Metallothioneins metal liganding

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

The first EXAFS studies of a cadmium environment in a protein were achieved for CdsZna- and Cdy-Metallothionein from rat liver (45). The two samples manifest identical EXAFS and the data are consistent with a shell of four sulfur atoms at —2.51 A. These results demonstrate that the inequivalence of cadmium atoms observed by ii Cd NMR studies of metallothioneins (46) does not arise from marked variations in atom type, coordination number, or metal-ligand distances within the metals first coordination sphere. 

The absorption spectrum of cadmium LADH differs markedly from that of the zinc or cobalt enzyme and perhaps bears upon the nature of the metal-binding ligands (Figure 19). An intense band centered at 245 m/i with a molar absorptivity per cadmium atom of 10,200 is shown by the difference spectrum at the bottom of the figure zinc LADH is employed as the reference. Notably, the molar absorptivity of this band is nearly 14,000, close to that reported for the cadmium mercaptide chromophores of metallothionein (23). This is consistent with the hypothesis that sulfhydryl groups may serve as metal ligands in LADH. 

A further complication in the identification of target sites and chemical forms of metals is the kinetic lability of coordinate covalent bonds. Metal ligands exchange rapidly in and out of the coordination sphere, in particular for first-row transition metals. This kinetic lability varies between metals, and, as indicated above, is influenced by the nature of the ligand, whether mono- or multidentate, and by the pH and ionic strength of its immediate environment. Copper, for example, forms relatively low affinity complexes with albumin or amino acids, but is tightly bound to ceruloplasmin. Similarly, mercury and cadmium form kinetically labile complexes with amino acids, glutathione, or albumin, but more stable chelates with metallothionein. 

Figure 2 The NMR chemical shifts from " Cd-substituted metalloproteins. The chemical shifts ate represented as bars. For proteins with representatives from different sources, like parvaibumin and metalloihionein, the width of the bar indicate the spread in shift, including two for each parvaibumin and seven for each metallothionein. For most of the entries metal ligands are shown S for cysteine, S for methionine, N for histidine, and O for all types of oxygen ligands. NumbCTs within brackets refer to references in the list of references.

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... 

The detection of metal-binding proteins, especially of Cd- and Hg-binding metallothioneins or of the merR protein, induced numerous studies of model compounds of Cd and Hg with more or less simple sulfur- and selenium-containing ligands. 

In mammals, as in yeast, several different metallothionein isoforms are known, each with a particular tissue distribution (Vasak and Hasler, 2000). Their synthesis is regulated at the level of transcription not only by copper (as well as the other divalent metal ions cadmium, mercury and zinc) but also by hormones, notably steroid hormones, that affect cellular differentiation. Intracellular copper accumulates in metallothionein in copper overload diseases, such as Wilson s disease, forming two distinct molecular forms one with 12 Cu(I) equivalents bound, in which all 20 thiolate ligands of the protein participate in metal binding the other with eight Cu(I)/ metallothionein a molecules, with between 12-14 cysteines involved in Cu(I) coordination (Pountney et ah, 1994). Although the role of specific metallothionein isoforms in zinc homeostasis and apoptosis is established, its primary function in copper metabolism remains enigmatic (Vasak and Hasler, 2000). 

It turns out that the six-membered Cu3( -8)3 rings are paradigmatic units. This type of ring system has been incorporated into current models of metallothioneins [low-molecular-weight proteins which are believed to play a key role in metal metabolism (cf. references in 136)]. The structural chemistry of the Ag complexes seems to be different. Monocyclic Ag(8 ) rings can be linked via bridging ligands as in [(86)Ag(88)Ag(86)] (133) or condensed as in [Ag2(86)2] " (28) (126). 

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... 

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.

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