Iron in Oxidative and Reduction Processes in Marine Sands

The iron content and the ratio of bivalent to total iron in the labile acid-soluble fraction of iron were studied in sublittoral sands of Vostok Bay, Sea of Japan. Iron oxidation and reduction rates in sand sediment were measured in the laboratory. Trivalent iron almost always prevailed in the acid-extractable fraction in the natural environment. The most oxidized state of iron occurred in spring and was associated with a low water temperature and a high seawater oxygen content; the most reduced iron was found in fall during periods of low hydrodynamics and low oxygen concentration. In sand, iron is mainly reduced by bacteria; this process is slow and can be inhibited by adding chloroform. Oxidation of iron is mainly a chemical process and cannot be stopped by chloroform. In sand, the content of redox equivalents such as trivalent iron is much greater than in dissolved oxygen of pore water. It is assumed that labile iron in sands acts as a redox buffer, is oxidized by dissolved pore water oxygen at the periods of advective mixing, and is slowly reduced by benthic bacteria during anaerobic conditions.     

Chemical aspects of siderophore mediated iron transport

In this mini-review we describe selected aspects of the coordination chemistry relevant to siderophore mediated iron transport and bioavailability. Specific emphasis is placed on a discussion of in vitro kinetic and thermodynamic data that are relevant to elucidating possible in vivo mechanisms for environmental iron acquisition by microbial cells.     

Iron Transport in Escherichia Coli: All has not been said and Done

During recent years new systems involved in iron transport were identified in the old workhorse Escherichia coli (and in other enterobacteria). This came as a bit of a surprise because one might think transport of this essential trace element was already thoroughly studied. Moreover, it appears that iron homeostasis consists not only of uptake but also of efflux of this potentially toxic redox-active metal. New findings in E. coli will be discussed and compared to the situation in other bacteria.     

Postweaning diarrhoea in pigs: an interaction between change to dry food and colibacillosis

Abstract Production of pyochelin by Pseudomonas species was measured in ethyl acetate extracts of culture supernatants. Pyochelin was purified by paper chromatography, followed by two-dimensional, thin-layer chromatography. Strains of P. cepacia , P. multivorans , and P. fluorescens produced pyochelin, while P. stutzeri , P. putida and P. maltophila were negative for pyochelin production. Outer membranes prepared from these strains had a protein of M _r 14000 which was shown to bind [⁵⁹Fe]pyochelin.     

Involvement of Vulnibactin and Exocellular Protease in Utilization of Transferrin- and Lactoferrin-Bound Iron by Vibrio vulnificus

In vitro growth experiments were conducted to evaluate the ability of vulnibactin, a siderophore produced by Vibrio vulnificus, to sequester transferrin- or lactoferrin-bound iron for growth. Comparative studies with the strain producing vulnibactin and its exocellular protease-deficient mutant revealed the involvement of the protease in addition to vulnibactin in effective utilization of iron ion (Fe³⁺) bound to transferrin and lactoferrin. It appears that the protease causes cleavage of these proteins, thereby making bound iron more accessible to vulnibactin.     

黄色条纹/黄色条纹样蛋白与植物中的铁转运功能

黄色条纹/黄色条纹样蛋白（yellow stripe/yellow stripe like,YS/YSL）在植物金属转运,尤其是在微量营养元素铁的分配和利用过程中发挥重要作用。近年来,植物的铁长距离运输机制已成为研究的热点之一。YS/YSL转运蛋白涉及铁的转移、运输、装载和卸载。鉴定并精确解析YS/YSL的功能对于阐明铁的长距离运输机制具有重要的意义。本文综述了YS/YSL的蛋白质性质、金属转运活性、底物特异性及其功能表达,以期为植物YS/YSL相关铁转运机制研究奠定基础,并为提高作物籽粒铁营养提供理论依据。     

Influences of Electron Donor,Bicarbonate, and Sulfate on Bioreduction Processes and Manganese/Copper Redistributions among Minerals in a Water-Saturated Sediment

Bioreduction processes have profound influences on mobility and bioavailability of metals in soils and sediments. In this study, a series of microcosm studies were conducted to investigate bioreduction progresses (ferric iron and sulfate reduction) and their influences on manganese and copper element redistributions with the change of microbial community under various geochemical conditions. Results indicated that ethanol stimulated higher rates of bioreduction processes than acetate did. High-concentration bicarbonate and sulfate addition inhibited iron reduction but not sulfate reduction. Sequential extraction revealed that the exchangeable and carbonate bonding-iron were increased in ethanol amendment, whereas those of copper were decreased. The elevated bicarbonate concentration and sulfate addition both influenced the mobility and redistribution of metals. 16S rRNA analysis indicated that ethanol amendment stimulated the growths of microbial iron and sulfate reducer. A high concentration of bicarbonate suppressed the growth of iron reducer Geobacteraceae, but showed limited effect on sulfate reducers. This study concluded that geochemical conditions such as electron donor, bicarbonate and sulfate concentration influenced the microbial community and led to changes in bioreduction processes and metal distributions in the anerobic sediments.     

A Transplasma Membrane Redox System in Phycomyces blakesleeanus: Properties of a Ferricyanide Reductase Activity Regulated by Iron Level and Vitamin K3

Intact Phycomyces blaskesleeanus mycelia are capable of reducing extracellular ferricyanide and this transmembrane reduction is an enzymatic process, which is enhanced by the presence of 10 mM lactate. It is modulated in response to intracellular iron levels and negatively regulated by iron and copper. It is inhibited by NEM, p CMB, iodoacetate, Zn2+, Cd2+, dicumarol, and capsaicine analog, but not by cloroquine, and activated by Ca2+, Mg2+, Na+, and K+. Ferricyanide reduction was concomitant with proton release into the extracellular medium, both processes being greatly promoted by vitamin K3 following hyperbolic saturation kinetics with regard to ferricyanide concentration. No stoichiometric proton release was observed with regard to ferricyanide reduction in the absence or the presence of vitamin K3. Proton release coupled with ferricyanide reductase activity does not appear to be due to H+-ATPase. The relevance of these findings to the relationship between the two processes is discussed.     

Redox conditions and iron chemistry in highland swamps of Burundi

Iron toxicity is a major soil constraint to rice (Oryza sative L.) cropping in highland swamps of Burundi. These swamps have a wide range of carbon content. This study aims at determining the influence of carbon content and redox conditions on the release of iron from Fe-bearing minerals. The pe-pH pairs distribution and oxalate dissolution data strongly suggest a control of Fe²⁺ activity by a pool of poorly crystallized ferric oxides. Flooding results in high values of KCl-extractable Fe (up to 22 cmolc kg^-1) being released from that pool. The iron release is positively correlated with organic matter. On the other hand, highly organic, peaty soils have large CEC and their adsorbed Fe fraction remains relatively low. As the exchangeable Fe fraction has previously been correlated with Fe toxicity to rice, we may conclude that very organic (> 25% C), peaty soils exhibit a lower Fe toxicity hazard than soils with intermediate carbon content (10–25%).     

Identification and mutational studies of conserved amino acids in the outer membrane receptor protein, FepA, which affect transport but not binding of ferric-enterobactin in Escherichia coli

Many gram-negative bacteria produce and excrete siderophores, which complex iron with high affinity in the environment. The ferric siderophore complexes are transported across the outer membrane by receptor proteins. This process requires energy and is TonB dependent and must involve conformational changes in the receptor proteins to allow the transport of the ferric siderophores from the extracellular binding site to the periplasm. There is a large variety in the structures, molecular weights and charges among the siderophores. It was therefore realized that when the sequences of the many different receptor proteins were compared, simultaneously, all identities and close similarities, found in this manner, could only be due to residues involved in the conformational changes and transport mechanism, common to all the proteins, and not be due to the specificity of ligand recognition. Once the crystal structures of FepA, FhuA and FecA became available, it was immediately clear that the sequence similarities which were found in the simultaneous alignment, were all localized in a few structural domains, which are identical in the three structures and can therefore be expected to be maintained in all the proteins in this family. One of these domains, tentatively named the lock region, consists of 10 residues with a central quadrupole formed by two arginines and two glutamates, from the plug region and the beta barrel. We mutated several of these residues in FepA. All showed normal binding in quantitative binding studies. Some showed normal transport as well, however, the majority showed moderate to severe defective transport with ferric enterobactin. The results therefore show the validity of the hypothesis that the simultaneous sequence alignment will select the residues involved in the transport function of the receptor proteins. In addition the results allow to relate the severity of the transport deficiency to be correlated with the structure of the lock region while it is also possible to propose a function of this region in the conformational changes of the protein during the transport of the ligand from the binding site to the periplasm.     

高等植物尿素代谢及转运的分子机理

尿素广泛存在于自然界中,是易于被许多生物（如植物）利用的生长氮源。该文通过概述尿素在不同生命系统中存在的基础生理意义及各类型尿素转运蛋白,讨论了植物细胞中尿素合成与分解的各种途径及尿素在植物氮营养、代谢和运输中的生理作用。迄今为止,在植物中已发现了2类转运尿素的膜蛋白,即MIPs和DUR3,它们分别在低亲和力、高亲和力尿素运输中发挥潜在作用。异源表达结果表明MIPs介导了尿素的被动迁移：而AtDUR3则参与拟南芥根系对尿素的吸收。对MIPs和DUR3转运尿素的酶学特征、亚细胞作用位点和表达调控状况等的研究表明：它们的分子生物学功能与植物的氮营养及氮素再分配和利用相关。     

花生下胚轴组织H~ 分泌与质膜氧化还原系统的某些特性

花生幼苗下胚轴细胞表面存在氧化NADH与还原Fe(CN)_6~(3-)的氧化还原系统(redox system),它的活性随反应介质pH的上升而增加,氧是该系统的天然电子受体,受DCCD抑制。在NADH与Fe(CN)_6~(3-)被氧化与还原的同时,组织的H~ 分泌明显受到促进,并随反应介质pH上升而增加。质膜H~ -ATPase专一抑制剂Na_3VO_4(0.1 mmol/L)对组织H~ 分泌抑制绝对量基本上不受介质pH的影响,表明质膜H~ -ATPase与redox system共同参与该组织的H~ 分泌,部分redox system支配的H~ 分泌是依赖氧的。     

Dependence of In Situ Bacterial Fe(II)-Oxidation and Fe(III)-Precipitation on Sequential Reactive Transport

A study was conducted to determine in situ rates of Fe(II) oxidation and Fe(III) precipitation along a 5.0 m reach of a ferruginous groundwater discharge zone under two distinct conditions; (i) the natural state featuring abundant flocculent mats of bacteriogenic iron oxides (BIOS) produced by Fe(II)-oxidizing bacteria, and (ii) after a manual washout of the streambed to remove the microbial mat. Examination of mat samples by differential interference contrast light microscopy revealed tangled meshworks of filamentous Leptothrix sheaths and helical Gallionella stalks intermixed with fine-grained hydrous ferric oxide (HFO) precipitates. The greatest accumulation of BIOS mat was 1.0 m downstream of the groundwater spring. Redox potential (Eh) increased sharply from 200 mV to over 300 mV over the last 2.0 m of the reach. Similarly, dissolved oxygen increased from < 10% saturation to almost 100% saturation over the last 2.0 m of the reach, whereas pH increased from 6.4 to 7.3. Pseudo-first-order rate constants determined on the basis of analytical solutions to sequential partial differential advection-dispersion-reaction equations for the linear Fe(II)→Fe(III)→HFO reaction network yielded in situ Fe(II) oxidation rate constants (k_ox) of 1.70 ± 0.20 min^?1 in natural conditions and 0.48 ± 0.14 min^?1 after washout. Corresponding Fe(III)-precipitation rates (k_p) before and after washout were 3.45 ± 0.10 min^?1 and 0.90 ± 0.01 min^?1, respectively. These values for k_ox and k_p are higher than estimates obtained from closed batch microcosm and laboratory experiments, underscoring the crucial dependence of in situ Fe(II) oxidation and Fe(III) precipitation rates on advective and dispersive mass transport. The results also highlight the influence that BIOS microbial mats exert on the reaction kinetics of the multiple heterogeneous reactions contributing not only to Fe(II)/Fe(III) redox transformations in groundwater discharge zones, but also the precipitation of HFO.     

A Simple Cranial Window Technique for Optical Monitoring of Cerebrocortical Microcirculation and NAD/NADH Redox State. Effect of Mitochondrial Electron Transport Inhibitors and Anoxic Anoxia

Abstract: Fluorescence of NADH and vascular volume of the brain cortex of chloralose-anesthetized cats were measured by surface fluororeflectometry. A cranial window and superfusion technique was elaborated for the topical inhibition of mitochondrial electron transport in the brain cortex by amytal (inhibits at site I) and cyanide (inhibits at site III). The changes in NAD/NADH redox state and CVV evoked by these electron transport inhibitors were compared with those elicited by anoxic anoxia. Amytal (10^-3-10^-1 M ) and cyanide (10^-5-10^-2 M ) resulted in a concentration-dependent and reversible increase in cortical NAD reduction and vascular volume, but the cerebrocortical vessels were almost completely dilatated long before maximum NAD reduction was reached. Cyanide at 10^-2 M increased cortical NAD reduction and vascular volume as much as anoxic anoxia. Amytal at 10^-1 M induced approximately half of the NAD reduction evoked by 10^-2 M cyanide or anoxic anoxia, but resulted in only slightly less vasodilatation than that following cyanide and anoxic anoxia. Since amytal inhibits mitochondrial electron transport at site I—and cyanide and anoxia at site III—but induces a comparable degree of vasodilatation, it is concluded that cytochrome oxidase cannot be the single molecular oxygen sensor in the brain cortex.     

Reductive Dissolution of Jarosite by Acidiphilium cryptum in Presence of Chelating Agents and Dissolved Iron

The mutual effects of Acidiphilium cryptum, nine ligands and dissolved iron on jarosite dissolution were studied in a 2³ full factorial experiment. Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and oxalic acid were able to dissolve jarosite chemically, but bacterial action enhanced this dissolution; from 88 ± 1, 46 ± 0 and 36 ± 0 µmol/L d to 129 ± 3, 177 ± 5 and 106 ± 14 µmol/L d, respectively. Oxalate and NTA exhibited a synergistic effect with dissolved iron and bacteria that may be explained by a model in which iron complexes behave as redox mediators in the microbial respiration in which jarosite, a solid substrate, serves as terminal electron acceptor.     

Iron Acquisition and Transport in Staphylococcus aureus

Pathogenic Gram-positive bacteria encounter many obstacles in route to successful invasion and subversion of a mammalian host. As such, bacterial species have evolved clever ways to prevent the host from clearing an infection, including the production of specialized virulence systems aimed at counteracting host defenses or providing protection from host immune mechanisms. Positioned at the interface of bacteria/host interactions is the bacterial cell wall, a dynamic surface organelle that serves a multitude of functions, ranging from physiologic processes such as structural scaffold and barrier to osmotic lysis to pathogenic properties, for example the deposition of surface molecules and the secretion of cytotoxins. In order to succeed in a battle with host defenses, invading bacteria need to acquire the nutrient iron, which is sequestered within host tissues. A cell-wall based iron acquisition and import pathway was uncovered in Staphylococcus aureus. This pathway, termed the isd or iron-responsive surface determinant locus, consists of a membrane transporter, cell wall anchored heme-binding proteins, heme/haptoglobin receptors, two heme oxygenases, and sortase B, a transpeptidase that anchors substrate proteins to the cell wall. Identification of the isd pathway provides an additional function to the already bountiful roles the cell wall plays in bacterial pathogenesis and provides new avenues for therapeutics to combat the rise of antimicrobial resistance in S. aureus. This review focuses on the molecular attributes of this locus, with emphasis placed on the mechanism of iron transport and the role of such a system during infection.     

Preprotein Import into Chloroplasts via the Toc and Tic Complexes Is Regulated by Redox Signals in Pisum sativum

The import of nuclear-encoded preproteins is necessary to maintain chloroplast function. The recognition and transfer of most precursor proteins across the chloroplast envelopes are facilitated by two membrane-inserted protein complexes, the translocons of the chloroplast outer and inner envelope （Toc and Tic complexes, respectively）. Several signals have been invoked to regulate the import of preproteins. In our study, we were interested in redox-based import regulation mediated by two signals： regulation based on thiols and on the metabolic NADP＋/NADPH ratio. We sought to identify the proteins participating in the regulation of these transport pathways and to characterize the preprotein subgroups whose import is redox-dependent. Our results provide evidence that the formation and reduction of disulfide bridges in the Toc receptors and Toc translocation channel have a strong influence on import yield of all tested preproteins that depend on the Toc complex for translocation. Furthermore, the metabolic NADP＋/NADPH ratio influences not only the composition of the Tic complex, but also the import efficiency of most, but not all, preproteins tested. Thus, several Tic subcomplexes appear to participate in the translocation of different preprotein subgroups, and the redox-active compo- nents of these complexes likely play a role in regulating transport.     

Transport of iron and manganese in relation to the shapes of their concentration-depth profiles

A model is presented which describes the transport of iron and manganese in the vicinity of a redox boundary. It is based on input of a particulate component, to form a point source, from which soluble species diffuse along a concentration gradient. The shapes of concentration-depth profiles in marine and freshwater sediments and water columns are reviewed and discussed in terms of the model. Transport, either entirely within a water column or within the sediment, may be simply treated because the rate of vertical transport can be regarded as constant. The discontinuity in the rate of vertical transport which occurs at the sediment-water interface can provide a complicated example of the model, especially when it coincides with the redox boundary. Authigenic mineral formation processes can modify the model, sometimes to such an extent that it becomes invalid. This is particularly true for soluble iron profiles in organically rich marine sediments. Sampling interval is critical to the resultant profile shape and must be relevant to the particular environment, e.g. metres in water columns and millimetres in sediments. The differences in the rates of reduction and oxidation of iron and manganese tend to modify both the position of the profile with respect to the redox-cline and its stage of development in a seasonally anoxic system. It is these factors which determine why most of the iron which reaches a sediment is permanently incorporated whereas manganese is re-released. This mechanism determines the average ratio of iron to manganese in sedimentary rocks. The development of peaked profile shapes in water columns implies that under certain conditions dissolved iron and manganese may be transported from the water column to the pore waters of the sediment.     

Transport and utilization of rhizoferrin bound iron in Mycobacterium smegmatis

Transport and metabolization of iron bound to the fungal siderophore rhizoferrin was analyzed by transport kinetics, Mössbauer and EPR spectroscopy. Saturation kinetics (v _max=24.4 pmol/(mg min), K _m=64.4M) and energy dependence excluded diffusion and provided evidence for a rhizoferrin transport system in M. smegmatis. Based on the spectroscopic techniques indications for intracellular presence of the ferric rhizoferrin complex were found. This feature could be of practical importance in the search of novel drugs for the treatment of mycobacterial infections. EPR and Mössbauer spectroscopy revealed different ferritin mineral cores depending on the siderophore iron source. This finding was interpreted in terms of different protein shells, i.e. two types of ferritins.     

Seasonal and Spatial Distribution of Iron Precipitating Heterotrophs in Water and Sediments of Fish Ponds of Differing Farming Managements

Population densities of primarily iron precipitating heterotrophs (metal-precipitating non oxidizing bacteria) were estimated in water and sediments of ponds being used for polyculture, monoculture and traditional systems of fish farming over a period of two and a half years. Spatial differences in microbial density in the ponds were related to the organic loading of the farm site and the type of farming. The highest bacterial populations occurred in November and the lowest in summer. Nitrite concentration was mainly responsible for such seasonal changes.