Structural determinants of concanavalin A specificity for oligosaccharides.

Iodinated glycopeptides (50 to 200 x lo3 cpm/pmol) have been utilized to examine the structural determinants affecting interaction with the saccharide binding site of concanavalin A by Scatchard plot analyses of saturation curves. Glycopeptides with association constants in the range of 4.5 X 10’ M-’ and 25 X 10’ Mm1 are retained by concanavalin A-Sepharose while glycopeptides with association constants in the range of 0.3 x lo6 M-’ and 4.0 X 10’ M-’ are not retained by concanavalin A-Sepharose. All of the glycopeptides examined have a core with the structure: Mancul + 3[Manal --+ G]Manpl + 4GlcNAcj?l+ 4GlcNAc + Asn. The presence of the 2 a-linked mannose residues is essential for interactions with the saccharide binding site that yield association constants of 4.5 X IO6 M-’ or greater. The presence of substituents at position C-2 of the a-linked mannose residues does not reduce the association constants below 4.5 x lo6 M-‘; however, sequential removal of peripheral sugars on branches arising from position C-2 results in a progressive increase in the association constants to a maximum of 20 to 23 X lo6 Mm1 for the core structure. Additional a-linked mannose residues do not result in a significant increase in the association constant as compared to the core sugars alone. The presence of a single fl1,4-linked N-acetylglucosamine residue on the cwl,6-linked mannose is sufficient to reduce the association constant to 3.3 x lo6 M-‘. Glycopeptides with an N-acetylglucosamine residue linked /31,4 to the b-linked mannose display an association constant of 2.0 X lo6 Mm1 or less if the cul,3-linked mannose bears a branch consisting of Gal@ --+ 4GlcNAcfil+ arising from position C-2. Removal of the galactose results in an increase of the association conStaXIt to 5.0 X lo6 M-‘.