Ziqi Gui

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Metallothioneins (MT) are a class of low molecular weight, cysteine rich proteins in almost all living things. These proteins have been found to bind a wide range of metals both in vivo and in vitro due to the unusually high cysteinyl sulfur content. The metallothioneins are believed to possess the ability to store and transport essential metals, such as Zn(II) and Cu(I), and also to detoxify toxic metals, such as Cd(II) and Hg(II). However, the precise physiological functions of these proteins still remain unclear to date. As silver(I)-metallothioneins have shown different stoichiometric ratios of Ag:MT from Zn(II)- and Cd(II)-metallothioneins in which 7 metals are bound to the protein, the focus of this research is on the elucidation of the structural nature of Ag-MT.;The binding of Ag(I) to rabbit liver Zn{dollar}\sb7{dollar}-MT has shown that the presence of the Zn(II) to protein prevents the incoming Ag(I) from forming an intermediate species with a well organized tertiary structure. As a result, a mixed species, {dollar}\rm Zn\sb2Ag\sb{lcub}12{rcub}{dollar}-MT, is formed in a distributed manner that gives poorly resolved spectroscopic features. However, Ag(I) binding to rabbit liver apo-MT 1 displayed a different pattern. At high temperatures (50{dollar}\sp\circ{dollar}C), Ag(I) binds to apo-MT 1 in a domain specific manner. A series of species, (Ag{dollar}\sb6)\sp{lcub}\beta{rcub}{dollar}-MT 1, {dollar}\rm(Ag\sb6)\sp{lcub}\alpha{rcub}(Ag\sb6)\sp{lcub}\beta{rcub},{dollar}-MT 1, and Ag{dollar}\sb{lcub}17{rcub}{dollar}-MT 1, as the thermodynamically favored products, have been characterized by both CD and emission spectroscopies. At room temperature, the binding takes place in a kinetically favored distributed manner, and only Ag{dollar}\sb{lcub}17{rcub}{dollar}-MT 1, as a species of final binding process, produces the well-defined spectroscopic bands.;The striking results obtained from both sulfur and silver K-edge EXAFS spectroscopy are that the local structure of Ag{dollar}\sb{lcub}12{rcub}{dollar}-MT 1 and Ag{dollar}\sb{lcub}17{rcub}{dollar}-MT 1 is very similar. Ag(I) in these two proteins adopts linear coordination. This new finding for the structural nature of the Ag{dollar}\sb{lcub}12{rcub}{dollar}-MT is unusual in view of the fact that the cluster structure of Ag{dollar}\sb{lcub}12{rcub}{dollar}-MT was widely believed to be similar to that of Cu{dollar}\sb{lcub}12{rcub}{dollar}-MT with a trigonal geometry based on the optical spectroscopic studies.;The sulfur K-edge XANES spectra indicate that the formal oxidation state of sulfur is S(-II) for all MT proteins. No oxidation of sulfur in any protein samples due to x-ray damage has been noticed. Small differences in the chemical shifts could not be observed due to the broad linewidths at the K-edge. Silver(I) proteins exhibit different XANES spectral features from Cd{dollar}\sb7{dollar}-, and Zn{dollar}\sb7{dollar}-MT 2, implying that the sulfur environment in the Ag-MT proteins is different from that in Cd-, and Zn-MT proteins.;Silver(I) binding to Cd{dollar}\sb7{dollar}-MT 1 is unusual in that the mixed Cd,Ag-MT 1 produces well-defined CD spectra. Ag(I) binds to Cd{dollar}\sb7{dollar}MT 1 in a distributed manner, and addition of the first 6 Ag(I) to Cd{dollar}\sb{lcub}7-{rcub}{dollar}MT 1 results in the loss of the CD spectral features representative of the Cd(II)-thiolate cluster in the {dollar}\alpha{dollar} domain. Formation of this mixed Cd,Ag-MT species with a distinct three-dimensional structure must involve a compromise between the tetrahedral geometry required for Cd(n) binding and linear coordination preferred for Ag(I) binding. All 7 Cd(II) ions are completely replaced by 17 Ag(I) and no Cd(II) is bound to the cysteinyl thiolate groups. This final species of Ag{dollar}\sb{lcub}17{rcub}{dollar}-MT 1 is quite stable with a linear coordination of Ag(I) in one domain. (Abstract shortened by UMI.)



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