Tannin complexation with metal ions and its implication on human health, environment and industry: An overview

Tannins, also known as plant polyphenols (PPs), are secondary metabolites widely existing in higher plants and are a kind of natural renewable resource with wide distribution, variety and quantity. Tannin has become an important class of fine chemicals due to the easily modified molecular structure and the properties of antibacterial and antioxidant, combining with protein and complexing with metal ion. Besides being used for tanning leather, tannins are also widely used in wood adhesive, concrete water-reducing agents, oil drilling fluid viscosity-reducing agents, pharmaceutical, mineral processing, water treatment, gas desulfurization, metal anticorrosion, wood anticorrosion, printing and dyeing, liquor clarification, oil antioxidant, daily chemical products and other products preparation. There are two groups of tannins: condensed tannins (CTs) (flavonoid-derived proanthocyanidins) and hydrolysable tannins (HTs) (gallic acid ester-derived). Tannins can form complexes with metals through the ortho-dihydroxyphenolic group(s), especially with transition metals. The structure-activity relationships, stoichiometry, and origin of the insolubility of which were emphasized. Furthermore, this paper proposed an in-depth discussion of the associations of tannins-metal complexes in human health, environment and industries.

Metalation calculators for E. coli strain JM109 (DE3): Aerobic, anaerobic and hydrogen peroxide exposed cells cultured in LB media

Three web-based calculators, and three analogous spreadsheets, have been generated that predict in vivo metal occupancies of proteins based on known metal affinities. The calculations exploit estimates of the availabilities of the labile buffered pools of different metals inside a cell. Here, metal availabilities have been estimated for a strain of E. coli that is commonly used in molecular biology and biochemistry research, for example in the production of recombinant proteins. Metal availabilities have been examined for cells grown in LB medium aerobically, anaerobically and in response to H2O2 by monitoring the abundance of a selected set of metal-responsive transcripts by qPCR. The selected genes are regulated by DNA-binding metal sensors that have been thermodynamically characterised in related bacterial cells enabling gene expression to be read-out as a function of intracellular metal availabilities expressed as free energies for forming metal complexes. The calculators compare these values with the free energies for forming complexes with the protein of interest, derived from metal affinities, to estimate how effectively the protein can compete with exchangeable binding sites in the intracellular milieu. The calculators then inter-compete the different metals, limiting total occupancy of the site to a maximum stoichiometry of 1, to output percentage occupancies with each metal. In addition to making these new and conditional calculators available, an original purpose of this article was to provide a tutorial which discusses constraints of this approach and presents ways in which such calculators might be exploited in basic and applied research, and in next-generation manufacturing.

Crystal structure and metal binding properties of the periplasmic iron component EfeM from Pseudomonas syringae EfeUOB/M iron-transport system

EfeUOB/M has been characterised in Pseudomonas syringae pathovar. syringae as a novel type of ferrous-iron transporter, consisting of an inner-membrane protein (EfeUPsy) and three periplasmic proteins (EfeOPsy, EfeMPsy and EfeBPsy). The role of an iron permease and peroxidase function has been identified for the EfeU and EfeB proteins, respectively, but the role of EfeO/M remains unclear. EfeMPsy is an 'M75-only' EfeO-like protein with a C-terminal peptidase-M75 domain (EfeOII/EfeM family). Herein, we report the 1.6 Å resolution crystal structure of EfeMPsy, the first structural report for an EfeM component of P. syringae pv. syringae. The structure possesses the bi-lobate architecture found in other bacterial periplasmic substrate/solute binding proteins. Metal binding studies, using SRCD and ICP-OES, reveal a preference of EfeMPsy for copper, iron and zinc. This work provides detailed knowledge of the structural scaffold, the metal site geometry, and the divalent metal binding potential of EfeM. This work provides crucial underpinning for a more detailed understanding of the role of EfeM/EfeO proteins and the peptidase-M75 domains in EfeUOB/M iron uptake systems in bacteria.

Metal-dependent electrochemical discrimination of DNA quadruplex sequences

Films of four different DNA quadruplex-forming (G4) sequences (c-KIT, c-MYC, HTelo, and BCL2) on gold surfaces were investigated by electrochemical impedance spectroscopy (EIS) to evaluate whether they evoke unique electrochemical responses that can be used for their identification. This could render EIS an alternative means for the determination of G4 sequences of unknown structure. Towards, this end, cation-dependent topology changes in the presence of either K⁺, K⁺ in combination with Li⁺, or Pb²⁺ in the presence of Li⁺ were first evaluated by circular dichroism (CD) spectroscopy, and electrochemical studies were performed subsequently. As a result, G4-sequence specific charge transfer resistance (R_CT) patterns were in fact observed for each G4 sequence, allowing their discrimination by EIS.

Plant food intake is associated with lower cadmium body burden in middle-aged adults

PURPOSE

Dietary intake is a primary source of cadmium (Cd) exposure in the non-smoking population. Plant foods containing metal-binding plant compounds such as polyphenols, phytates, and phytochelatins may reduce Cd bioavailability and result in lower Cd body burden. In this study, we investigated the association between plant food intake and urinary creatinine-adjusted Cd (uCd), a well-established marker of Cd body burden.

METHODS

Participants were from a cross-sectional sample of 1901 adults in the REasons for Geographic and Racial Differences in Stroke (REGARDS) cohort. Dietary intake was assessed with a food frequency questionnaire. We created a 12-point plant food score (PFS) based on reported intake across seven categories (fruits, vegetables, legumes, nuts/seeds, whole grains, tea, and wine). Higher scores indicated higher consumption and diversity of plant food intake. Multivariable linear regression models were used to estimate the association between PFS and uCd. Due to the influence of age and smoking on Cd status, stratified analyses were conducted.

RESULTS

Mean PFS was 5.4 (SD 2.2) and mean uCd was 0.53 µg/g creatinine (SD 0.39). In adjusted models, PFS was not associated with uCd (p > 0.05). In stratified analyses, PFS was inversely associated with uCd (p = 0.047) with a 1-point higher PFS associated with 0.018 µg/g lower uCd among middle-aged (45-59) adults. No significant association was observed between PFS and uCd in older (≥ 60) adults. The association of PFS and uCd did not differ by smoking status.

CONCLUSION

Our findings suggest higher plant food intake is associated with lower Cd body burden in middle-aged but not older adults.

Inactivation of NikR from Helicobacter pylori by a bismuth drug

The NikR protein is an essential DNA regulator of Helicobacter pylori, a human pathogen, which infects almost half of the world's population. Herein, we comprehensively characterized the interaction of a bismuth drug with Helicobacter pylori NikR. We show that Bi(III) can occupy the high-affinity Ni(II) site of NikR. The highly-conserved residue Cys107 at this site is critical for Bi(III) binding. Importantly, such a binding disassembles physiologically functional NikR tetramer into inactive dimer, leading to abrogation of the DNA-binding capability of NikR. Bi(III)-binding also significantly disturbs regulatory function of Helicobacter pylori NikR in vivo. Therefore, NikR might serve as a potential intracellular target of a bismuth drug.

Structure, kinetics, molecular and redox properties of a cytosolic and developmentally regulated fungal catalase-peroxidase

CAT-2, a cytosolic catalase-peroxidase (CP) from Neurospora crassa, which is induced during asexual spore formation, was heterologously expressed and characterized. CAT-2 had the Met-Tyr-Trp (M-Y-W) adduct required for catalase activity. Its K_M for H₂O₂ was micromolar for peroxidase and millimolar for catalase activity. A E_m = -158 mV reduction potential value was obtained and the Soret band shift suggested a mixture of low and high spin ferric iron. CAT-2 EPR spectrum at 10 K indicated an axial and a rhombic component. With peroxyacetic acid (PAA), formation of Compound I* was observed with EPR. CAT-2 homodimer crystallographic structure contained two K⁺ ions; Glu107 residues were displaced to bind them. CAT-2 showed the essential amino acid residues for activity in similar positions to other CPs. CAT-2 Arg426 is oriented towards the M-Y-W adduct, interacting with the deprotonated Tyr238 hydroxyl group. A perhydroxy modification of the indole nitrogen of Trp90 was oriented toward the catalytic His91. In contrast to cytochrome c peroxidase and ascorbate peroxidase, the catalase-peroxidase heme propionates are not exposed to the solvent. Together with other N. crassa enzymes that utilize H₂O₂ as a substrate, CAT-2 has many tryptophan and proline residues at its surface, probably related to H₂O₂ selection in water.

Diffuse binding of Zn(2+) to the denatured ensemble of Cu/Zn superoxide dismutase 1

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Nearly complete backbone assignments for monomeric SOD1 were obtained.

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Monomeric SOD1 is unstructured in 9 M urea.

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Zn²⁺-binding to monomeric SOD1 occurs through diffuse coordination to most His residues.

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The binding geometry of Zn²⁺ is different than in the native, folded SOD1.

The stability and structural properties of the metalloprotein superoxide dismutase 1 (SOD1) are found to depend critically on metal ions. Native SOD1 monomers coordinate one structural Zn²⁺ and one redox-active Cu^2+/1+ to the active site. To do this, the Zn²⁺ ions need to interact with the SOD1 protein on the denatured side of the folding barrier, prior to the formation of the folding nucleus. In this study, we have examined at residue level the nature of this early Zn²⁺ binding by NMR studies on the urea denatured-state of SOD1. Nearly complete backbone chemical shift assignments were obtained in 9 M urea at physiological pH, conditions at which NMR studies are scarce. Our results demonstrate that SOD1 is predominantly unstructured under these conditions. Chemical-shift changes upon Zn²⁺ titration show that denatured SOD1 retains a significant affinity to Zn²⁺ ions, even in 9 M urea. However, the Zn²⁺ interactions are not limited to the native metal-binding ligands in the two binding sites, but are seen for all His residues. Moreover, the native Cu^2+/1+ ligand H46 seems not to bind as well as the other His residues, while the nearby non-native H43 does bind, indicating that the binding geometry is relaxed. The result suggests that the Zn²⁺-binding observed to catalyze folding of SOD1 in physiological buffer is initiated by diffuse, non-specific coordination to the coil, which subsequently funnels by ligand exchange into the native coordination geometry of the folded monomer. Altogether, this diffuse binding is a result with fundamental implications for folding of metalloproteins in general.