PERSPECTIVES mitochondrial electron transport chain4.In OPINION chloroplasts, Fe–S clusters are required for the function of photosystem I, ferredoxin and the 5 cytochrome b6f complex . Iron–sulphur cluster biogenesis and The ability of Fe–S clusters to coordinate ligands also allows them to facilitate various mitochondrial iron homeostasis enzymatic functions2,3,6.Mitochondrial aconi- tase is part of the citric acid cycle, which provides free energy for ATP generation. In Tracey A. Rouault and Wing-Hang Tong aconitase, a single iron atom of the [4Fe–4S] cluster facilitates the dehydration–hydration Abstract | Iron–sulphur clusters are Fe–S proteins in eukaryotes reaction that reversibly converts citrate to important cofactors for proteins that are Fe–S proteins are an ancient, ubiquitous and isocitrate by ligating the hydroxyl group of involved in many cellular processes, functionally diverse class of metalloproteins the substrate and activating this group for including electron transport, enzymatic that are found in organisms from bacteria to elimination (FIG. 1b).The sulphur atoms of catalysis and regulation. The enzymes that humans1,2.The unique features of these Fe–S clusters can also be involved in catalysis catalyse the formation of iron–sulphur remarkably versatile cofactors enable Fe–S pro- (FIG. 1b).Fe–S clusters also provide some clusters are widely conserved from bacteria teins to transfer electrons, catalyse enzymatic enzymes with structural stability. The Fe–S to humans. Recent studies in model reactions, and function as regulatory proteins clusters in several DNA repair enzymes, systems and humans reveal that (FIG. 1).The underlying architectural element of including the endonuclease III homologue iron–sulphur proteins have important roles in the Fe–S cluster is the [2Fe–2S] rhomb that is NTH1 (REF. 7) and the MutY/MYH family mitochondrial iron homeostasis and in the formed by two tetrahedrally coordinated iron of DNA glycosylases8,are required for the pathogenesis of the human disease atoms with two bridging sulphides, and com- recognition and repair of DNA damage. Friedreich ataxia. plexes that contain up to eight iron atoms can The sensitivity of Fe–S clusters towards – be conceptualized as an elaboration of the species such as H2O2,NO,and O2 also allows Iron–sulphur (Fe–S) clusters are indispens- basic [2Fe–2S] unit (FIG. 1a).The combination them to function as important regulatory sen- able participants in three processes that are of the chemical reactivity of iron and sulphur, sors that respond to oxidative stress and intra- central to life on earth — respiration, photo- together with variations of cluster composi- cellular iron levels in bacteria9 and mammals10 synthesis and nitrogen fixation. These ancient tion, oxidation, spin states and the local pro- (FIG. 1c).Mammalian cells encode two aconi- modular cofactors consist of iron atoms that tein environment, enables Fe–S clusters to tases: the mitochondrial enzyme (mentioned are directly coordinated to inorganic sulphide function in numerous distinct biological roles. above) that functions in the citric acid cycle, or the cysteinyl sulphurs of the associated Known mammalian Fe–S proteins and their and a cytosolic enzyme, iron regulatory proteins. Fe–S clusters are biologically versa- functions are listed in TABLE 1,but this list is protein-1 (IRP1), that is bifunctional11.In its tile and can function in a wide variety of cru- probably far from complete, not only because [4Fe–4S] cluster form, IRP1 functions as an cial electron transfer and biosynthetic of the limitations of sequence-based methods aconitase. When the Fe–S cluster is absent, for processes. In addition, Fe–S proteins have for predicting potential Fe–S proteins, but example, as a result of Fe depletion or the pres- important regulatory functions. Work in both also because Fe–S clusters often spontaneously ence of reactive oxygen species, IRP1 binds yeast and mammalian systems supports the disassemble during aerobic purification. mRNAs that contain a specific stem–loop conclusion that defects in the enzymes Fe–S proteins have a wide range of reduc- sequence that is known as the iron-responsive involved in Fe–S cluster assembly lead to sig- tion potentials (–700 mV to 400 mV; REF. 3) element (IRE). When IRP1 binds to the nificant mitochondrial iron overload and fail- and are among the most important electron 5′ untranslated region of ferritin mRNA, ure. However, the mechanism by which the carriers in nature. They are integral compo- the translation of ferritin — an iron storage compromise of Fe–S cluster synthesis leads to nents of respiratory and photosynthetic elec- protein — is repressed. When IRP1 binds to mitochondrial iron overload is unknown. tron transfer chains, in which groups of Fe–S the 3′ untranslated region of transferrin recep- Here,we discuss important features of clusters function as a linear series of redox tor mRNA, which produces an iron-uptake Fe–S chemistry and biology, with emphasis centres or electron reservoirs4,5.Up to 12 dif- protein, the transcript is protected from degra- on the potential importance of Fe–S proteins ferent Fe–S clusters can be found in the dation. As a result, IRP1 binding to mRNAs in the regulation of mammalian cellular can decrease iron sequestration and increase and mitochondrial iron metabolism. Fe–S cellular iron uptake. However, the Fe–S cluster proteins function as important regulatory of IRP1 is stable at the low oxygen concentra- proteins in systems ranging from bacteria to “…mitochondrial iron tions in mammalian cells12, and IRP1 is not a mammals. We propose that mitochondrial homeostasis might be a key contributor to iron regulation in healthy 11,13 iron homeostasis might be a highly regulated highly regulated process in animals .Nevertheless, IRP1 can be induced process in which an Fe–S protein functions as to bind mRNA by stimuli such as H2O2,NO or – (REF.14) a sensor for the mitochondrial iron status or which an Fe–S protein O2 ,which indicates that it might be aids in the formation of such a sensor. An important in pathophysiology. improved understanding of the relationship functions as a sensor for the between Fe–S cluster synthesis and mito- mitochondrial iron status Mechanism of Fe–S cluster assembly chondrial iron overload is important in As early as the 1970s, studies showed that Fe–S understanding the pathogenesis of the or aids in the formation of clusters can be synthesized in vitro1,but it was human disease Friedreich ataxia. such a sensor.” not until the late 1980s that pioneering work NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 6 | APRIL 2005 | 345 © 2005 Nature Publishing Group PERSPECTIVES by Dean and co-workers identified the first extensive genetic and biochemical studies the unfolding and refolding of scaffold pro- enzymes that catalyse Fe–S cluster formation that have been carried out in E. coli and in teins during cluster assembly, or might func- in bacteria15.To date, at least three operons — Saccharomyces cerevisiae15,19,20 (FIG. 2). IscS is tion in the release of the Fe–S cluster during nif, isc and suf — have been shown to encode a pyridoxal-phosphate-dependent cysteine its transfer to the target protein (FIG. 2). genes that are involved in bacterial Fe–S cluster desulphurase. Nucleophilic attack on the cys- Studies in cyanobacteria17,21, plants18,22 and biosynthesis (for a review, see REF.15). The nif- teine sulphur of the substrate, by an active site animals23–25 support the idea that the basic specific genes in Azotobacter vinelandii are cysteine residue in IscS, results in the forma- assembly mechanisms are conserved across involved in the assembly of Fe–S clusters in tion of an enzyme-bound persulphide species. So, because of the sequence and func- nitrogenase. Escherichia coli contains the isc (S–IscS). Sulphur transfer and iron acquisi- tional similarities, whenever possible in fol- and suf gene clusters that are involved in tion leads to assembly of [2Fe–2S] and lowing sections, we apply the nomenclature the general maturation of Fe–S proteins. [4Fe–4S] clusters on the scaffold proteins that was originally proposed for the E. coli isc Homologues of the bacterial isc genes have IscU, Nfu and IscA. Cluster assembly requires operon to the mammalian Fe–S cluster been identified in yeast, plants and animals15,16, the reduction of the persulphide sulphur, and assembly proteins and genes. In bacteria, Fe–S and homologues of the bacterial suf genes redox proteins such as ferredoxin and gluta- cluster assembly takes place in the cytoplasm, are found in cyanobacteria and plants16–18. redoxin might be required to complete cluster whereas in mammals and plants, Fe–S cluster Most of the insights into the mechanism formation. IscU is a substrate for the chaper- assembly takes place in several compartments of Fe–S cluster assembly have come from the one proteins HscA/HscB, which might assist (discussed later). a Native Fe–S clusters [2Fe–2S] [4Fe–4S] FeMo-cofactor P-cluster A-cluster C-cluster b Substrate binding and catalysis S-adenosyl c Cluster interconversion/disassembly NH2 N N N N Cluster disassemby Cluster interconversion Isocitrate O COO– [4Fe–4S] O OH OH COO– COO– Aconitase [4Fe–4S] S-adenosylmethionine-dependent [3Fe–4S] [2Fe–2S] enzyme [4Fe–4S] Figure 1 | The structural and chemical versatility of iron–sulphur (Fe–S) clusters. a | The Fe–S cluster types that are most common are the [2Fe–2S] and [4Fe–4S] clusters. More complex Fe–S clusters are found in the active site of nitrogenase (the FeMo-cofactor and P-clusters) and in carbon monoxide dehydrogenase (A- and C-clusters), and these clusters can be thought of as an elaboration of these basic units. The clusters shown are colour coded by atom type: iron, red; sulphur, yellow; molybdenum (Mo), brown; nickel, green; and copper, blue.