The combining site of anti-I Ma (group 1).

Evidence is provided that the trisaccharide beta DGal(1----4)beta DGlcNAc(1----6)beta DGal is bound by the monoclonal anti-I Ma antibody beginning with a basically nonpolar cleft at the surface of the protein which comes into contact with a weakly polar region of the trisaccharide that extends from the C-5 methylene group of the reducing unit about the surfaces involving the acetamido grouping and the OH-3' of the beta DGlcNAc unit intramolecularly hydrogen bonded to the O-5'' of the nonreducing beta DGal unit and up to the C-5'' methylene group. The combining site then terminates with a polar grouping at its periphery which is disposed to react with OH-6'' and likely OH-4'' in a highly specific manner. The hydroxyl groups at positions 1, 2, 3, 4, 6', 3'' and 4'' remain in contact with the aq. phase. This conclusion was deduced from the relative potencies as inhibitors of a wide number of synthetic compounds that bear varying structural relationships to the trisaccharide. It appears that the stability of the complex is mainly related to attractive interactions between two large complementary and essentially hydrophobic surfaces relative to those when these surfaces are exposed to water.