Electron spin echo envelope modulation studies of lectins: evidence for a conserved Mn(2+)-binding site.

Electron spin echo envelope modulation (ESEEM) experiments have been used to investigate the Mn(2+)-binding site in a series of lectins including concanavalin A, pea lectin (Pisum sativum), isolectin A from lentil (Lens culinaris), soybean agglutinin (Glycine max), Erythrina indica lectin, and Lotus tetragonolobus isoelectin 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.