Metallothionein three-metal-thiolate cluster

Fig. 39. The three-metal-thiolate cluster in the j8-domain of rat liver metallothionein II (C. D. Stout, personal communication).

Figure 6 Metallothionein metal coordination. (A,B) The four- and three-metal clusters of Mj-MT where M = Zn or Cd with tetrahedral MS., coordination. (C,D) Bidentate and monodentate chelation ofAu(l) by metallothionein as observed in Cd,Zn,Au-MT and (TmSAu)20 MT, respectively. (E,F) Isomorphous substitution and thiolate-bridged coordination of gold(l) in MT, which have not been reported to date. The formulae under structure C-F are the limiting cases for saturation of the protein by each mode of gold coordination...

Metallothioneins of mammals are single-stranded low molecular proteins with 61 - 68 amino acids in the structure. Cysteine accounts for one third of amino acid moieties eysteine moieties are located in the conservative sequences such as cys-x-cys, cys-x-y-cys and cys-cys (x and y are amino acids different from cysteine). Metal ions are boimd to sulfhydiyl groups of cysteine moieties that create tetraedric conformation of thiolated clusters in the ease of divalent ions [12]. MT has the highest affinity to Cu (stability constant lO - lO ), then to Cd (lO - lO ) and Zn (lO " - lO") and it is not able to bind Cu. Eighteen metal ions that can be bound by MT have been shown however, only Cu", Cd, Pb, Hg, Ag and Bi are able to displace boimded Zn from the structure of MT. Overall, it can coordinate up to 12 mono- or 7 divalent ions. Its tertiary structure is based on the presence of two domains, a and (3 (Fig. 1). a-domain (C-terminal) is more stable and contains four binding sites for divalent ions of heavy metals, P-domain (N-terminal) can bind three divalent metal ions [13]. 

---