Fe-S clusters, fragile sentinels of the cell

Iron-sulfur (Fe-S) clusters are ubiquitous cofactors present in a myriad of proteins controlling processes as diverse as DNA replication, photosynthesis, respiration and gene regulation. Their assembly and delivery into apo-proteins are catalysed by different multi-protein systems conserved throughout prokaryotes and eukaryotes. Because so many cellular processes are dependent upon Fe-S proteins, alteration of the Fe-S clusters or of the systems that make them has profound impact on cellular physiology. The present review aims at covering and discussing those situations wherein these highly efficient redox sensitive cofactors turn from faithful sentinels into enfeebled assistants or, worse, into dangerous insiders.     

Redox regulation of cellular functions

Maintenance of normal intracellular redox status plays an important role in such processes as DNA synthesis, gene expression, enzymatic activity, and others. In addition, it is clear that changes in the redox status of intracellular content and individual molecules, resulting from stress or intrinsic cellular activity, are involved in the regulation of different processes in cells. Small changes in intracellular levels of reactive oxygen species participate in intracellular signaling. Thiol-containing molecules, such as glutathione, thioredoxins, glutaredoxins, and peroxiredoxins, also play an important role in maintaining redox homeostasis and redox regulation. This review attempts to summarize the current knowledge about redox regulation in different cell types.     

Iron-sulfur clusters in mitochondrial metabolism: Multifaceted roles of a simple cofactor

Iron-sulfur metabolism is essential for cellular function and is a key process in mitochondria. In this review, we focus on the structure and assembly of mitochondrial iron-sulfur clusters and their roles in various metabolic processes that occur in mitochondria. Iron-sulfur clusters are crucial in mitochondrial respiration, in which they are required for the assembly, stability, and function of respiratory complexes I, II, and III. They also serve important functions in the citric acid cycle, DNA metabolism, and apoptosis. Whereas the identification of iron-sulfur containing proteins and their roles in numerous aspects of cellular function has been a long-standing research area, that in mitochondria is comparatively recent, and it is likely that their roles within mitochondria have been only partially revealed. We review the status of the field and provide examples of other cellular iron-sulfur proteins to highlight their multifarious roles.     

Zinc coordination environments in proteins determine zinc functions.

Estimates of the number of zinc proteins in humans are now possible and a functional annotation of the zinc proteome can begin. The catalytic and structural roles of zinc in hundreds of enzymes and thousands of so-called "zinc finger" protein domains have provided a molecular basis for the numerous biological functions of this essential element. Additional, regulatory functions of zinc/protein interactions are being recognized. They include roles of the zinc ion in signal transduction, in controlling the architecture of protein complexes, and in redox-active zinc sites, where the binding and release of zinc is under redox control. Moreover, a considerable number of proteins participate in cellular zinc homeostasis, e.g. membrane transporters, and cellular storage, sensor, and trafficking proteins. These proteins have evolved with mechanisms to handle zinc ions rather specifically and selectively. They perform their functions with a remarkably modest set: One redox state of the zinc ion and nitrogen, oxygen, and sulfur ligands from the side chains of histidine, glutamate/aspartate, and cysteine, respectively. By permutation of the ligands in this set, the functional potential of the zinc ion has been fully explored. Different coordination environments modulate the chemical characteristics of the zinc ion, control the kinetics of its binding, and allow it to be either metabolically active or inert. Insights into all these functions are building an understanding of why zinc is so critical for such a multitude of life processes.     

The heme synthesis and degradation pathways: role in oxidant sensitivity. Heme oxygenase has both pro- and antioxidant properties

The heme biosynthetic and catabolic pathways generate pro- and antioxidant compounds, and consequently, influence cellular sensitivity to oxidants. Heme precursors (delta-aminolevulinic acid, porphyrins) generate reactive oxygen species (ROS), from autoxidation and photochemical reactions, respectively. Heme, an essential iron chelate, serves in respiration, oxygen transport, detoxification, and signal transduction processes. The potential toxicity of heme and hemoproteins points to a critical role for heme degradation in cellular metabolism. The heme oxygenases (HOs) provide this function and participate in cellular defense. This hypothesis emerges from the observation that the activation of HO-1 is an ubiquitous cellular response to oxidative stress. The reaction products of HO activity, biliverdin, and its subsequent metabolite bilirubin, have antioxidant properties. Furthermore, iron released from HO activity stimulates ferritin synthesis, which ultimately provides an iron detoxification mechanism that may account for long-term cytoprotection observed after HO induction. However, such models have overlooked potential pro-oxidant consequences of HO activity. The HO reaction releases iron, which could be involved in deleterious reactions that compete with iron reutilization and sequestration pathways. Indeed, the induction of HO activity may have both pro- and antioxidant sequelae depending on cellular redox potential, and the metabolic fate of the heme iron.     

严格厌氧菌铁氧还蛋白的研究进展

铁氧还蛋白(ferredoxin,Fd)是一类含有铁硫簇的小分子蛋白质,广泛存在于自然界中,参与生物体内的呼吸、发酵、固氮、二氧化碳固定和制氢等生理过程.Fd对于严格厌氧的细菌尤为重要,是因为这类细菌的能量代谢高度依赖于低氧化还原电势的生物组分,而Fd能够利用铁硫中心灵活调节其氧还电势,适应低电势需求.本文选取厌氧细菌...     

低氧与铁代谢相关蛋白

氧和铁这两种元素对生命活动十分重要. 低氧诱导因子(hypoxia-inducible factors, HIFs)作为转录因子,参与一系列靶基因的表达调控以适应低氧. 铁参与 DNA合成、氧气运输、代谢反应等多种细胞活动,过量游离铁会通过Haber-Weiss或 Fenton反应产生毒性自由基. 细胞通过与铁吸收、存储和利用有关的多种铁代谢相 关蛋白之间的协同作用来维持铁稳态. 与铁稳态相关的一些基因是HIFs的靶基因或 者间接受低氧调控,包括转铁蛋白、转铁蛋白受体、二价金属转运体1、铁调素、膜 铁转运蛋白、血浆铜蓝蛋白、铁蛋白等,而胞内铁浓度的改变能影响HIFs的表达. 本文就低氧与铁代谢相关蛋白的关系,尤其是低氧对铁代谢相关蛋白的调节作一综 述.     

Iron, oxidative stress and the example of solar ultraviolet A radiation

Iron has outstanding biological importance as it is required for a wide variety of essential cellular processes and, as such, is a vital nutrient. The element holds this central position by virtue of its facile redox chemistry and the high affinity of both redox states (iron II and iron III) for oxygen. These same properties also render iron toxic when its redox-active chelatable 'labile' form exceeds the normal binding capacity of the cell. Indeed, in contrast to iron bound to proteins, the intracellular labile iron (LI) can be potentially toxic especially in the presence of reactive oxygen species (ROS), as it can lead to catalytic formation of oxygen-derived free radicals such as hydroxyl radical that ultimately overwhelm the cellular antioxidant defense mechanisms and lead to cell damage. While intracellular iron homeostasis and body iron balance are tightly regulated to minimise the presence of potentially toxic LI, under conditions of oxidative stress and certain pathologies, iron homeostasis is severely altered. This alteration manifests itself in several ways, one of which is an increase in the intracellular level of potentially harmful LI. For example acute exposure of skin cells to ultraviolet A (UVA, 320-400 nm), the oxidising component of sunlight provokes an immediate increase in the available pool of intracellular LI that appears to play a key role in the increased susceptibility of skin cells to UVA-mediated oxidative membrane damage and necrotic cell death. The main purpose of this overview is to bring together some of the new findings related to intracellular LI distribution and trafficking under physiological and patho-physiological conditions as well as to discuss mechanisms and consequences of oxidant-induced alterations in the intracellular pool of LI, as exemplified by UVA radiation.     

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.     

The Presence of Multiple Cellular Defects Associated with a Novel G50E Iron-Sulfur Cluster Scaffold Protein (ISCU) Mutation Leads to Development of Mitochondrial Myopathy

Iron-sulfur (Fe-S) clusters are versatile cofactors involved in regulating multiple physiological activities, including energy generation through cellular respiration. Initially, the Fe-S clusters are assembled on a conserved scaffold protein, iron-sulfur cluster scaffold protein (ISCU), in coordination with iron and sulfur donor proteins in human mitochondria. Loss of ISCU function leads to myopathy, characterized by muscle wasting and cardiac hypertrophy. In addition to the homozygous ISCU mutation (g.7044G→C), compound heterozygous patients with severe myopathy have been identified to carry the c.149G→A missense mutation converting the glycine 50 residue to glutamate. However, the physiological defects and molecular mechanism associated with G50E mutation have not been elucidated. In this report, we uncover mechanistic insights concerning how the G50E ISCU mutation in humans leads to the development of severe ISCU myopathy, using a human cell line and yeast as the model systems. The biochemical results highlight that the G50E mutation results in compromised interaction with the sulfur donor NFS1 and the J-protein HSCB, thus impairing the rate of Fe-S cluster synthesis. As a result, electron transport chain complexes show significant reduction in their redox properties, leading to loss of cellular respiration. Furthermore, the G50E mutant mitochondria display enhancement in iron level and reactive oxygen species, thereby causing oxidative stress leading to impairment in the mitochondrial functions. Thus, our findings provide compelling evidence that the respiration defect due to impaired biogenesis of Fe-S clusters in myopathy patients leads to manifestation of complex clinical symptoms.     

CpNifS-dependent iron-sulfur cluster biogenesis in chloroplasts

Iron-sulfur (Fe-S) clusters are important prosthetic groups in all organisms. The biosynthesis of Fe-S clusters has been studied extensively in bacteria and yeast. By contrast, much remains to be discovered about Fe-S cluster biogenesis in higher plants. Plant plastids are known to make their own Fe-S clusters. Plastid Fe-S proteins are involved in essential metabolic pathways, such as photosynthesis, nitrogen and sulfur assimilation, protein import, and chlorophyll transformation. This review aims to summarize the roles of Fe-S proteins in essential metabolic pathways and to give an overview of the latest findings on plastidic Fe-S assembly. The plastidic Fe-S biosynthetic machinery contains many homologues of bacterial mobilization of sulfur (SUF) proteins, but there are additional components and properties that may be plant-specific. These additional features could make the plastidic machinery more suitable for assembling Fe-S clusters in the presence of oxygen, and may enable it to be regulated in response to oxidative stress, iron status and light.     

Iron-sulfur clusters: Formation,perturbation, and physiological functions

Iron-sulfur proteins occur in all life forms. Ferredoxins and Rieske proteins each contain a (2Fe2S) cluster whereas photosystem I (PSI) contains three (4Fe4S) clusters. Essential enzymes such as sulfite reductase, nitrite reductase, nitrogenase, glutamate synthase, aconitase, succinate dehydrogenase, ferredoxin/thioredoxin reductase, as well as many other vital proteins, each contain a (4Fe4S) cluster. Iron-sulfur clusters are formed enzymatically from cysteinyl-sulfur and ferritin-sequestered iron. Many iron-sulfur clusters are inactivated by O₂ and/or reactive oxygen species (ROS) such as O₂^•−. Perhaps 0.1 % of the electrons passing through either the mitochondrial electron transport chain or PSI result in the formation of O₂^•−. Many plant stresses increase ROS formation, which subsequently may perturb iron-sulfur clusters. Plants have evolved three different superoxide dismutases (SODs) to control the internal concentrations of harmful ROS. Possible roles of functional and non-functional iron-sulfur clusters in the coordination of metabolic activities of stressed and non-stressed plants are discussed.     

Redox signalling via the cellular thiolstat

Research conducted during the last two decades has provided evidence for the existence of an extensive intracellular redox signalling, control and feedback network based on different cysteine-containing proteins and enzymes. Together, these proteins enable the living cell to sense and respond towards external and internal redox changes in a measured, gradual, appropriate and mostly reversible manner. The (bio)chemical basis of this regulatory 'thiolstat' is provided by the complex redox chemistry of the amino acid cysteine, which occurs in vivo in various sulfur chemotypes and is able to participate in different redox processes. Although our knowledge of the biological redox behaviour of sulfur (i.e. cysteine or methionine) is expanding, numerous questions still remain. Future research will need to focus on the individual proteins involved in this redox system, their particular properties and specific roles in cellular defence and survival. Once it is more fully understood, the cellular thiolstat and its individual components are likely to form prominent targets for drug design.     

Iron transport: emerging roles in health and disease.

In the theater of cellular life, iron plays an ambiguous and yet undoubted lead role. Iron is a ubiquitous core element of the earth and plays a central role in countless biochemical pathways. It is integral to the catalysis of the redox reactions of oxidative phosphorylation in the respiratory chain, and it provides a specific binding site for oxygen in the heme binding moiety of hemoglobin, which allows oxygen transport in the blood. Its biological utility depends upon its ability to readily accept or donate electrons, interconverting between its ferric (Fe3+) and ferrous (Fe2+) forms. In contrast to these beneficial features, free iron can assume a dangerous aspect catalyzing the formation of highly reactive compounds such as cytotoxic hydroxyl radicals that cause damage to the macromolecular components of cells, including DNA and proteins, and thereby cellular destruction. The handling of iron in the body must therefore be very carefully regulated. Most environmental iron is in the Fe3+ state, which is almost insoluble at neutral pH. To overcome the virtual insolubility and potential toxicity of iron, a myriad of specialized transport systems and associated proteins have evolved to mediate regulated acquisition, transport, and storage of iron in a soluble, biologically useful, non-toxic form. We are gradually beginning to understand how these proteins individually and in concert serve to maintain cellular and whole body homeostasis of this crucial yet potentially harmful metal ion. Furthermore, studies are increasingly implicating iron and its associated transport in specific pathologies of many organs. Investigation of the transport proteins and their functions is beginning to unravel the detailed mechanisms underlying the diseases associated with iron deficiency, iron overload, and other dysfunctions of iron metabolism.     

铁硫蛋白的生物合成及相关疾病

铁硫蛋白是以铁硫簇为辅基,相对分子质量较小的一类蛋白质．它广泛存在于各种生物体内,参与电子传递、能量代谢以及基因表达调控等重要生理过程．其生物合成过程复杂,并且从细菌到人类高度保守．在真核细胞内,铁硫蛋白的组装由线粒体铁硫簇组装系统（mitochondrial iron sulfur cluster assembly system,mitochondrial ISC assembly system）和细胞质铁硫簇组装器（cytosolic iron sulfur cluster assembly,CIA）完成．研究发现,铁硫蛋白的合成异常可导致弗里德赖希共济失调(friedreich ataxia,FRDA)、遗传性肌病和铁粒幼细胞性贫血等多种罕见疾病,这些疾病严重影响个体的生活质量和寿命．因此,深入了解铁硫蛋白的结构和生物合成过程,对研究其生物学功能与相关疾病的诊断和治疗有重要意义．     

Cellular regulation of iron assimilation

Cells of plants, most microorganisms, and animals require well-defined amounts of iron for survival, replication, and differentiation. The metal is an important component of such processes as synthesis of DNA, RNA, and chlorophyll; electron transport; oxygen metabolism; and nitrogen fixation. Because of the insolubility of iron in aerobic environments at neutral and alkaline pH values, cells have had to devise specific strategies to assimilate the metal. These include (1) development of systems for reducing ferric ions to the more soluble ferrous ions at the cell surface, (2) employment of small carrier molecules (termed siderophores) that have high affinity for ferric ions and receptor proteins for the ferrated molecules, and (3) use of transferrin and other proteins that can transport ferric ions. Excessive amounts of iron are toxic, however, and intracellular storage capacity is limited and efflux mechanisms generally are lacking. Thus, cells have had to develop methods of preventing over-accumulation of the metal. These include use of (1) oxygen to convert ferrous to ferric ions, (2) small molecules that can bind ferrous ions, termed siderophraxes, and (3) proteins that, when combined with ferrous ions, repress the expression of iron transport genes. Often, one organism can prevent growth of neighbors by restricting their access to iron. In other cases, cells assist each other by sharing iron acquisition systems or by restricting influx of excess iron. Homeostatic control of other essential trace metals also is required for optimal cell function. Nevertheless, since iron thus far has received most attention, it serves as the model of mineral metabolism. Moreover, many of the observations made on control of iron metabolism suggest possible applications in prevention and management of plant and animal infections as well as of neoplastic diseases, arthropathy, and cardiomyopathy. This review will focus on (1) problems at the cellular level of iron acquisition, storage, and exclusion; and (2) the strategies devised by cells of plants, microorganisms, and animals to solve these problems.     

DNA damage responses to oxidative stress

The DNA damage response is a hierarchical process. DNA damage is detected by sensor proteins such as the MRN complex that transmit the information to transducer proteins such as ATM and ATR, which control the damage response through the phosphorylation of effector proteins. The extent of the DNA damage determines cell fate: cell cycle arrest and DNA repair or the activation of apoptotic pathways. In aerobic cells, reactive oxygen species (ROS) are generated as a by-product of normal mitochondrial activity. If not properly controlled, ROS can cause severe damage to cellular macromolecules, especially the DNA. We describe here some of the cellular responses to alterations in the cellular redox state during hypoxia or oxidative stress. Oxidative damage in DNA is repaired primarily via the base excision repair (BER) pathway which appears to be the simplest of the three excision repair pathways. To allow time for DNA repair, the cells activate their cell cycle checkpoints, leading to cell cycle arrest and preventing the replication of damage and defective DNA.