RESUMO
Cadmium-113 nuclear magnetic resonance studies are presented on the 113Cd-substituted protein concanavalin A (Con A). This protein has two different conformations, locked and unlocked, as described previously by Brown et al. [Brown, R. D., III, Brewer, C. F., & Koenig, S. H. (1977) Biochemistry 16, 3883]. 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 three resonances: a free cadmium resonance (68 ppm), a resonance assigned to 113 Cd occupying the manganese site (46 ppm), and a resonance assigned to 113Cd occupying the calcium site (--125 ppm). In addition, Pb(II) is shown to bind to the calcium site and Zn(II) is shown to have high affinity for both sites. Data have been presented in previous literature that support a model in which Con A binds monosaccharides in a different manner than it binds oligosaccharides. However, if this difference exists, it does not affect the metal binding sites. Also, some heterogeneity in Con A has been reported in the literature; however, we have shown that the presence of these heterogeneities does not affect the 113 Cd NMR parameters.
