Immunochemical studies on blood groups—LXIX. The conformation of the trisaccharide determinant in the combining site of anti-I Ma (group 1)

Abstract

Further studies of the inhibition of the monoclonal anti-I Ma (group 1) antibody, in quantitative precipitin assays, by synthetic oligosaccharides derived from N-acetyllactosamine (LacNAc) are reported. The results confirm that the antibody binds about the βLacNAc-OCH 2CHO-portions of such structures as βLacNAc(1→6)β dGal or βLacNAc(1→6)α dGalNAc. The trisaccharides βLacNAc(1→6)-7-deoxy- d- glycero- d-galacto -heptose ( 7) and βLacNAc(1→6)-7-deoxy- l- glycero- d- galacto-heptose ( 8), in contrast to βLacNAc(1→6) dGal ( 1), were found ( 1HNMR) to possess well-defined conformations. Since 7 proved to be a 2.2 times better inhibitor than 1, whereas 8 was a poor inhibitor, it is concluded that the antibody binds 1 in the conformation which is preferred by 7. On this basis, it is noted that 1 would reside in a conformation that possesses a region amenable to hydrophobic bonding that extends between the C-5 groupings of the two dGal units and over the acetamido group of the central β dGlcNAc residue with the 3-hydroxyl of this unit hydrogen bonded to O-5 of the terminal β dGal unit. It is suggested that it is this region which becomes bound to a hydrophobic site of the antibody. Thus, nine of the hydroxyl groups in 1 would remain available for interaction with the solvent water and it would follow that the combining site is an hydrophobic cleft.