Activation of concanavalin A by Cd2+

Role of second metal ion in establishing active conformations of concanavalin A

The stoichiometry of Mn2+ binding to concanavalin A was found to be influenced by temperature, pH, and the presence or absence of saccharide. Demetalized concanavalin A binds one Mn2+ (S1 site) at 5 degrees C, pH 6.5, and two Mn2+ at 25 degrees C (S1 and S2 sites). The association constants for Mn2+ are 6.2 x 10(5) and 3.7 x 10(4) M-1 for the S1 and S2 sites, respectively, at 25 degrees C. Concanavalin A with one Mn2+ bound per monomer remains in an open conformation and exhibits a relatively high water proton relaxation rate. Concanavalin A with two Mn2+ ions remains in a closed conformation characterized by a lower relaxation rate. The rate of binding of the second Mn2+ to concanavalin A as determined by ESR and the rate of conversion of open form to closed form (folding over) as determined by proton relaxation rate measurements gave an identical rate constant of 80.0 +/- 5.8 M-1 h-1 at 17 degrees C. Ca2+, Sr2+, and high levels of methyl alpha-D-mannopyranoside also induce folding of concanavalin A. Ca2+ is not catalytic but stoichiometric in causing the folding. Mn2+ in the S1 site can be displaced by Ni2+, Co2+, and Zn2+, and Mn2+ in the S2 site can be displaced by Ca2+ and Sr2+. Concanavalin A with Ni2+, Co2+, Zn2+, or Mn2+ in the S1 site and Ca2+ or Sr2+ in the S2 site has a higher affinity for methylumbelliferyl alpha-D-mannopyranoside than Ni-Mn-, Co-Mn-, Zn-Mn-, and Cd-Cd-concanavalin A.

A fluorimetric study of the lanthanides binding to concanavalin A

The binding of Tb3+ and other lanthanides to Con A has been studied by sensitized Tb3+ luminescence, by quenching of intrinsic fluorescence and by activity measurements. In all the experimental conditions tested, it was found that holo and apo Con A bind lanthanide ions at a site different from the binding sites of the constitutive metals, Mn2+ and Ca2+. The bound lanthanide did not affect the saccharide binding ability and the hemoagglutinating ability of Con A. The intrinsic fluorescence of Con A is quenched by the binding of Tb3+ and Gd3+. The same quenching is obtained by shifting the pH of Con A from pH 6.5 to 4.5. It is proposed that H+ and Ln3+ completely quench a tryptophan, perhaps the residue 88 or 182.

43Ca and 67Zn NMR spectra of Ca2+, Zn2+-concanavalin A solutions

The half-band width of 43Ca NMR of free aqueous Ca2+ was scarcely increased by adding more than equal molar apo-concanavalin A(apo-Con A), suggesting that slow chemical exchange, koff less than 10 s-1, occurs for the Ca2+ ion from Con A. In contrast with the 43Ca NMR findings, the half-band width of 67Zn NMR of free aqueous Zn2+ was markedly increased by adding apo-Con A. The 67Zn NMR half-band width of Zn2+ in the presence of apo-Con A was decreased by adding excess Ca2+, but was increased by adding excess D-mannose. These changes of the half-band width were influenced mutually by D-mannose or Ca2+, respectively. The broadened half-band widths of Zn2+ in the presence of Con A were decreased by adding Mn2+, suggesting that Mn2+ was substituted for Zn2+ at a metal binding site of Con A.