The fine structure of the combining site of human mAb IgMNOV to poly-alpha(2----8)linked NeuNAc, the epitope of the group B meningococcal and E. coli K1 polysaccharides, has been probed using RIA and ELISA. Inhibition by oligomers ranging from 2 to 12 residues was used to assay binding to IgMNOV by group B meningococcal polysaccharide preparations (GBMP) or by poly(A). The inhibitory properties of the oligomers were almost identical in both assays of the binding of GBMP to horse IgM (H46). This evidence and the finding that both GBMP and poly(A) precipitated IgMNOV equally per unit weight indicated that the epitope of poly(A) must mimic an equivalent epitope on GBMP despite the absence of any apparent common structural features in the two molecules. Unlike most carbohydrate-anticarbohydrate systems in which the site is saturated by oligomers of up to six or seven sugars, all the anti-alpha(2----8)NeuNAc systems above required much larger oligomers. Because these oligomers are larger than the maximum size of an antibody site the epitope must be conformationally controlled, and this has been confirmed by nuclear magnetic resonance spectroscopy. However, despite the above similarities, GBMP and poly(A) were differentiated in that only GBMP bound to H46. Smaller linear molecules obtained by delipidating the GBMP, as well as periodate-oxidized GBMP with its nonreducing end oxidized or linked covalently to BSA, bound to and precipitated IgMNOV and H46. This showed that, despite their differences, terminal nonreducing ends were not involved and that both epitopes were located in the conformationally controlled inner residues of the GBMP. The difference thus must reside in the ability of IgMNOV and H46 to recognize different structural aspects of the same conformationally controlled inner residues. The ELISA data indicate that both IgMNOV and H46 have groove-type sites that bind exclusively to an epitope located on the acidic side of the inner residues. The differences determining the ability of IgMNOV and the failure of H46 to cross-react with poly(A), poly(I), and denatured DNA, may depend on differences in the degree of protonation required by each antibody, and this may be clarified by a study of the effects of pH on the precipitin behavior of IgMNOV and H46.

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