Evidence for two types of binding sites in cadmium metallothionein determined by perturbed angular correlation of γ rays

Recent 113Cd NMR as well as ESR and magnetic susceptibility measurements on Co(II)-metallothionein strongly suggest the existence of a unique metal-thiolate cluster structure in this protein. However, additional knowledge on the details of molecular architecture of the seven metal-binding sites in metallothionein (approx.6-7 g-atoms/mol, usually Zn2+ and Cd2+) is required in order to establish a realistic model of the molecule and to understand its function. Since metallothionein is the only known protein in which cadmium is found naturally, perturbed angular correlation of gamma rays (PAC) employing 111Cd nuclei offers itself particularly for such a study. Analysis of PAC spectra using 111Cd has proven to be useful in the structural elucidation of specific metal sites in metalloproteins under both crystalline and noncrystalline conditions. We report here PAC measurements performed on fully and partially saturated 111Cd metallothionein