Nuclear magnetic resonance investigation of cadmium 113 substituted pea and lentil lectins.

The lentil (LcH) and pea (PSA) lectins, which are members of the class of D-glucose/D-mannose binding lectins, are Ca2+ X Mn2+ metalloproteins that require the metal ions for their saccharide binding and biological activities. We have prepared a variety of Cd2+ derivatives of PSA and LcH, with Cd2+ in either the transition metal (S1) or calcium (S2) sites, or in both. Thus, Cd2+ X Zn2+, Cd2+ X Mn2+, and Ca2+ X Cd2+ derivatives were prepared, in addition to the Cd2+ X Cd2+ derivatives which we have recently reported. This is the first report of stable mixed metal Cd2+ complexes of lectins. The physical and saccharide binding properties of the Cd2+ derivatives of both lectins were characterized by a variety of physiochemical techniques and found to be the same as those of the corresponding native proteins. 113Cd NMR spectra of mono- and disubstituted 113Cd2+ complexes of LcH and PSA were recorded and compared with 113Cd NMR data for concanavalin A (ConA) (Palmer, A.R., Bailey, D.B., Behnke, W.D., Cardin, A.D., Yang, P.P., and Ellis, P.D. (1980) Biochemistry 19, 5063-5070). The data for the PSA and LcH derivatives were found to be very similar, indicating close homology of their metal ion binding sites. 113Cd resonances at 44.6 ppm and -129.4 ppm for 113Cd2+ X 113Cd2+ X LcH, and at 46.6 and -130.4 for the corresponding PSA derivative, are chemical shifts very similar to those observed for 113Cd2+ X 113Cd2+ X ConA. Assignment of the resonances to the transition metal (S1) and calcium (S2) sites were unambiguous since the Ca2+ X 113Cd2+ and 113Cd2+ X Zn2+ derivatives of both lectins showed single resonances characteristic of the S1 and S2 sites, respectively. The results indicate that, unlike ConA, 113Cd2+ binds tightly to PSA and LcH. Binding of monosaccharide to both lectins induce small (2 ppm) upfield shifts in their S2 113Cd resonances, in contrast to the larger shift (8 ppm) observed in ConA. The 113Cd2+ X Mn2+ complexes of PSA and LcH fail to show a 113Cd resonance characteristic of these derivatives, which provides evidence for the close proximity of the metal ions in the two proteins. The present findings indicate that the coordinating ligand atoms to the metal ions at the S1 and S2 sites in LcH, PSA, and ConA are the same.