With the aim of extending our knowledge on the reaction pathways of Zn-metallothionein (MT) and apo-MT species in the presence of Hg(II), we monitored the titration of Zn 7 -MT, Zn 4 -aMT and Zn 3 -bMT proteins, at pH 7 and 3, with either HgCl 2 or Hg(ClO 4 ) 2 by CD and UV-vis spectroscopy. Detailed analysis of the optical data revealed that standard variables, such as the pH of the solution, the binding ability of the counter-ion (chloride or perchlorate), and the time elapsed between subsequent additions of Hg(II) to the protein, play a determinant role in the stoichiometry, stereochemistry and degree of folding of the Hg-MT species.Despite the fact that the effect of these variables is unquestionable, it is difficult to generalize. Overall, it can be concluded that the reaction conditions [pH, time elapsed between subsequent additions of Hg(II) to the protein] affect the structural properties more substantially than the stoichiometry of the Hg-MT species, and that the role of the counter-ion becomes particularly apparent on the structure of overloaded Hg-MT.Keywords: mercury(II) binding; mercury-metallothionein; metallothionein; a-metallothionein; b-metallothionein.Mercury thiolates provide representative examples of the structural diversity shown by the extensive family of metal thiolates [1][2][3][4]. The most striking features of mercury thiolates in the solid phase are the different structures obtained when Hg(II) is co-ordinated to very similar thiolate ligands [5,6] and the distinctive behavior of Hg(II) towards a particular thiolate compared with that of Zn (II) or Cd(II) [7], which has been referred to as the zinc family paradox [3]. Moreover, correlations between solid-state and solution complexes cannot be easily established. Overall, the diverse co-ordination preferences of Hg(II) ions (mainly tetrahedral, trigonal-planar and digonal) and their coexistence in polynuclear complex species, the various ligation modes of the thiolate ligands (i.e. terminal, l 2 -bridging or l 3 -bridging) and the possibility of secondary Hg(II)-sulfur interactions [8] make it difficult to anticipate the structure of a particular mercury thiolate complex [1,3,9]. This results from the interplay of not only the above factors, but also the reaction conditions. Of these, the presence of additional coordinating species, such as halide ions, make the bonding situation for mercury even less straightforward than in the case of homoleptic mercury thiolates [10,11].The biological chemistry of mercury is dominated by co-ordination to cysteine thiolate groups in agreement with the preference of this metal ion for the soft sulfur ligands. The high binding constants for binding of Hg(II) to cysteine residues account for the irreversible replacement of essential metals (Zn, Cu) in cysteine-containing metalloproteins and thus for the high toxicity of mercury to living systems. Within the same context of the highly favored thermodynamically Hg-S bond, resistance to Hg(II) toxicity in several bacteria is based on an ensemble of prote...
In this study, performed in Mediterranean brackish ponds during spring season, we assessed the effects of biotic interactions and abiotic factors on the size and taxonomic structure of the phytoplankton and zooplankton. We used a taxonomic and a size diversity index as a descriptor of the community structure. We predicted that the size diversity of each trophic level would be mainly related to biotic interactions, such as size-based fish predation (in the case of zooplankton) and food resource availability (in the case of phytoplankton), whereas taxonomic diversity would be more affected by abiotic variables (e.g., conductivity, pond morphology). Our results showed a negative relationship between phytoplankton size diversity and food resource availability leading to low size diversities under food scarcity due to dominance of small species. Conductivity also negatively affected the phytoplankton size diversity, although slightly. Regarding zooplankton size diversity, none of predictors tested seemed to influence this index. Similar fish size diversities among ponds may prevent a significant effect of fish predation on size diversity of zooplankton. As expected, taxonomic diversity of phytoplankton and zooplankton was related to abiotic variables (specifically pond morphometry) rather than biotic interactions, which are usually body size dependent, especially in these species-poor brackish environments.
To elucidate the chemical interactions underlying the role of metallothioneins (MTs) in reducing the cytotoxicity caused by MeHg(II), we monitored in parallel by electronic absorption and CD spectroscopies the stepwise addition of MeHgCl stock solution to mammalian Zn 7 -MT1 and the isolated Zn 4 -aMT1 and Zn 3 -bMT1 fragments. The incorporation of MeHg + into Zn 7 -MT and Zn 3 -bMT entails total displacement of Zn (II) Mercury is a widespread contaminant that enters the environment from a variety of sources including industrial processes and hazardous waste sites. The ability of aquatic micro-organisms to convert metallic mercury into the methylmercury(II) cation (MeHg + ) is the key to its accumulation in fish, which then become a common source of exposure of humans to MeHg + [1,2]. Whereas the damaging pathological and biochemical consequences of MeHg + in humans have long been known, current studies are focusing on the effects of MeHg + on the central nervous system [3] and male fertility [4]. In both cases, a role for metallothioneins (MTs) in attenuating the cytotoxicity caused by MeHg + has been proposed [5][6][7]. A main feature of MTs, a family of ubiquitous low molecular mass proteins, is their extremely high content of cysteine residues. These bind to metal centers enabling them to serve as a heavymetal-detoxification system [8]. Considering the abundance of MTs in the central nervous system and the preference of Hg(II) ions for soft sulfur ligands, the study of MeHg-MT species from a chemical perspective is warranted.Although the interaction of MTs with Hg(II) ions has long been established [9], elucidation of the binding features of Hg-MT species has been hampered by the inherent difficulties of Hg(II) thiolate chemistry, which mainly arise from the diverse coordination preferences of Hg(II) and the various ligation modes of the thiolate ligands [10,11]. Nevertheless, the analysis of Hg(II) binding to MTs has been intensively studied [9]. In contrast, the chemistry of MeHg(II)-MT complexes has attracted much less attention. Earlier studies found MT to have no significant role in the detoxification of MeHg + [12] and to be unable to bind to MeHg + either in vivo or in vitro [13]. Subsequent attempts to induce brain MT by exposure to MeHg + gave inconsistent results: MT concentrations remained unchanged in rats [14,15], whereas MT and mRNA concentrations increased in MeHg + -treated rat neonatal astrocyte cultures [16]. However, there is mounting evidence that induction of MTs in astrocytes attenuates and even reverses the cytotoxicity caused by MeHg + [5,6], indicating binding of MeHg + by an astrocyte-specific MT isoform, MT1 [17].Existing data on Hg(II)-MT species cannot be extended to MeHg-MT complexes mainly because of the different coordination chemistry of Hg(II) and MeHg + towards thiolate ligands and thus towards the cysteine residues responsible for metal coordination in MTs. The coexistence of digonal, trigonal-planar and tetrahedral coordination geometries together with the presence of se...
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