MOLECULAR RECOGNITION .12. THE BINDING OF THE H-HUMAN BLOOD-GROUP DETERMINANTS AND CONGENERS BY A LECTIN OF GALACTIA-TENUIFLORA

The H-type 2 human blood group-related trisaccharide (alpha-L-Fuc-(1c-->2b)-beta-D-Gal-(1b-->4a)-beta-D-GlcNAc-OMe (52)) is bound by the anti-H lectin of Galactia tenuiflora very differently than by the lectin I of Ulex europaeus. The reason why the Galactia lectin binds the H-type 1 related trisaccharide (alpha-L-Fuc-(1c-->2b)-beta-D-Gal-(1b-->3a)-beta-D-GlcNAc-OMe (5)) more strongly and methyl alpha-L-fucopyranoside much more weakly than does the Ulex lectin is that, for the Galactia lectin, the hydroxyl groups at positions 3a, 3b, 4b, and 4c are indispensable to complex formation whereas it is the hydroxyl groups at positions 3b, 2c, 3c, and 4c which provide the key polar intractions in the case of the Ulex lectin. The H-type 2 (52) . Galactia lectin complex appears to have the hydroxyl groups at positions 6b, 2c, and 3c at or near the periphery of the combining site and the three key hydroxyl groups hydrogen bonded to the protein deep within the combining site and sheltered from water. The CH3O-1a, NHAc-2a, and CH2OH-6a groups likely remain in the aqueous phase remote from the surface of the protein.