Showing papers on "Zinc published in 1995"

Cloning and functional characterization of a mammalian zinc transporter that confers resistance to zinc.

Abstract: A cDNA encoding a zinc transporter (ZnT-1) was isolated from a rat kidney cDNA expression library by complementation of a mutated, zinc-sensitive BHK cell line. This cDNA was used to isolate the homologous mouse ZnT-1 gene. The proteins predicted for these transporters contain six membrane-spanning domains, a large intracellular loop and a C-terminal tail. ZnT-1 is homologous to zinc and cobalt resistance genes of yeast. Immunocytochemistry with an antibody to a myc epitope added to the C-terminus of ZnT-1 revealed localization to the plasma membrane. Transformation of normal cells with a mutant ZnT-1 lacking the first membrane-spanning domain conferred zinc sensitivity on wild-type cells, suggesting that ZnT-1 functions as a multimer. Deletion of the first two membrane-spanning domains resulted in a non-functional molecule, whereas deletion of the C-terminal tail produced a toxic phenotype. Mutant cells have a slightly higher steady-state level of intracellular zinc and high basal expression of a zinc-dependent reporter gene compared with normal cells. Mutant cells have a lower turnover of 65Zn compared with normal cells or mutant cells transformed with ZnT-1. We propose that ZnT-1 transports zinc out of cells and that its absence accounts for the increased sensitivity of mutant cells to zinc toxicity.

Protein motifs 5. Zinc fingers.

Abstract: The term zinc finger was first used to describe a 30-residue, repeated sequence motif found in an unusually abundant Xenopus transcription factor. It was proposed that each motif is folded around a central zinc ion to form an independent minidomain and that adjacent zinc fingers are combined as modules to make up a DNA-binding domain with the modules "gripping" the DNA (hence the term finger). We now know that these proposals were correct and that these DNA-binding motifs are found in many eukaryotic DNA-binding proteins. More recently, crystal structures of three different complexes between zinc finger domains and their target DNA binding sites have revealed a remarkably simple mode of interaction with DNA. The simplicity of the zinc finger structure, and of its interaction with DNA, is a very striking feature of this protein domain. After the discovery of the zinc finger motif, patterns of potential zinc ligands have been found in several other proteins, some of which also bind to DNA. Structural studies of these domains have revealed how zinc can stabilize quite diverse protein architectures. In total, 10 such small zinc-binding domains have been studied structurally. These form a diverse collection, but each in turn has been termed a zinc finger motif-although clearly what they have in common is only their zinc-binding property, which stabilizes an apparently autonomously folded unit.

The 3-D structure of a zinc metallo-beta-lactamase from Bacillus cereus reveals a new type of protein fold.

Abstract: The 3-D structure of Bacillus cereus (569/H/9) beta-lactamase (EC 3.5.2.6), which catalyses the hydrolysis of nearly all beta-lactams, has been solved at 2.5 A resolution by the multiple isomorphous replacement method, with density modification and phase combination, from crystals of the native protein and of a specially designed mutant (T97C). The current model includes 212 of the 227 amino acid residues, the zinc ion and 10 water molecules. The protein is folded into a beta beta sandwich with helices on each external face. To our knowledge, this fold has never been observed. An approximate internal molecular symmetry is found, with a 2-fold axis passing roughly through the zinc ion and suggesting a possible gene duplication. The active site is located at one edge of the beta beta sandwich and near the N-terminal end of a helix. The zinc ion is coordinated by three histidine residues (86, 88 and 149) and a water molecule. A sequence comparison of the relevant metallo-beta-lactamases, based on this protein structure, highlights a few well-conserved amino acid residues. The structure shows that most of these residues are in the active site. Among these, aspartic acid 90 and histidine 210 participate in a proposed catalytic mechanism for beta-lactam hydrolysis.

Biosorption of heavy metals by Saccharomyces cerevisiae.

Abstract: Abundant and common yeast biomass has been examined for its capacity to sequester heavy metals from dilute aqueous solutions. Live and non-living biomass of Saccharomyces cerevisiae differs in the uptake of uranium, zinc and copper at the optimum pH 4-5. Culture growth conditions can influence the biosorbent metal uptake capacity which normally was: living and non-living brewer's yeast: U > Zn > Cd > Cu; non-living baker's yeast: Zn > (Cd) > U > Cu; living baker's yeast: Zn > Cu approximately (Cd) > U. Non-living brewer's yeast biomass accumulated 0.58 mmol U/g. The best biosorbent of zinc was non-living baker's yeast (approximately 0.56 mmol Zn/g). Dead cells of S. cerevisiae removed approximately 40% more uranium or zinc than the corresponding live cultures. Biosorption of uranium by S. cerevisiae was a rapid process reaching 60% of the final uptake value within the first 15 min of contact. Its deposition differing from that of other heavy metals more associated with the cell wall, uranium was deposited as fine needle-like crystals both on the inside and outside of the S. cerevisiae cells.

Zinc binding in proteins and solution: A simple but accurate nonbonded representation

Abstract: Force field parameters that use a combination of Lennard-Jones and electrostatic interactions are developed for divalent zinc and tested in solution and protein simulations, It is shown that the parameter set gives free energies of solution in good agreement with experiment. Molecular dynamics simulations of carboxypeptidase A and carbonic anhydrase are performed with these zinc parameters and the CHARMM 22 beta all-atom parameter set, The structural results are as accurate as those obtained in published simulations that use specifically bonded models for the zinc ion and the AMBER force field, The inclusion of longer-range electrostatic interactions by use of the Extended Electrostatics model is found to improve the equilibrium conformation of the active site. It is concluded that the present parameter set, which permits different coordination geometries and ligand exchange for the zinc ion, can be employed effectively for both solution and protein simulations of zinc-containing systems. (C) 1995 Wiley-Liss, Inc.

Submicrometer zinc oxide particles: Elaboration in polyol medium and morphological characteristics

Abstract: A novel and easy route for preparing submicrometer particles of zinc oxide, involving hydrolysis of zinc salt in a polyol medium, is proposed. Zinc acetate dihydrate and diethyleneglycol appear to be the best candidates for obtaining a high yield of particles with well-defined morphological characteristics. Monodisperse spherical particles in the submicrometer range (0.2−0.4 μm) have been obtained for a salt concentration less than 0.1 mol 1−1. The particle size depends mainly on the heating rate. The particles are microporous (surface area: 80 m2 g−1) and are formed by aggregation of small crystallites (10 nm). Calcination at moderate temperature drastically reduces this porosity without significant interparticle sintering. At higher concentration, no aggregation occurs and tiny single crystallite particles are obtained.

Atomic Layer Epitaxy Growth of TiN Thin Films

Abstract: TiN thin films were grown on soda lime glass substrates by atomic layer epitaxy. Two different chemical schemes were studied: a direct reaction between the alternately supplied TiCl{sub 4} and NH{sub 3}, and a process employing a reducing zinc pulse given after the TiCl{sub 4} dose. The latter process was found to result in films with lower electrical resistivity. Resistivities as low as 50 {micro}{Omega} cm were measured for films grown with zinc at 500 C while those prepared without zinc had resistivities of about 250 {micro}{Omega} cm. In both cases the contents of chlorine residues were below the detection limit of Rutherford backscattering spectrometry (RBS), i.e., about 0.5 atom percent. In addition to RBS and electrical measurements, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, nuclear resonance broadening, and reflectance spectra measurements were employed for the film characterization. The possible reasons for the major drawback of the process, the low growth rate of only about 0.2 {angstrom}/cycle, are discussed.

Structural analysis of zinc substitutions in the active site of thermolysin.

Abstract: Native thermolysin binds a single catalytically essential zinc ion that is tetrahedrally coordinated by three protein ligands and a water molecule. During catalysis the zinc ligation is thought to change from fourfold to fivefold. Substitution of the active-site zinc with Cd2+, Mn2+, Fe2+, and Co2+ alters the catalytic activity (Holmquist B, Vallee BL, 1974, J Biol Chem 249:4601-4607). Excess zinc inhibits the enzyme. To investigate the structural basis of these changes in activity, we have determined the structures of a series of metal-substituted thermolysins at 1.7-1.9 A resolution. The structure of the Co(2+)-substituted enzyme is shown to be very similar to that of wild type except that two solvent molecules are liganded to the metal at positions that are thought to be occupied by the two oxygens of the hydrated scissile peptide in the transition state. Thus, the enhanced activity toward some substrates of the cobalt-relative to the zinc-substituted enzyme may be due to enhanced stabilization of the transition state. The ability of Zn2+ and Co2+ to accept tetrahedral coordination in the Michaelis complex, as well as fivefold coordination in the transition state, may also contribute to their effectiveness in catalysis. The Cd(2+)- and Mn(2+)-substituted thermolysins display conformational changes that disrupt the active site to varying degrees and could explain the associated reduction of activity. The conformational changes involve not only the essential catalytic residue, Glu 143, but also concerted side-chain rotations in the adjacent residues Met 120 and Leu 144. Some of these side-chain movements are similar to adjustments that have been observed previously in association with the "hinge-bending" motion that is presumed to occur during catalysis by the zinc endoproteases. In the presence of excess zinc, a second zinc ion is observed to bind at His 231 within 3.2 A of the zinc bound to native thermolysin, explaining the inhibitory effect.

Zinc Oxide for Weanling Piglets

Abstract: Digestive disorders and impaired performance are common problems among weanling piglets. The purpose of the present study was to examine the effect on health and performance of feeding different levels of zinc (as zinc oxide) for one, two or three weeks after weaning to piglets weaned at 28 days of age. A supplement of 2500 ppm zinc for two weeks after weaning reduced the incidence and severity of non-specific post-weaning diarrhoea by up to 50%. The daily gain, but not feed intake and feed efficiency, was significantly influenced by the dietary zinc level. Piglets fed 2500 ppm zinc showed the best performance. The beneficial effect of a temporary supply of 2500 and 4000 ppm zinc on post-weaning diarrhoea was found to be coincident with an increased alkaline phosphatase activity and zinc concentration in serum. The frequency of post-weaning diarrhoea was shown to be affected by the dietary content of calcium and copper.

Wheat genotypes differ in Zn efficiency when grown in chelate-buffered nutrient solution

Abstract: Ten Triticum aestivum and two Triticum turgidum conv. durum genotypes were grown in chelate-buffered nutrient solution at Zn supplies ranging from deficient to sufficient (free Zn activities from 2 to 200 pM, pZn from 11.7 to 9.7). The critical level of Zn ion activity in solution for healthy growth of wheat plants was around 40 pM. Genotypes differed in the growth response: those classified as Zn-efficient suffered less reduction of shoot growth and did not change the rate of root growth at a Zn supply quite deficient for Zn-inefficient genotypes. Root growth of Zn-inefficient genotypes increased at deficient Zn supply. The shoot/root ratio was the most sensitive parameter of Zn efficiency; Zn-efficient genotypes showed less reduction in the ratio when grown at deficient compared to sufficient Zn supply. Classification of wheat genotypes into Zn-efficient and Zn-inefficient groups after screening in chelate-buffered nutrient solution corresponded well with classification obtained in field experiments on Zn-deficient soil.

Solubility and mobility of copper, zinc and lead in acidic environments

Abstract: Understanding the chemical speciation of metals in solution is necessary for evaluating their toxicity and mobility in soils. Soil samples from the Powder River Basin, Wyoming were extracted with distilled deionised H2O. Soil water extracts were subjected to chemical speciation to determine the relative distribution and chemical forms of copper (Cu), zinc (Zn) and lead (Pb) in acidic environments. As pyrite oxidised, the pH decreased from 6.6 to 2.4, concentration of dissolved sulfate (ST) increased from 259 to 4,388 mg L-1 and concentration of dissolved organic carbon (DOC) decreased from 56.9 to 14.4 mg L-1. Dissolved Cu concentrations ranged from 0.06 to 0.42 mg L-1 and dissolved Zn concentrations ranged from 0.084 to 4.60 mgL-1. Dissolved concentrations of Pb were found to be 0.003 to 0.046 mg L-1. Chemical speciation indicated that at near neutral pH, dissolved metal concentration in soil water extracts was dominated by DOC- metal complexes. At low pH, dissolved metal concentration in soil water extracts was dominated by free ionic forms (e.g. Cu2+, Zn2+, Pb2+) followed by ion pairs (e.g. CuSO0 4, ZnSO0 4, PbSO0 4). Results obtained in this study suggest that as soil pH decreased, the availability and mobility of metal ions increased due to the chemical form in which these metal ions are present in soil solutions.