Anthony G. Wedd

Metal ion ligands in hyperaccumulating plants

Metal-hyperaccumulating plants have the ability to take up extraordinary quantities of certain metal ions without succumbing to toxic effects. Most hyperaccumulators select for particular metals but the mechanisms of selection are not understood at the molecular level. While there are many metal-binding biomolecules, this review focuses only on ligands that have been reported to play a role in sequestering, transporting or storing the accumulated metal. These include citrate, histidine and the phytosiderophores. The metal detoxification role of metallothioneins and phytochelatins in plants is also discussed.

Unification of the Copper(I) Binding Affinities of the Metallo-chaperones Atx1, Atox1, and Related Proteins DETECTION PROBES AND AFFINITY STANDARDS

Literature estimates of metal-protein affinities are widely scattered for many systems, as highlighted by the class of metallo-chaperone proteins, which includes human Atox1. The discrepancies may be attributed to unreliable detection probes and/or inconsistent affinity standards. In this study, application of the four CuI ligand probes bicinchoninate, bathocuproine disulfonate, dithiothreitol (Dtt), and glutathione (GSH) is reviewed, and their CuI affinities are re-estimated and unified. Excess bicinchoninate or bathocuproine disulfonate reacts with CuI to yield distinct 1:2 chromatophoric complexes [CuIL2]3− with formation constants β2 = 1017.2 and 1019.8 m−2, respectively. These constants do not depend on proton concentration for pH ≥7.0. Consequently, they are a pair of complementary and stable probes capable of detecting free Cu+ concentrations from 10−12 to 10−19 m. Dtt binds CuI with KD ∼10−15 m at pH 7, but it is air-sensitive, and its CuI affinity varies with pH. The CuI binding properties of Atox1 and related proteins (including the fifth and sixth domains at the N terminus of the Wilson protein ATP7B) were assessed with these probes. The results demonstrate the following: (i) their use permits the stoichiometry of high affinity CuI binding and the individual quantitative affinities (KD values) to be determined reliably via noncompetitive and competitive reactions, respectively; (ii) the scattered literature values are unified by using reliable probes on a unified scale; and (iii) Atox1-type proteins bind CuI with sub-femtomolar affinities, consistent with tight control of labile Cu+ concentrations in living cells.

Metal Binding Affinities of Arabidopsis Zinc and Copper Transporters: Selectivities Match the Relative, but Not the Absolute, Affinities of their Amino-Terminal Domains

HMA2, HMA4, and HMA7 are three of the eight heavy metal transporting P(1B)-type ATPases in the simple plant Arabidopsis thaliana. The first two transport Zn(2+), and the third transports Cu(+). Each protein contains soluble N-terminal metal-binding domains (MBDs) that are essential for metal transport. While the MBD of HMA7 features a CxxC sequence motif characteristic of Cu(I) binding sites, those of HMA2 and HMA4 contain a CCxxE motif, unique for plant Zn(2+)-ATPases. The three MBDs HMA2n (residues 1-79), HMA4n (residues 1-96), and HMA7n (residues 56-127) and an HMA7/4n chimera were expressed in Escherichia coli. The chimera features the ICCTSE motif from HMA4n inserted in place of the native MTCAAC motif of HMA7n. Binding affinities for Zn(II) and Cu(I) of each MBD were determined by ligand competition with a number of chromophoric probes. The challenges of using these probes reliably were evaluated, and the relative affinities of the MBDs were verified by independent cross-checks. The affinities of HMA2n and HMA4n for Zn(II) are higher than that of HMA7n by a factor of 20-30, but the relative affinities for Cu(I) are inverted by a factor of 30-50. These relativities are consistent with their respective roles in metal selection and transportation. Chimera HMA7/4n binds Cu(I) with an affinity between those of HMA4n and HMA7n but binds Zn(II) more weakly than either parent protein does. The four MBDs bind Cu(I) more strongly than Zn(II) by factors of >10(6). It is apparent that the individual MBDs are not able to overcome the large thermodynamic preference for Cu(+) over Zn(2+). This information highlights the potential toxicity of Cu(+) in vivo and why copper sensor proteins are approximately 6 orders of magnitude more sensitive than zinc sensor proteins. Metal speciation must be controlled by multiple factors, including thermodynamics (affinity), kinetics (including protein-protein interactions), and compartmentalization. The structure of Zn(II)-bound HMA4n defined by NMR confirmed the predicted ferredoxin betaalphabetabetaalphabeta fold. A single Zn atom was modeled onto a metal-binding site with protein ligands comprising the two thiolates and the carboxylate of the CCxxE motif. The observed (113)Cd chemical shift in [(113)Cd]HMA4n was consistent with a Cd(II)S(2)OX (X = O or N) coordination sphere. The Zn(II) form of the Cu(I) transporter HMA7n is a monomer in solution but crystallized as a polymeric chain [(Zn(II)-HMA7n)(m)]. Each Zn(II) ion occupied a distorted tetrahedral site formed from two Cys ligands of the CxxC motif of one HMA7n molecule and the amino N and carbonyl O atoms of the N-terminal methionine of another.

PHOTOSENSITIZED DEGRADATION OF DNA BY DAUNOMYCIN

Abstract Visible irradiation of DNA‐daunomycin solutions resulted in a decrease of viscosity of the DNA and an increase of the rate of denaturation of DNA in formaldehyde. These changes are consistent with the induction of single‐strand breaks in the DNA, some of which pair to cause fragmentation of the DNA. The DNA damage increases with drug: nucleotide ratio up to 0.2 and is diminished beyond that range. The damage also increases with ionic strength up to 0.6 M and is diminished above that value. These results suggest that the non‐intercalated form of the drug is involved in the photosensitization process. Radicals that are produced accompanying the degradation have been trapped by 5,5‐dimeth‐yl‐l‐pyrroline‐1‐oxide and identified as hydroxyl radicals from their ESR spectrum. The DNA photosensitized damage is completely inhibited when hydroxyl radicals are removed by the spin‐trap, suggesting a direct role for the hydroxyl radicals in the DNA photosensitized degradation process. The implications of the photosensitized DNA damage and the production of hydroxyl radicals in this process are discussed with respect to the medical uses and chemotherapeutic role of daunomycin.

A C-terminal domain of the membrane copper pump Ctr1 exchanges copper(I) with the copper chaperone Atx1.

A cloned C-terminal domain of the yeast high-affinity copper uptake pump Ctr1 exchanges Cu(I) rapidly with the yeast copper chaperone Atx1: 10(-2) < Kex < 10(+2).

Binding, Transport and Storage of Metal Ions in Biological Cells

Overview Sodium: Its Role in Bacterial Metabolism Potassium Magnesium Calcium Vanadium Chromium Molybdenum and Tungsten Manganese Iron in Eukarya Iron Uptake and Homeostasis in Prokaryotic Microorganisms Iron-Sulfur Clusters Ferritin and its Role in Iron Homeostasis Cobalt and Nickel Platinum Binding, Transport and Storage of Copper in Prokaryotes Binding, Transport, and Storage of Copper in Mitochondria Binding, Transport and Storage of Copper in Eukaryotes Silver Gold Metallothioneins Zinc Cadmium Mercury Antimony and Bismuth Actinides in Biological Systems Aluminium Binding, Transport and Storage of Lead

Photochemical oxidation of water and reduction of polyoxometalate anions at interfaces of water with ionic liquids or diethylether

Photoreduction of [P2W18O62]6-, [S2Mo18O62]4-, and [S2W18O62]4- polyoxometalate anions (POMs) and oxidation of water occurs when water–ionic liquid and water–diethylether interfaces are irradiated with white light (275–750 nm) or sunlight. The ionic liquids (ILs) employed were aprotic ([Bmim]X; Bmim = (1-butyl-3-methylimidazolium,X = BF4,PF6) and protic (DEAS = diethanolamine hydrogen sulphate; DEAP = diethanolamine hydrogen phosphate). Photochemical formation of reduced POMs at both thermodynamically stable and unstable water–IL interfaces led to their initial diffusion into the aqueous phase and subsequent extraction into the IL phase. The mass transport was monitored visually by color change and by steady-state voltammetry at microelectrodes placed near the interface and in the bulk solution phases. However, no diffusion into the organic phase was observed when [P2W18O62]6- was photo-reduced at the water–diethylether interface. In all cases, water acted as the electron donor to give the overall process: 4POM + 2H2O + hν → 4POM- + 4H+ + O2. However, more highly reduced POM species are likely to be generated as intermediates. The rate of diffusion of photo-generated POM- was dependent on the initial concentration of oxidized POM and the viscosity of the IL (or mixed phase system produced in cases in which the interface is thermodynamically unstable). In the water-DEAS system, the evolution of dioxygen was monitored in situ in the aqueous phase by using a Clark-type oxygen sensor. Differences in the structures of bulk and interfacial water are implicated in the activation of water. An analogous series of reactions occurred upon irradiation of solid POM salts in the presence of water vapor.

The challenges of using a copper fluorescent sensor (CS1) to track intracellular distributions of copper in neuronal and glial cells

Copper is an essential biometal involved in critical cell functions including respiration. However, the mechanisms controlling its sub-cellular localization during health and disease remain poorly understood. This is partially due to the difficulty of detecting a metal ion that is bound tightly to metallo-chaperone and detoxification molecules in the cell. A BODIPY-based Cu fluorescent probe CS1 (Cu sensor 1) has been applied in innovative attempts to visualize monovalent Cu pools within cells (Zeng et al., J. Am. Chem. Soc., 2006, 128, 10–11). Inspired by this work, we sought to use CS1 to identify sub-cellular localization of Cu delivered to M17 neuronal or U87MG glial cells by a cell-permeable bis(thiosemicarbazonato)Cu(II) complex, CuII(gtsm). This complex increases cellular Cu concentrations by factors of 10–100 when compared to treatment with equivalent concentrations of CuCl2 (Donnelly et al., J. Biol. Chem., 2008, 283, 4568–4577). However, we were unable to identify any specific increase in CS1 fluorescence in neurons or glia treated with CuCl2 or with CuII(gtsm), despite controls revealing a large increase in total cellular Cu with the latter treatment. Further in vitro characterization of CS1 suggests that, consistent with its relatively weak affinity for CuI (KD ≈ 10−11 M), it is unlikely to compete with endogenous proteins with sub-picomolar affinities, nor with glutathione, the endogenous redox buffer essential for functional maintenance of many proteins, including those that bind CuI. Moreover, we show that CS1 is localized predominantly to lysosomes and that the observed background fluorescence may be attributed to increased concentrations of apo-CS1 in this organelle or to the probe gaining access to CuI made available via recycling of nutrient Cu in the acidic lysosome. It was possible to observe a consistent increase in CS1 fluorescence in neuronal cells exposed to stress. For example, treatment with buthionine sulfoximine decreased cellular glutathione levels and led to enhanced CS1 fluorescence, but the total cellular Cu levels did not correlate with the increased fluorescence. In addition, cells treated with reagents that are known to alter cellular pH homeostasis provided an enhanced fluorescence. Our findings demonstrate that the source of enhanced CS1 fluorescence in Cu-supplemented cells must be interpreted with caution. It may be a consequence of altered cell pH, compromised vesicle maturation, increased CS1 uptake and/or trapping of CS1 in the lysosomal compartment.

Ionic liquid-enhanced photooxidation of water using the polyoxometalate anion [P2W18O62](6-) as the sensitizer.

Simple polyoxometalate anions are known to be photoreduced in molecular solvents in the presence of 2-propanol or benzyl alcohol. The use of ionic liquids (ILs) as the solvent is now reported to also allow the photooxidation of water to be achieved. In particular, the photochemistry of the classic Dawson polyoxometalate salt K(6)[P(2)W(18)O(62)] has been studied in detail when water is present in the aprotic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and the protic IL, diethanolamine hydrogen sulfate (DEAS). In these and other ILs, irradiation with white light (wavelength 275-750 nm) or UV light (wavelength 275-320 nm) leads to overall reduction of the [P(2)W(18)O(62)](6-) anion to [P(2)W(18)O(62)](7-) and concomitant oxidation of water to dioxygen and protons. The modified structure of bulk water present in ILs appears to facilitate its oxidation. Analogous results were obtained in aqueous solutions containing the protic IL as an electrolyte. The photoproducts (reduced polyoxometalate anion, dioxygen, and protons) were identified by, respectively, voltammetry, a Clark electrode, and monitoring of pH. The formal reversible potentials E(0)(F) for [P(2)W(18)O(62)](6-/7-/8-/9-/10-) couples are much more positive than in molecular solvents. The [P(2)W(18)O(62)](8-) and more reduced anions, if formed as intermediates, would efficiently reduce photoproducts H(+) or dioxygen to produce [P(2)W(18)O(62)](7-), rather than reform to [P(2)W(18)O(62)](6-). Thus, under photoirradiation conditions [P(2)W(18)O(62)](7-) acts as a kinetic sink so that in principle indirect splitting of water to produce dioxygen and dihydrogen can be achieved. The equivalent form of photooxidation does not occur in liquid water or in molecular solvents such as MeCN and MeCN/CH(2)Cl(2) containing added water, but does occur for solid K(6)[P(2)W(18)O(62)] in contact with water vapor.

Coupled Electron- and Proton-Transfer Processes in the Reduction of α-[P2W18O62]6- and α-[H2W12O40]6- As Revealed by Simulation of Cyclic Voltammograms

Quantitative analysis of the complex problem of coupled electron- and proton-transfer steps during reduction of the polyoxo anions α-[P(2)W(18)O(62)](6)(-) and α-[H(2)W(12)O(40)](6)(-) in aqueous NaCl (0.5 M) has been achieved by simulation of cyclic voltammograms (Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem. 1994, 66, 589A) over wide ranges of anion concentration, pH, and scan rate. Since there are too many unknown parameters to attempt a one-step global form of simulation, a systematic, stepwise approach has been adopted by progressively accessing regimes of increasing voltammetric complexity. This protocol allows experimental behavior in each system over 5 orders of magnitude in proton concentration to be simulated by estimation of three protonation constants combined with experimentally determined reversible half-wave potentials for the two one-electron processes involved. Fast electron transfer and protonation kinetics are assumed. The importance of the values chosen for the diffusion coefficients of the proton and polyoxo anion species is considered. The simulations account for the fact that pairs of one-electron processes coalesce to give an apparent two-electron process in the pH range 1-6 for reduction of both anions.