Thiolate ligands in metallothionein confer redox activity on zinc clusters

Abstract

We postulate a novel and general mechanism in which the redox-active sulfur donor group of cyst(e)ine confers oxidoreductive characteristics on stable zinc sites in proteins. Thus, the present, an earlier, and accompanying manuscripts Metallothionein (MT) is a unique metalloprotein in which cysteine constitutes one-third of its amino acids and histidine and aromatic amino acids all are completely absent. All 20 cysteines bind seven zinc atoms such that each metal atom has a complement of four cysteine ligands. It is important to emphasize that the by now well known multinuclear cluster network, identified by both x-ray diffraction and NMR spectroscopy only less than a decade ago (1,2), shows the exceptional structural arrangement that was hitherto unknown in zinc/sulfur chemistry and has thus far been encountered solely in biological material such as MT. The zinc/cysteinyl interactions in the two clusters are of two different types: they are either bridging or terminal cysteine thiolates. In the ␤-domain cluster, three bridging and six terminal cysteine thiolates provide a coordination environment that is formally identical for each of the three zinc atoms. In the ␣-domain cluster, there are two different zinc sites; two of them have one terminal ligand and three bridging ligands, respectively, while the other two have two terminal and two bridging ligands.We have performed and advocated experiments to relate the structure of MT to its possible function(s) on the basis of the nature of zinc coordination in MT (3-5). We have suggested that the characteristics of the cluster motif might be the key to the mode of cellular zinc distribution (6). MT binds zinc with high thermodynamic stability [K d ϭ 1.4 ϫ 10 Ϫ13 M for human MT at pH 7.0 (7)] while simultaneously providing a mechanism for kinetic lability whereby zinc can be released at rates that are orders of magnitude greater than those observed for zinc metalloenzymes. Zinc, as well as cadmium, is known to undergo rapid inter-and intracluster exchange (8, 9).MT apparently binds zinc with higher affinity than do many other proteins. For MT to serve as a source for the distribution of zinc, mechanisms would be required that could regulate the binding and release of the metal. It has been shown that an interaction of MT with glutathione disulfide (GSSG) or other biological disulfides releases zinc (10, 11) and that the combination of reduced glutathione (GSH) and GSSG enhances transfer of zinc from MT to an apoenzyme (12). This has led us to infer that the reactivity and redox behavior of the sulfur ligands in the MT clusters are crucial for the dynamic state of zinc. We proposed that the zinc-sulfur cluster chemistry might be sensitive to changes of the cellular redox state and that oxidizing conditions induce the transfer of zinc from its binding sites in MT to those of lower affinity in other proteins.We here provide additional support for this concept and show that a number of compounds, including some of potential biological importance, can oxidize the ...

p38 Activation Is Required Upstream of Potassium Current Enhancement and Caspase Cleavage in Thiol Oxidant-Induced Neuronal Apoptosis

“…Indeed, NO has been shown previously to trigger zinc release from metallothionein (Pearce et al, 2000), a metalloprotein that also releases zinc after DTDP oxidation (Jiang et al, 1998;Maret and Vallee, 1998). We also find evidence that the intracellular signaling cascades activated by DTDP appear to be similar to those observed by other researchers after exposure to extracellular zinc.…”

Effects of thioredoxin reductase-1 deletion on embryogenesis and transcriptome

“…This, in turn, might lead to reduced flux through glutathione-dependent enzymes, including GST, and reduced GST activity might stimulate feedback induction of GST mRNA accumulation [56]. Similarly, the iron-thiolate centers in MTs confer redox sensitivity onto heavy metal-binding by the protein [57][58][59]. As a result, txnrd1 disruption might antagonize heavy metal chelation by MTs, resulting in elevated levels of free intracellular zinc, a potent inducer of mt gene expression [60,61], and causing the observed increase in MT mRNA levels.…”

Nutrient-gene interactions: a single nutrient and hundreds of target genes

“…Although zinc does not directly participate in redox reactions, it contributes to the protection of biological structures from damage by free radicals (Pearce et al, 2000;Powell, 2000). This may be mediated in the first place by its effects on the expression level of metallothioneins (MTs) that act secondarily as free radical scavengers or by its co-activation function in SOD, which protects cellular thiols and reduces the generation of free radicals from iron-dependent processes (Maret, 1994(Maret, , 1995Maret and Vallee, 1998;Maret et al, 1999). These functions become apparent during zinc deficiency, with markedly increased levels of lipid peroxidation in plasma membranes as well as in mitochondrial and microsomal membranes.…”

“…Metallothioneins (MTs) are essential to intracellular zinc homeostasis by sequestration of the metal and thereby controlling available free zinc (Sato et al, 1984;Beyersmann and Haase, 2001;Coyle et al, 2002). The cysteine sulfur ligands in the cluster structure of MTs can be oxidized and reduced with concomitant changes in bound zinc (Maret, 1994(Maret, , 1995Maret and Vallee, 1998;Maret et al, 1999). There are four known functional MT genes, MT-I to MT-IV, that show different expression patterns (Ebadi et al, 1995;Vallee, 1995).…”