Physical studies of lanthanide binding to concanavalin A.

The binding of lanthanide ions to concanavalin A has been studied using circular dichroic, magnetic circular dichroic and fluorescence spectroscopies. A Scatchard analysis of the fluorescence enhancement of Tb3+ binding reveal the presence of two classes of sites. One strong binding site (S3) is present per protomer which is not affected by addition of transition metals or calcium. There are at least three additional weak binding sites per protomer, one of which may be S1. There is virtually no energy transfer from Tb3+ to Co2+ in cobalt-substituted concanavalin A. The S3 site, therefore, must be at least 39 A distant from S1. The circular dichroic spectrum of Co2+ concanavalin A is altered by addition of Gd3+ in a manner consistent with competition for S1. The magnetic circular dichroic spectrum of Pr3+ bound to concanavalin A is not affected by excess Ca2+ or the addition of specific carbohydrate effectors. There is no evidence for lanthanide ion binding to S2.