Metallothionein tetrahedral coordination

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...

These complexes are known with planar and non-planar M3X3 cores, and with tetrahedral or square-planar metal coordination. The X3M3 cycle in chair conformation and with tetrahedral metal coordination occurs in (p-SMe)3B3X6 (X = Cl, Br)38 and is proposed in the form of (p-Scys)3Cd3(Scys)6 for one of the two clusters with cysteinate coordination in metallothionein proteins.39 The twist-boat conformation occurs in S3Sn3Me640 and probably in the related compounds S3M3R6 (M = Ge, Sn, Pb R = Me, Bu, Ph).41 The same structural unit with distorted twist-boat stereochemistry occurs in the (p-S)3Fe3(Scys)5L cluster recently discovered in several ferredoxins.15 42... 

Metallothioneins are cysteine-rich proteins capable of binding a range of metal ions, such as Zn, Cu, Cd, and Hg, via thiolate coordination. The structure of a Zn/Cd metallothionein shows a triangular array of metals, each coordinated tetrahedrally to thiolate cysteines. 

For -t-2 cations such as zinc(II) and cadmium(ll) each metallothionein molecule contains up to seven metal atoms. X-ray studies indicate that the metal atoms are in approximately tetrahedral sites bound to the cysteine sulfur atoms. The soft mer-cury(II) ion has a higher affinity for sulfur and will displace cadmium from metallothionein. At first the mercury ions occupy tetrahedral sites but as the number increases, the geometries of the metal sites and protein change until about nine Hg(Il) atoms are bound in a linear (S—Hg—S) fashion.92 Up to twelve + 1 cations such as copper(l) and silver(I) can bind per molecule, indicating a coordination number lower than four, probably three (see Problem 12.34),... 

The presence of Hg, Cd, Cu and Zn induces the production of thioneins in the liver and kidneys of mammals. Between 4 and 12 metal centres can be bound by one thionein Zn Hg, Cd centres are likely to be in tetrahedral environments, while Cu may be 3-coordinate. The structure of the Cd/Zn-containing metallothionein isoform II from rat liver has been determined by X-ray difiraction, and Figure 28.6a illustrates the folded protein chain consisting of 61 amino acid residues of which 20 are Cys groups. One Cd + and two Zn centres are bound in... 

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