Conformation Control of Peptides by Metal Ions. Coordination Conformation Correlation Observed in a Model for Cys-X-Y-Cys/M(2+) in Proteins

Minimal motif peptide structure of metzincin clan zinc peptidases in micelles

It is well known that the functions of metalloproteins generally originate from their metal-binding motifs. However, the intrinsic nature of individual motifs remains unknown, particularly the details about metal-binding effects on the folding of motifs; the converse is also unknown, although there is no doubt that the motif is the core of the reactivity for each metalloprotein. In this study, we focused our attention on the zinc-binding motif of the metzincin clan family, HEXXHXXGXXH; this family contains the general zinc-binding sequence His-Glu-Xaa-Xaa-His (HEXXH) and the extended GXXH region. We adopted the motif sequence of stromelysin-1 and investigated the folding properties of the Trp-labeled peptides WAHEIAHSLGLFHA (STR-W1), AWHEIAHSLGLFHA (STR-W2), AHEIAHSLGWFHA (STR-W11), and AHEIAHSLGLFHWA (STR-W14) in the presence and absence of zinc ions in hydrophobic micellar environments by circular dichroism (CD) measurements. We accessed successful incorporation of these zinc peptides into micelles using quenching of Trp fluorescence. Results of CD studies indicated that two of the Trp-incorporated peptides, STR-W1 and STR-W14, exhibited helical folding in the hydrophobic region of cetyltrimethylammonium chloride micelle. The NMR structural analysis of the apo STR-W14 revealed that the conformation in the C-terminus GXXH region significantly differred between the apo state in the micelle and the reported Zn-bound state of stromelysin-1 in crystal structures. The structural analyses of the qualitative Zn-binding properties of this motif peptide provide an interesting Zn-binding mechanism: the minimum consensus motif in the metzincin clan, a basic zinc-binding motif with an extended GXXH region, has the potential to serve as a preorganized Zn binding scaffold in a hydrophobic environment.

Computational studies of the coordination stereochemistry, bonding, and metal selectivity of mercury

Knowledge of the bonding and selectivity of organic mercury, [H3C-Hg]+ (MeHg+), and inorganic Hg2+ for protein and DNA functional groups is important for understanding the mechanism of heavy metal poisoning. Herein, we elucidate (1) the differences between inorganic Hg2+ and organic MeHg+ in their interactions with different ligands of biological interest, (2) the protein and DNA functional groups that Hg2+ and MeHg+ target in aqueous solution, and (3) the likelihood of "soft" Hg2+ displacing the "borderline" Zn2+ bound to "harder" nitrogen/oxygen-containing side chains such as His and Asp/Glu. The results reveal that, relative to Hg2+, the lower positive charge on MeHg+ results in a longer and weaker bond with a given ligand, in accord with the observed kinetic lability of MeHg+ complexes. They also indicate that negatively charged or polar amino acid side chains containing S-/O-/S/N donors could coordinate to both organic MeHg+ and inorganic Hg2+. In addition, Gua and Cyt could also coordinate to MeHg+ and disrupt Gua...Cyt base pairing. A key novel finding is that Hg2+ is a far better electron acceptor than Zn2+, and can thus accept more negative charge from the Zn ligands than the native Zn2+, thus enhancing Hg-ligand interactions and enabling Hg2+ to displace the native cofactor from zinc essential enzymes and "structural" Zn proteins. The results herein support several possible mechanisms for Hg poisoning. Ways that mercury poisoning could be prevented in cells are discussed.

Complexation of mercury(II) in soil organic matter: EXAFS evidence for linear two-coordination with reduced sulfur groups

The chemical speciation of inorganic mercury (Hg) is to a great extent controlling biologically mediated processes, such as mercury methylation, in soils, sediments, and surface waters. Of utmost importance are complexation reactions with functional groups of natural organic matter (NOM), indirectly determining concentrations of bioavailable, inorganic Hg species. Two previous extended X-ray absorption fine structure (EXAFS) spectroscopic studies have revealed that reduced organic sulfur (S) and oxygen/ nitrogen (O/N) groups are involved in the complexation of Hg(II) to humic substances extracted from organic soils. In this work, covering intact organic soils and extending to much lower concentrations of Hg than before, we show that Hg is complexed by two reduced organic S groups (likely thiols) at a distance of 2.33 A in a linear configuration. Furthermore, a third reduced S (likely an organic sulfide) was indicated to contribute with a weaker second shell attraction at a distance of 2.92-3.08 A. When all high-affinity S sites, corresponding to 20-30% of total reduced organic S, were saturated, a structure involving one carbonyl-O or amino-N at 2.07 A and one carboxyl-O at 2.84 A in the first shell, and two second shell C atoms at an average distance of 3.14 A, gave the best fit to data. Similar results were obtained for humic acid extracted from an organic wetland soil. We conclude that models that are in current use to describe the biogeochemistry of mercury and to calculate thermodynamic processes need to include a two-coordinated complexation of Hg(II) to reduced organic sulfur groups in NOM in soils and waters.

New model potentials for sulfur–copper(I) and sulfur–mercury(II) interactions in proteins: Fromab initio to molecular dynamics

We have developed new force field and parameters for copper(I) and mercury(II) to be used in molecular dynamics simulations of metalloproteins. Parameters have been derived from fitting of ab initio interaction potentials calculated at the MP2 level of theory, and results compared to experimental data when available. Nonbonded parameters for the metals have been calculated from ab initio interaction potentials with TIP3P water. Due to high charge transfer between Cu(I) or Hg(II) and their ligands, the model is restricted to a linear coordination of the metal bonded to two sulfur atoms. The experimentally observed asymmetric distribution of metal ligand bond lengths (r) is accounted for by the addition of an anharmonic (r3) term in the potential. Finally, the new parameters and potential, introduced into the CHARMM force field, are tested in short molecular dynamics simulations of two metal thiolates fragments in water. (Brooks BR et al. J Comput Chem 1983, 4, 1987.1).

Relation between Intramolecular NH···S Hydrogen Bonds and Coordination Number in Mercury(II) Complexes with Carbamoylbenzenethiol Derivatives

A novel series of bis(carbamoylthiophenolato)mercury(II) complexes, [Hg(S-RNHCOC6H4)2] (1, R = 2-t-Bu; 2, R = 2-CH3; 3, R = 2-C6H5CH2; 4, R = 4-t-Bu), and a tetrakis(carbamoylthiophenolato)mercury(II) complex, (NEt4)2[Hg-(S-2-CH3NHCOC6H4)4] (5), were synthesized and characterized by 1H NMR, IR, 199Hg NMR, and crystallographic analyses. The bis(carbamoylthiophenolato)mercury complexes 1-3 do not have intramolecular NH...S hydrogen bonds between the amide NH group and the sulfur atom coordinated to mercury, whereas the tetrakis(thiophenolato)mercury complex 5 does have an intramolecular NH...S hydrogen bond. A relatively weak NH...S hydrogen bond in 5 can be seen in the 1H NMR spectra and the IR spectra in chloroform and in the solid state. The 199Hg NMR spectra in bis(carbamoylthiophenolato)mercury complexes 1-4 show a downfield shift, with an increase in the flow of electrons to mercury(II) from the oxygen atom due to the intramolecular Hg...O bonding interaction. Conversely, the 199Hg NMR spectra in 5 show a high-field shift with a decrease in the flow of electrons to mercury(II) from the sulfur atom due to the intramolecular NH...S hydrogen bond.

Is the cytoplasmic loop of MerT, the mercuric ion transport protein, involved in mercury transfer to the mercuric reductase?

In MerT, the mercury transporter, a first cysteine pair, located in the first trans-membrane helix, receives mercury from the periplasm. Then, a second cysteine pair, housed in a cytoplasmic loop connecting the second and the third trans-membrane helices, is thought to transfer the metal to another cysteine pair located in the N-terminal extension of the mercuric reductase. We found that a 23-amino acid synthetic peptide corresponding to the cytoplasmic loop can bind one mercury atom per molecule and that this mercury atom can be transferred specifically to MerAa. The solution structure of Hg-bound ppMerT has been solved by 1H NMR spectroscopy.

Novel model peptide for Atx1-like metallochaperones

The novel cyclodecapeptide c(GMTCSGCSRP) is able to bind soft metals with a selectivity for Hg(2+) and Cu(+) over Pb(2+), Cd(2+) and Zn(2+), and is demonstrated to be an excellent structural model of the binding loop of the copper metallochaperone Atx1 in its apo and mercury loaded forms.

Homoleptic group 12 metal bis(mercaptoimidazolyl)borate complexes M(Bm(R))2 (M = Zn, Cd, Hg)

The sodium salt of the bis(2-mercapto-1-methylimidazolyl)borate anion [Bm(Me)](-) and those of the new bis(2-mercapto-1-alkylimidazolyl)borates [Bm(R)](-) (R = Bz, Bu(t), p-Tol) have been readily obtained from NaBH(4) and the appropriate 2-mercapto-1-alkylimidazoles. To contrast the binding preferences of the group 12 metals in a sulfur-rich environment, the four complete series of homoleptic complexes M[Bm(R)](2) (M = Zn, Cd, Hg), including the first bis(mercaptoimidazolyl)borate derivatives of cadmium and mercury, have been prepared. X-ray diffraction studies of Cd[Bm(Me)](2) and M[Bm(tBu)](2) (M = Zn, Cd, Hg) show the presence of distorted tetrahedral [MS(4)] central cores supplemented by two weak vicinal M.H-B bonds, interactions which appear to be a common feature in the coordination chemistry of Bm(R) ligands. In the case of zinc, it has been found that only in the presence of bulky ligands, as in Zn[Bm(tBu)](2), may an unexpected expansion in the coordination number from four to six be induced. This observation suggests the viability of octahedral intermediates in the processes whereby certain zinc enzymes transfer or exchange metal ions.