Mercury(II) binding to metallothioneins

Abstract

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...

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