Cadmium-113 nuclear magnetic resonance investigation of metal binding sites in concanavalin A

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Abstract
Cadmium-113 NMR studies are presented on the 113Cd-substituted protein concanavalin A (Con A). This protein has 2 different conformations, locked and unlocked, as described previously by Brown et al. The unlocked form of Con A gives one 113Cd resonance, indicating rapid exchange of the metal ions between the solution and the binding sites. Solutions of the locked form show 3 resonances: a free Cd resonance (68 ppm), a resonance assigned to 113Cd occupying the Mn site (46 ppm) and a resonance assigned to 113Cd occupying the Ca site (-125 ppm). Pb(II) binds to the Ca site, and Zn(II) has high affinity for both sites. Data presented in previous literature support a model in which Con A binds monosaccharides in a different manner than it binds oligosaccharides. If this difference exists, it does not affect the metal binding sites. Some heterogeneity in Con A was reported in the literature; the presence of these heterogeneities does not affect the 113Cd NMR parameters.