Clusters metallothioneins

Mammalian metallothioneins typically bind seven metal ions in cluster structures, with bridging sulfur groups, as seen in the x-ray structure of the Cd5Zn2MT complex (86). It is therefore difficult to develop a simple formation-constant description for the binding of metal ions to MT (87), considering that protonation-deprotonation equilibria of the free protein itself should also be taken into account. However, the usefulness of Table VIII as a guide to the affinity of metal ions for mercapto donor ligands is seen in that the ability of metal ions to... 

Metallothioneins (MT) are unique 7-kDa proteins containing 20 cysteine molecules bounded to seven zinc atoms, which form two clusters with bridging or terminal cysteine thiolates. A main function of MT is to serve as a source for the distribution of zinc in cells, and this function is connected with the MT redox activity, which is responsible for the regulation of binding and release of zinc. It has been shown that the release of zinc is stimulated by MT oxidation in the reaction with glutathione disulfide or other biological disulfides [334]. MT redox properties led to a suggestion that MT may possesses antioxidant activity. The mechanism of MT antioxidant activity is of a special interest in connection with the possible antioxidant effects of zinc. (Zinc can be substituted in MT by some other metals such as copper or cadmium, but Ca MT and Cu MT exhibit manly prooxidant activity.)... 

A brief historical note on the structure of the iron-sulfur clusters in ferredoxins is relevant. After the first analytical results revealed the presence of (nearly) equimolar iron and acid-labile sulfur, it was clear that the metal center in ferredoxins did not resemble any previously characterized cofactor type. The early proposals for the Fe S center structure were based on a linear chain of iron atoms coordinated by bridging cysteines and inorganic sulfur (Blomstrom et al., 1964 Rabino-witz, 1971). While the later crystallographic analyses of HiPIP, PaFd, and model compounds (Herskovitz et al., 1972) demonstrated the cubane-type structure of the 4Fe 4S cluster, the original proposals have turned out to be somewhat prophetic. Linear chains of sulfide-linked irons are observed in 2Fe 2S ferredoxins and in the high-pH form of aconitase. Cysteines linked to several metal atoms are present in metallothionein. The chemistry of iron-sulfur clusters is rich and varied, and undoubtedly many other surprises await in the future. 

Fig. 37. (a and b) Two perpendicular views of rat liver metallothionein II, where the metal clusters are highlighted (C. D. Stout, personal communication). 

It is interesting that a unique secondary structural element, designated the half-turn, was indentified in preliminary NMR studies of rabbit metallothionein-2 (Wagner etal., 1986). The half-turn element is defined as a type II turn with (f>3 rotated from 90° to -90° its occurrence in the metallothionein-2 structure arises from the constraints placed on the relatively short polypeptide chain by the metal clusters. Although these constraints are not well understood and are certainly difficult to predict, the continued biophysical study of metallothionein-2 will certainly improve our understanding of protein-metal cluster interactions. 

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... 

The two clusters differ in their metal-binding properties. Mammalian Cd, Zn metallothionein contains four Cd ions in cluster A and two Cd ions and one Zn ion in cluster B. 1157,1159 Calf liver metallothionein contains three Cu ions in cluster B with Cd-displaceable zinc ions in cluster A.1160 Studies on the binding of zinc and cadmium to human liver metallothionein show that binding occurs first in cooperative fashion to cluster A, followed by cooperative binding to cluster B. Incubation of the 7Cd metallothionein resulted in loss of Cd ions from cluster B initially.1161... 

Several studies on models have been reported, both on the binding of a range of metals to apometallothionein and the design of ligands, particularly with Cys-X-Cys and Cys-X-Y-Cys arrangements as found for metallothionein.1163,1164 All metal derivatives appear to bind in metal-thiolate clusters.1149 Platinum has also been found bound to metallothionein in rat tissues following treatment with cisplatin and the trans isomer.1165... 

The protein metallothionein (MT) has a MW of 6000 and is rich in thiolate groups. It binds up to seven zinc(II), cadmium(II) and cobalt(II) ions. The seven ions are organized in two clusters. The so-called M4S11 cluster is shown in... 

Fig. 6.15. Schematic structure of the M4S11 cluster in metallothioneins (M = Zn, Cd, Co) [45-48]. There is no bridging ligand between metal 1 and metal 4.

Metallothionein proteins are the most abundant intracellular, metal-binding proteins (Andrews, 2000). Four metallothionein isoforms have been identified in the mouse (MT-I-MT-IV) and are clustered within 50 kilobases of each other. In humans, there is one MTII gene and a cluster of MT I genes on chromosome 16 (Searl et al., 1984 West et al., 1990 Heuchel et al., 1995). The mammalian metallothioneins generally consist of 61 amino acids and 20 of these are cysteines (Heuchel et al., 1995). These cysteines are important for the binding of such bivalent metal ions, such as zinc, copper and cadmium. 

Polypeptide Fold in the Two Metal Clusters of Metallothionein-2 by Nuclear Magnetic Resonance and Distance Geometry. 

Fig. 2. Proteins that bind Cu(I). (a) Saccharomyces cerevisiae metallothionein (Cupl, pdb code laqr). Cupl binds up to seven Cu(I) ions (medium gray spheres) using 10 cysteine sulfur atoms (light spheres) in a polythiolate cluster (Peterson et al., 1996). All bonds shorter than 2.8 A are shown as dotted lines, (b) Cucumis sativus stellacyanin (pdb code Ijer). Both Cu(l) and Cu(ll) are bound by a pseudo-trigonal planar arrangement of (His)2Cys residues with an axial Gin ligand (Hart et al., 1996). In other cupredoxins such as plastocyanin, a Met residue is the axial ligand (Adman, 1991).

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