Location of Mn2+ in concanavalin A containing only a Mn2+ ion

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.

Manganese, calcium, and saccharide binding to concanavalin A, as studied by ultrafiltration

The binding of the ligands Mn2+, Ca2+, and methyl alpha-D-glucopyranoside to concanavalin A, purified as described (A.J. Sophianopoulos and J.A. Sophianopoulos (1981) Prep. Biochem. 11, 413-435), was studied by ultrafiltration in 0.2 M NaCl, pH 5.2 and pH 6.5 to 7, and at 23 to 25 degrees C. The association constant (Ka) of methyl alpha-D-glucopyranoside to concanavalin A was (2 +/- 0.2) X 10(3) M-1, both at pH 5.2 and 7. At pH 5.2 and in the absence of Ca2+, the Ka of Mn2+ to concanavalin A was (5 +/- 1) X 10(3) M-1, and in the presence of 1 mM Ca2+, the Ka was (9.1 +/- 2.1) X 10(5) M-1. At pH 6.5 Mn2+ bound to concanavalin A with a Ka of (7.3 +/- 1.8) X 10(5) M-1, and the binding affinity was virtually independent of the presence of Ca2+. Experiments of binding of 4-methylumbelliferyl alpha-D-mannopyranoside to concanavalin A indicated that at pH 5.2, binding of a single Mn2+ per concanavalin A monomer was sufficient to induce a fully active saccharide binding site. Ca2+ is not necessary for such activation, but rather it increases the affinity of concanavalin A for binding Mn2+.