Mapping of subsites in the combining area of monoclonal anti-galactan immunoglobulin A J539.

Monoclonal immunoglobulin A J539 binds beta-(1----6)-D-galactopyranans. Measurement of the affinity of its Fab' fragment for a series of galacto oligosaccharides--some of which carried deoxyfluoro groups--has made it possible to assign a binding mode of the polysaccharide that has the reducing end oriented from the heavy (H) chain toward the light (L) chain. In addition, the values obtained for the affinity constants of the immunoglobulin with these oligosaccharides, as well as the maximal values obtained for the intrinsic ligand-induced fluorescence, permit a deduction about the relative affinity of the protein's four subsites for each galactose residue of the tetrasaccharide fragment it can bind. If these subsites are labeled C, A, B, and D, going from the H-chain toward the L-chain across the face of the immunoglobulin combining area, then the order of decreasing affinity is A greater than B greater than C greater than D.