Study of the structure of the transition metal-binding site of concanavalin A by extended X-ray absorption fine-structure spectroscopy

We have measured the extended X-ray absorption fine-structure spectra of Zn-concanavalin A and of Zn,Ca-concanavalin A on the high-energy side of the K absorption edge of Zn. Analysis of the spectra shows that, in Zn-concanavalin A, the Zn is hexaco-ordinate with a mean metal-to-ligand distance of 2.06 ± 0.l Å. Formation of Zn,Ca-concanavalin A by binding of Ca2+ at the calcium-specific site is accompanied by a decrease of the co-ordination number of the Zn from six to five with virtually no change in metal-to-ligand distance (2.02 ± 0.1 Å). The geometry of the hexaco-ordinate and pentaco-ordinate complexes and the mechanism of their interconversion are discussed.