Magnetic resonance studies of concanavalin A: assignment of histidine resonances in 220 MHz proton spectrum of complexes with Co2+ and Zn2+.

The low field regions of the 220 MHz proton magnetic resonance spectra of concanavalin A (Con A) complexes with metal ions show well resolved resonances from the C2 protons of histidine side chains. Shifts of these resonances are observed when Zn2+ ions at the transition metal ion binding site, S1, are replaced by CO2 ions. The magnitude of these shifts can be used to determine the orientation of axis of anisotropy of the Co2+ ligand field since the distances from the C2 protons of the histidines to S1 can be computed from the crystal structure coordinates. Assignment of the separate peaks in the spectrum of the Con A-Co2+-Ca2+ complex and of the Con A-Zn2+-Ca2+ complex, to particular histidines in the amino acid sequence of Con A then follows. The refined crystal coordinates of both Reeke et al. (Reeke, G. N., Jr., Becker, J. W., and Edelman, G. M. (1975) J. Biol. Chem, 250, 1525-1547) and of Hardman and Ainsworth (private communication) have been used. These two sets of coordinates both yield orientations for the axis of anisotropy which are approximately in the direction of the His 24 nitrogen ligand.