Divalent cadmium and lead and the trivalent lanthanides bind in the transition metal site (S1) of concanavalin A and induce saccharide binding to the protein in the presence of calcium. Partial activation of the protein in the presence of lanthanides alone indicates these ions bind into both transition metal (S1) and calcium sites (S2). The activity of a lanthanide-protein derivative may be increased by the addition of either calcium or a transition metal ion. The saccharide binding activity decreases in the order Zn2+> Ni2+> Co2+> Mn2+> Cd2+ reflecting the order of binding constants for these ions in the transition metal site. Like the lanthanides, divalent cadmium substitutes for both the transition metal ion and calcium ion to partially activate the protein. Divalent lead substitutes only for the transition metal ion and partially activates the protein upon adding calcium. The data are consistent with a model in which saccharide binding activity is independent of the metal size in SI but critically dependent upon metal size in S2.
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