one Mn2+, one Ca2+, and one carbohydrate binding-site,172,278-280 believed, until recently, to be situated in a prominent cavity in the molecular model.280 Two such subunits are joined with their flattened bases proximal, to form roughly ellipsoidal domes. The dimers pair across one another, to form tetramers as depicted in Fig. 2. 

—Schematic Representation of Con A Tetramer.22 [Ca, Mn, and S indicate the position of the Ca2+, Mn2+, and carbohydrate-specific binding-sites, respectively. I localizes the position of the hydrophobic binding-site present in crystals of the 1222 space group (reproduced by permission of Nature).] 

The most striking feature of the con A secondary structure is its high content ( 50%) of j8-pleated sheet, a finding supported by c.d. and o.r.d. studies.281-2853 A theoretical study of j3-sheet-breaking residues was 

Two further studies, one by Villafranca and Viola,309 who carried out 13C spin-lattice relaxation measurements on the interaction of methyl a-D-glucopyranoside with con A, and the other by Alter and Mag-nuson,310 who characterized the binding of 2-deoxy-2-(trifluoroacetamido)-D-glucose to the lectin, placed the saccharide at 1.0 to 1.4 nm from the Mn2+ion, in agreement with the results of Brewer and coworkers.307,308 Villafranca and Viola s model309 placed C-l, C-2, and C-6 closest to the Mn2+ (see Fig. 3). On the other hand, support for the cavity proposed by Becker and coworkers280 comes from p.m.r.-spectral measurements of the longitudinal relaxation-times of the methyl proton of methyl a-D-mannopyranoside with Zn2+- and Gd2+-substituted con A.311,312 

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Table VI presents comparative inhibitory data on four D-mannose(D-glucose)-binding lectins.213 Such studies are exceptionally useful in defining fine differences in the specificity of a group of lectins having similar, but not identical, sugar-binding specificity.

D-Mannose, D-glucose, and 2-acetamido-2-deoxy-D-glucose have been coupled to albumin to form pseudoglycoproteins for use in a study of receptor-mediated binding of glycoproteins by alveolar macrophages. Fluoresceinyl derivatives of D-mannose, lactose, 2-acetamido-2-deoxy-a-D-galactose, and L-fucose have been covalently attached to albumin and used for direct visualization of membrane lectins in situ. ... 

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