Electron Spin Echo Envelope Modulation Studies of Lectins: Evidence for A Conserved Mn²⁺-Binding Site

Electron spin echo envelope modulation (ESEEM) experiments have been used to investigate the Mn²⁺-binding site in a series of lectins including concanavalin A, pea lectin (Pisum satiυum), isolectin A from lentil (Lens culinaris), soybean agglutinin (Glycine max), Erythrina indica lectin, and Lotus tetragonolobus isolectin A. Together with model studies, the results provide direct evidence for a single nitrogen atom of a conserved residue bonded directly to Mn²⁺ in all of them. ESEEM measurements of the lectins exchanged with deuterium oxide, together with model studies, provide evidence for the presence of two water molecules coordinated to the Mn²⁺ in all of the proteins. In contrast to concanavalin A, the absence of solvent exchange at the Mn²⁺ site in the pea and lentil lectins demonstrated by nuclear magnetic relaxation dispersion measurements [Bhattacharyya, L., Brewer, C. F., Brown, R. D., III, & Koenig, S. H. (1985) Biochemistry 24, 4985–4990] must therefore be due to slow exchange of the water ligands of the bound Mn²⁺. Binding of saccharides was observed to have little effect on the structural features of the Mn²⁺ site in the lectins as determined by ESEEM.