Proton nuclear magnetic resonance studies of soybean lectin-monosaccharide interactions: computer analysis of complex...

1H NMR was used to quantify soybean lectin binding to monosaccharides, using presaturation of HOD plus a spin-echo sequence to observe sugar -NHCOCH3 and -OCH3 to below 0.01 mM. Binding is in the very-slow-exchange limit; there is no broadening or shifting and only unbound sugar is observed for pH 5 to 8 and 25 to 75 degrees C. Preliminary results were consistent with those previously reported for methyl 2-acetamido-2-deoxy-alpha-D-galactopyranoside (Me alpha-D-GalNAcp) (K = 3 X 10(4) liters mol-1 with four sites per tetramer). More detailed studies, however, gave concave Scatchard plots for methyl 2-acetamido-2-deoxy-alpha- and beta-D-galactopyranoside, (Me alpha- and Me beta-D-GalNAcp), best fitted using K1 values of (6-12) X 10(4) liters mol-1, K2 values less than or equal to 0.5 X 10(4) liters mol-1, and four sites of each type in D2O or 80% H2O at 25 degrees C and pH 7.2. Data for methyl alpha-D-galactopyranoside were fitted with K = 0.5 X 10(4) liters mol-1 and eight sites of the same K. Monosaccharides may be binding in the recently reported "hydrophobic sites" of soybean lectin. Both methyl 2-acetamido-2-deoxy-beta-D-galactofuranoside (Me beta-D-GalNAcf) and methyl 2-acetamido-2-deoxy-alpha-D-glucopyranoside (Me alpha-D-GlcNAcP) showed some binding by 1H NMR; K's were similar to those for the high-affinity sugars, but the occupancy was much lower. The soybean lectin in this study was saturated in Ca2+ (greater than or equal to 4 mol/tetramer), but low in Mn2+, with Mn2+ plus Mg2+ less than 4. We report new melting points for D-N-acetylgalactosamine, Me alpha-D-GlcNAcp, Me beta-D-GalNAcp, and Me beta-D-GalNAcf, and a fully listed program for fitting curved Scatchard plots using Apple IIc and IIe computers.