Dynamic metal-thiolate cluster structure of metallothioneins

Abstract

113Cd-NMR studies have established that vertebrate and invertebrate metallothioneins contain two unique metal-thiolate clusters. However, it proved to be difficult to account theoretically for all features of the 113Cd-NMR spectra. In a reinvestigation of these features using chromatographically homogeneous 113Cd7-metallothionein we have identified a total of seven 113Cd resonances and have confirmed the massive intensity difference among these signals. From the effects of variations in temperature, ionic strength, and magnetic field on one-dimensional 113Cd-NMR spectra and from two-dimensional J-resolved 113Cd-NMR spectrum it was concluded that the seven 113Cd signals are composed of several overlapping multiplets and indicative of a dynamic organization of the metal-thiolate clusters. The same nonrigid structure is also indicated individually by recent two-dimensional correlated (COSY) 1H-NMR studies of 113Cd7-metallothionein. While showing 113Cd-1H coupling for 19 of the 20 cysteine residues, these studies have evidence for only two "bridging" cysteine ligands of the clusters instead of the expected eight suggesting interferences by metal-thiolate exchange process at the 1H-NMR time scale. Analogous indirect evidence for structural mobility of the clusters comes from EXAFS measurements of Zn7-metallothionein and from measurements of the perturbed angular correlation of gamma-rays (PAC) emis-sion of 111mCd-metallothionein. Thus, while the EXAFS spectra revealed back-scattering from the thiolate ligands, lattice movements within the cluster is believed to preclude back-scattering from neighboring metals. Similarly, in the PAC time spectra the damping of the major oscillatory component was attributed to inordinately large charge fluctuation in the immediate environment of the 111mCd nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)