BBA - Molecular Cell Research (v.1823, #9)

Special issue: Cell Biology of Metals by Jonathan D. Gitlin; Roland Lill (1405).

Quantifying the amount and defining the location of metal ions in cells and organisms are critical steps in understanding metal homeostasis and how dyshomeostasis causes or is a consequence of disease. A number of recent advances have been made in the development and application of analytical methods to visualize metal ions in biological specimens. Here, we briefly summarize these advances before focusing in more depth on probes for examining transition metals in living cells with high spatial and temporal resolution using fluorescence microscopy. This article is part of a Special Issue entitled: Cell Biology of Metals.► We discuss methods to map the location and dynamics of metal ions in cells. ► Metal ions can exist in free and bound forms. ► We discuss techniques for defining free and total metal ion content of cells. ► We highlight recent developments in fluorescent probes for metal ions. ► These probes can be used in live cell imaging.
Keywords: Metal ion homeostasis; Fluorescence imaging; Fluorescent sensors; Imaging metal ions in cells;

The taste of heavy metals: Gene regulation by MTF-1 by Viola Günther; Uschi Lindert; Walter Schaffner (1416-1425).
The metal-responsive transcription factor-1 (MTF-1, also termed MRE-binding transcription factor-1 or metal regulatory transcription factor-1) is a pluripotent transcriptional regulator involved in cellular adaptation to various stress conditions, primarily exposure to heavy metals but also to hypoxia or oxidative stress. MTF-1 is evolutionarily conserved from insects to humans and is the main activator of metallothionein genes, which encode small cysteine-rich proteins that can scavenge toxic heavy metals and free radicals. MTF-1 has been suggested to act as an intracellular metal sensor but evidence for direct metal sensing was scarce. Here we review recent advances in our understanding of MTF-1 regulation with a focus on the mechanism underlying heavy metal responsiveness and transcriptional activation mediated by mammalian or Drosophila MTF-1. This article is part of a Special Issue entitled: Cell Biology of Metals.► MTF-1 activates target gene expression in response to heavy metals. ► Oxidative stress and hypoxia can also induce MTF-1 dependent gene expression. ► MTF-1 is conserved from insects to mammals. ► Drosophila and mammalian MTF-1 share several key functions. ► Nevertheless, there is little sequence similarity outside of the zinc finger domain.
Keywords: MTF-1; Heavy metal homeostasis; Zinc; Copper; Metallothionein; Transcription;

Ferroportin-mediated iron transport: Expression and regulation by Diane M. Ward; Jerry Kaplan (1426-1433).
The distinguishing feature between iron homeostasis in single versus multicellular organisms is the need for multicellular organisms to transfer iron from sites of absorption to sites of utilization and storage. Ferroportin is the only known iron exporter and ferroportin plays an essential role in the export of iron from cells to blood. Ferroportin can be regulated at many different levels including transcriptionally, post-transcriptionally, through mRNA stability and post-translationally, through protein turnover. Additionally, ferroportin may be regulated in both cell-dependent and cell-autonomous fashions. Regulation of ferroportin is critical for iron homeostasis as alterations in ferroportin may result in either iron deficiency or iron overload. This article is part of a Special Issue entitled: Cell Biology of Metals.► Ferroportin is the only known iron exporter. ► Malregulation of ferroportin leads to human disease, iron limitation or iron overload. ► Ferroportin is regulated transcriptionally, post-transcriptionally, and post-translationally. ► Post-translational regulation can occur by hepcidin-dependent and hepcidin-independent mechanisms.
Keywords: Ceruloplasmin; Ferroportin; Hepcidin; Homeostasis; Internalization; Iron;

Hepcidin and iron homeostasis by Tomas Ganz; Elizabeta Nemeth (1434-1443).
Despite fluctuations in dietary iron intake and intermittent losses through bleeding, the plasma iron concentrations in humans remain stable at 10–30 μM. While most of the iron entering blood plasma comes from recycling, appropriate amount of iron is absorbed from the diet to compensate for losses and maintain nontoxic amounts in stores. Plasma iron concentration and iron distribution are similarly regulated in laboratory rodents. The hepatic peptide hepcidin was identified as the systemic iron-regulatory hormone. In the efferent arc, hepcidin regulates intestinal iron absorption, plasma iron concentrations, and tissue iron distribution by inducing degradation of its receptor, the cellular iron exporter ferroportin. Ferroportin exports iron into plasma from absorptive enterocytes, from macrophages that recycle the iron of senescent erythrocytes, and from hepatocytes that store iron. In the more complex and less well understood afferent arc, hepatic hepcidin synthesis is transcriptionally regulated by extracellular and intracellular iron concentrations through a molecular complex of bone morphogenetic protein receptors and their iron-specific ligands, modulators and iron sensors. Through as yet undefined pathways, hepcidin is also homeostatically regulated by the iron requirements of erythroid precursors for hemoglobin synthesis. In accordance with the role of hepcidin-mediated iron redistribution in host defense, hepcidin production is regulated by inflammation as well. Increased hepcidin concentrations in plasma are pathogenic in iron-restrictive anemias including anemias associated with inflammation, chronic kidney disease and some cancers. Hepcidin deficiency causes iron overload in hereditary hemochromatosis and ineffective erythropoiesis. Hepcidin, ferroportin and their regulators represent potential targets for the diagnosis and treatment of iron disorders and anemias. This article is part of a Special Issue entitled: Cell Biology of Metals.► Hepatic peptide hepcidin regulates iron absorption, plasma concentration and tissue distribution. ► It binds to its receptor/iron exporter ferroportin and causes its internalization and degradation. ► Hepcidin is regulated by iron, inflammation and erythroid activity. ► Major iron disorders are caused by dysregulation of hepcidin. ► Molecular analysis of the hepcidin–ferroportin system allows targeting for diagnosis and therapy.
Keywords: Iron overload; Iron deficiency; Anemia;

Murine mutants in the study of systemic iron metabolism and its disorders: An update on recent advances by Thomas B. Bartnikas; Mark D. Fleming; Paul J. Schmidt (1444-1450).
Many past and recent advances in the field of iron metabolism have relied upon the use of mouse models of disease. These models have arisen spontaneously in breeder colonies or have been engineered for global or conditional ablation or overexpression of select genes. Full phenotypic characterization of these models typically involves maintenance on iron-loaded or -deficient diets, treatment with oxidative or hemolytic agents, breeding to other mutant lines or other stresses. In this review, we focus on systemic iron biology and the contributions that mouse model-based studies have made to the field. We have divided the field into three broad areas of research: dietary iron absorption, regulation of hepcidin expression and cellular iron metabolism. For each area, we begin with an overview of the current understanding of key molecular and cellular determinants then discuss recent advances. Finally, we conclude with brief comments on prospects for future study. This article is part of a Special Issue entitled: Cell Biology of Metals.► Mouse models have proven invaluable to the study of iron metabolism. ► Recent models target dietary absorption, hepcidin regulation and cellular metabolism. ► Optimal use of models involves a combination of genetic and environmental approaches.
Keywords: Mouse; Iron; Model; Dietary iron absorption; Hepcidin; Cellular iron metabolism;

NGAL-Siderocalin in kidney disease by Neal Paragas; Andong Qiu; Maria Hollmen; Thomas L. Nickolas; Prasad Devarajan; Jonathan Barasch (1451-1458).
Kidney damage induces the expression of a myriad of proteins in the serum and in the urine. The function of these proteins in the sequence of damage and repair is now being studied in genetic models and by novel imaging techniques. One of the most intensely expressed proteins is lipocalin2, also called NGAL or Siderocalin. While this protein has been best studied by clinical scientists, only a few labs study its underlying metabolism and function in tissue damage. Structure–function studies, imaging studies and clinical studies have revealed that NGAL-Siderocalin is an endogenous antimicrobial with iron scavenging activity. This review discusses the “iron problem” of kidney damage, the tight linkage between kidney damage and NGAL-Siderocalin expression and the potential roles that NGAL-Siderocalin may serve in the defense of the urogenital system. This article is part of a Special Issue entitled: Cell Biology of Metals.► NGAL is expressed by the kidney during acute kidney injury. ► Urinary NGAL correlates with the severity of acute kidney injury. ► NGAL binds iron with a bacterial siderophore, enterochelin, with an extremely high affinity. ► NGAL binds iron with an endogenous metabolite, catechol, with high affinity.
Keywords: NGAL; Siderocalin; UTI; Acute Kidney Injury; Catechol; Enterochelin;

Cellular and mitochondrial iron homeostasis in vertebrates by Caiyong Chen; Barry H. Paw (1459-1467).
Iron plays an essential role in cellular metabolism and biological processes. However, due to its intrinsic redox activity, free iron is a potentially toxic molecule in cellular biochemistry. Thus, organisms have developed sophisticated ways to import, sequester, and utilize iron. The transferrin cycle is a well-studied iron uptake pathway that is important for most vertebrate cells. Circulating iron can also be imported into cells by mechanisms that are independent of transferrin. Once imported into erythroid cells, iron is predominantly consumed by the mitochondria for the biosynthesis of heme and iron sulfur clusters. This review focuses on canonical transferrin-mediated and the newly discovered, non-transferrin mediated iron uptake pathways, as well as, mitochondrial iron homeostasis in higher eukaryotes. This article is part of a Special Issue entitled: Cell Biology of Metals.► Tf-mediated Fe assimilation. ► Mitochondrial Fe homeostasis. ► Tf-independent Fe assimilation.
Keywords: Iron; Transferrin; Non-transferrin bound iron; Mitochondrion; Mitoferrin;

Mammalian iron metabolism and its control by iron regulatory proteins by Cole P. Anderson; Macy Shen; Richard S. Eisenstein; Elizabeth A. Leibold (1468-1483).
Cellular iron homeostasis is maintained by iron regulatory proteins 1 and 2 (IRP1 and IRP2). IRPs bind to iron-responsive elements (IREs) located in the untranslated regions of mRNAs encoding protein involved in iron uptake, storage, utilization and export. Over the past decade, significant progress has been made in understanding how IRPs are regulated by iron-dependent and iron-independent mechanisms and the pathological consequences of IRP2 deficiency in mice. The identification of novel IREs involved in diverse cellular pathways has revealed that the IRP–IRE network extends to processes other than iron homeostasis. A mechanistic understanding of IRP regulation will likely yield important insights into the basis of disorders of iron metabolism. This article is part of a Special Issue entitled: Cell Biology of Metals.► IRP1 and IRP2 are the principal regulators of mammalian cellular iron homeostasis. ► IRPs bind to iron-responsive elements (IREs) located in the untranslated regions of mRNAs involved in iron uptake, storage, utilization and export. ► IRPs are post-translationally regulated by iron and reactive oxygen and nitrogen species. ► The identification of novel IREs reveals the presence of an expanded IRP–IRE network beyond cellular iron homeostasis. ► IRP deficiency in mice disrupts iron homeostasis and leads to hematological, neurodegenerative and metabolic disorders.
Keywords: Iron; IRP; Iron-responsive element; RNA-binding protein; Iron–sulfur protein; Post-transcriptional regulation;

Protein degradation and iron homeostasis by Joel W. Thompson; Richard K. Bruick (1484-1490).
Regulation of both systemic and cellular iron homeostasis requires the capacity to sense iron levels and appropriately modify the expression of iron metabolism genes. These responses are coordinated through the efforts of several key regulatory factors including F-box and Leucine-rich Repeat Protein 5 (FBXL5), Iron Regulatory Proteins (IRPs), Hypoxia Inducible Factor (HIF), and ferroportin. Notably, the stability of each of these proteins is regulated in response to iron. Recent discoveries have greatly advanced our understanding of the molecular mechanisms governing iron-sensing and protein degradation within these pathways. It has become clear that iron's privileged roles in both enzyme catalysis and protein structure contribute to its regulation of protein stability. Moreover, these multiple pathways intersect with one another in larger regulatory networks to maintain iron homeostasis. This article is part of a Special Issue entitled: Cell Biology of Metals.► Regulation of iron homeostasis requires the capacity to sense iron levels. ► Iron levels affect changes in expression of proteins mediating iron metabolism. ► The stability of many proteins is governed through regulated protein degradation. ► Mechanisms linking iron sensing and protein degradation are becoming clearer.
Keywords: F-box and Leucine-rich Repeat Protein 5; Hemerythrin domain; Iron homeostasis; Iron Regulatory Proteins; Ferroportin; Hypoxia Inducible Factor;

The role of mitochondria in cellular iron–sulfur protein biogenesis and iron metabolism by Roland Lill; Bastian Hoffmann; Sabine Molik; Antonio J. Pierik; Nicole Rietzschel; Oliver Stehling; Marta A. Uzarska; Holger Webert; Claudia Wilbrecht; Ulrich Mühlenhoff (1491-1508).
Mitochondria play a key role in iron metabolism in that they synthesize heme, assemble iron–sulfur (Fe/S) proteins, and participate in cellular iron regulation. Here, we review the latter two topics and their intimate connection. The mitochondrial Fe/S cluster (ISC) assembly machinery consists of 17 proteins that operate in three major steps of the maturation process. First, the cysteine desulfurase complex Nfs1–Isd11 as the sulfur donor cooperates with ferredoxin–ferredoxin reductase acting as an electron transfer chain, and frataxin to synthesize an [2Fe–2S] cluster on the scaffold protein Isu1. Second, the cluster is released from Isu1 and transferred toward apoproteins with the help of a dedicated Hsp70 chaperone system and the glutaredoxin Grx5. Finally, various specialized ISC components assist in the generation of [4Fe–4S] clusters and cluster insertion into specific target apoproteins. Functional defects of the core ISC assembly machinery are signaled to cytosolic or nuclear iron regulatory systems resulting in increased cellular iron acquisition and mitochondrial iron accumulation. In fungi, regulation is achieved by iron-responsive transcription factors controlling the expression of genes involved in iron uptake and intracellular distribution. They are assisted by cytosolic multidomain glutaredoxins which use a bound Fe/S cluster as iron sensor and additionally perform an essential role in intracellular iron delivery to target metalloproteins. In mammalian cells, the iron regulatory proteins IRP1, an Fe/S protein, and IRP2 act in a post-transcriptional fashion to adjust the cellular needs for iron. Thus, Fe/S protein biogenesis and cellular iron metabolism are tightly linked to coordinate iron supply and utilization. This article is part of a Special Issue entitled: Cell Biology of Metals.► Iron–sulfur protein biogenesis in mitochondria is a conserved and essential process. ► The process can be sub‐divided into three major biosynthetic steps. ► The process serves as a major regulator for cellular iron homeostasis. ► The process is relevant for neurodegenerative, hematological and metabolic diseases.
Keywords: Metal cofactor; Chaperone; Glutaredoxin; Desulfurase; Ferredoxin; Frataxin;

Metabolic remodeling in iron-deficient fungi by Caroline C. Philpott; Sébastien Leidgens; Avery G. Frey (1509-1520).
Eukaryotic cells contain dozens, perhaps hundreds, of iron-dependent proteins, which perform critical functions in nearly every major cellular process. Nutritional iron is frequently available to cells in only limited amounts; thus, unicellular and higher eukaryotes have evolved mechanisms to cope with iron scarcity. These mechanisms have been studied at the molecular level in the model eukaryotes Saccharomyces cerevisiae and Schizosaccharomyces pombe, as well as in some pathogenic fungi. Each of these fungal species exhibits metabolic adaptations to iron deficiency that serve to reduce the cell's reliance on iron. However, the regulatory mechanisms that accomplish these adaptations differ greatly between fungal species. This article is part of a Special Issue entitled: Cell Biology of Metals.►Fungi adapt to iron deficiency by increasing iron uptake and decreasing iron use. ►Budding yeast rely on transcriptional and post-transcriptional regulators. ►Other fungi rely on a pair of transcriptional repressors. ►Different mechanisms are used to achieve similar metabolic changes.
Keywords: Aspergillus; Saccharomyces cerevisiae; Schizosaccharomyces pombe; Iron; Heme; Iron–sulfur cluster;

Getting a sense for signals: Regulation of the plant iron deficiency response by Maria N. Hindt; Mary Lou Guerinot (1521-1530).
Understanding the Fe deficiency response in plants is necessary for improving both plant health and the human diet, which relies on Fe from plant sources. In this review we focus on the regulation of the two major strategies for iron acquisition in plants, exemplified by the model plants Arabidopsis and rice. Critical to our knowledge of Fe homeostasis in plants is determining how Fe is sensed and how this signal is transmitted and integrated into a response. We will explore the evidence for an Fe sensor in plants and summarize the recent findings on hormones and signaling molecules which contribute to the Fe deficiency response. This article is part of a Special Issue entitled: Cell Biology of Metals.► We summarize two strategies for Fe acquisition in plants. ► Strategy I response appears to be controlled by two transcriptional networks: FIT and POPEYE. ► Hormones are implicated in positive and negative control of the Fe deficiency response.
Keywords: Iron deficiency response; Iron reduction; Iron chelation; Iron regulated transcription factor; Iron sensor; Hormone;

The ins and outs of algal metal transport by Crysten E. Blaby-Haas; Sabeeha S. Merchant (1531-1552).
Metal transporters are a central component in the interaction of algae with their environment. They represent the first line of defense to cellular perturbations in metal concentration, and by analyzing algal metal transporter repertoires, we gain insight into a fundamental aspect of algal biology. The ability of individual algae to thrive in environments with unique geochemistry, compared to non-algal species commonly used as reference organisms for metal homeostasis, provides an opportunity to broaden our understanding of biological metal requirements, preferences and trafficking. Chlamydomonas reinhardtii is the best developed reference organism for the study of algal biology, especially with respect to metal metabolism; however, the diversity of algal niches necessitates a comparative genomic analysis of all sequenced algal genomes. A comparison between known and putative proteins in animals, plants, fungi and algae using protein similarity networks has revealed the presence of novel metal metabolism components in Chlamydomonas including new iron and copper transporters. This analysis also supports the concept that, in terms of metal metabolism, algae from similar niches are more related to one another than to algae from the same phylogenetic clade. This article is part of a Special Issue entitled: Cell Biology of Metals.► Comparative genomic approach reveals the diversity of metal metabolism among algae. ► Algal genomes encode transporters related to plants, animals and bacteria. ► Similarity networks are used to predict functions for putative metal transporters. ► Some transporters are fused to other domains.
Keywords: FEA1; CTR; CDF; Chloroplast; Zinc; Iron;

The zinc homeostasis network of land plants by Scott Aleksander Sinclair; Ute Krämer (1553-1567).
The use of the essential element zinc (Zn) in the biochemistry of land plants is widespread, and thus comparable to that in other eukaryotes. Plants have evolved the ability to adjust to vast fluctuations in external Zn supply, and they can store considerable amounts of Zn inside cell vacuoles. Moreover, among plants there is overwhelming, but yet little explored, natural genetic diversity that phenotypically affects Zn homeostasis. This results in the ability of specific races or species to thrive in different soils ranging from extremely Zn-deficient to highly Zn-polluted. Zn homeostasis is maintained by a tightly regulated network of low-molecular-weight ligands, membrane transport and Zn-binding proteins, as well as regulators. Here we review Zn homeostasis of land plants largely based on the model plant Arabidopsis thaliana, for which our molecular understanding is most developed at present. There is some evidence for substantial conservation of Zn homeostasis networks among land pants, and this review can serve as a reference for future comparisons. Major progress has recently been made in our understanding of the regulation of transcriptional Zn deficiency responses and the role of the low-molecular-weight chelator nicotianamine in plant Zn homeostasis. Moreover, we have begun to understand how iron (Fe) and Zn homeostasis interact as a consequence of the chemical similarity between their divalent cations and the lack of specificity of the major root iron uptake transporter IRT1. The molecular analysis of Zn-hyperaccumulating plants reveals how metal homeostasis networks can be effectively modified. These insights are important for sustainable bio-fortification approaches. This article is part of a Special Issue entitled: Cell Biology of Metals.► Zn homeostasis factors ► Zn deficiency response regulator ► systemic signalling ► Novel imaging tools
Keywords: Zn; Fe; Metal; Hyperaccumulator plant; Arabidopsis; Metal regulation;

Cell biology of molybdenum in plants and humans by Ralf R. Mendel; Tobias Kruse (1568-1579).
The transition element molybdenum (Mo) needs to be complexed by a special cofactor in order to gain catalytic activity. With the exception of bacterial Mo-nitrogenase, where Mo is a constituent of the FeMo-cofactor, Mo is bound to a pterin, thus forming the molybdenum cofactor Moco, which in different variants is the active compound at the catalytic site of all other Mo-containing enzymes. In eukaryotes, the most prominent Mo-enzymes are nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and the mitochondrial amidoxime reductase. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also requires iron, ATP and copper. After its synthesis, Moco is distributed to the apoproteins of Mo-enzymes by Moco-carrier/binding proteins. A deficiency in the biosynthesis of Moco has lethal consequences for the respective organisms. In humans, Moco deficiency is a severe inherited inborn error in metabolism resulting in severe neurodegeneration in newborns and causing early childhood death. This article is part of a Special Issue entitled: Cell Biology of Metals.► Mo needs to be complexed by a special cofactor in order to gain catalytic activity. ► The molybdenum cofactor (Moco) forms the active site of all eukaryotic Mo enzymes. ► Mo enzymes are an essential part of the global carbon, nitrogen and sulfur cycles. ► Moco is synthesized by a conserved pathway that consists of four steps. ► Iron and copper are involved in Mo metabolism in eukaryotes.
Keywords: Molybdenum cofactor; Molybdenum deficiency; Sulfite oxidase; Nitrate reductase; Xanthine dehydrogenase; Aldehyde oxidase;

Charting the travels of copper in eukaryotes from yeast to mammals by Tracy Nevitt; Helena Öhrvik; Dennis J. Thiele (1580-1593).
Throughout evolution, all organisms have harnessed the redox properties of copper (Cu) and iron (Fe) as a cofactor or structural determinant of proteins that perform critical functions in biology. At its most sobering stance to Earth's biome, Cu biochemistry allows photosynthetic organisms to harness solar energy and convert it into the organic energy that sustains the existence of all nonphotosynthetic life forms. The conversion of organic energy, in the form of nutrients that include carbohydrates, amino acids and fatty acids, is subsequently released during cellular respiration, itself a Cu-dependent process, and stored as ATP that is used to drive a myriad of critical biological processes such as enzyme-catalyzed biosynthetic processes, transport of cargo around cells and across membranes, and protein degradation. The life-supporting properties of Cu incur a significant challenge to cells that must not only exquisitely balance intracellular Cu concentrations, but also chaperone this redox-active metal from its point of cellular entry to its ultimate destination so as to avert the potential for inappropriate biochemical interactions or generation of damaging reactive oxidative species (ROS). In this review we chart the travels of Cu from the extracellular milieu of fungal and mammalian cells, its path within the cytosol as inferred by the proteins and ligands that escort and deliver Cu to intracellular organelles and protein targets, and its journey throughout the body of mammals. This article is part of a Special Issue entitled: Cell Biology of Metals.► Copper is an essential metal for human development and health. ► Yeast and mammals share dedicated mechanisms to maintain copper homeostasis. ► Insights into how organisms orchestrate a response to copper availability contributes towards the understanding of human disease.
Keywords: Copper; Homeostasis; Yeast; Mammal; Transporter; Chaperone;

Role of metal in folding and stability of copper proteins in vitro by Maria E. Palm-Espling; Moritz S. Niemiec; Pernilla Wittung-Stafshede (1594-1603).
Metal coordination is required for function of many proteins. For biosynthesis of proteins coordinating a metal, the question arises if the metal binds before, during or after folding of the polypeptide. Moreover, when the metal is bound to the protein, how does its coordination affect biophysical properties such as stability and dynamics? Understanding how metals are utilized by proteins in cells on a molecular level requires accurate descriptions of the thermodynamic and kinetic parameters involved in protein–metal complexes. Copper is one of the essential transition metals found in the active sites of many key proteins. To avoid toxicity of free copper ions, living systems have developed elaborate copper-transport systems that involve dedicated proteins that facilitate efficient and specific delivery of copper to target proteins. This review describes in vitro and in silico biophysical work assessing the role of copper in folding and stability of copper-binding proteins. Examples of proteins discussed are: a blue-copper protein (Pseudomonas aeruginosa azurin), members of copper-transport systems (bacterial CopZ, human Atox1 and ATP7B domains) and multi-copper ferroxidases (yeast Fet3p and human ceruloplasmin). The consequences of interactions between copper proteins and platinum-complexes are also discussed. This article is part of a Special Issue entitled: Cell Biology of Metals.► In vitro thermodynamic and kinetic observations are complementary to structural and functional data. ► If copper binds prior to folding, active protein is formed 3 orders of magnitude faster. ► Conserved residues in Atox1 increase stability, add flexibility and guide target interaction. ► Multi-copper oxidases unfold via intermediates and the copper ions are removed in steps. ► The cancer agent cisplatin interacts with Atox1 and triggers slow protein unfolding.
Keywords: Protein folding; Azurin; Atox1; Ceruloplasmin; Wilson disease protein; Cisplatin;

Structure, function, and assembly of heme centers in mitochondrial respiratory complexes by Hyung J. Kim; Oleh Khalimonchuk; Pamela M. Smith; Dennis R. Winge (1604-1616).
The sequential flow of electrons in the respiratory chain, from a low reduction potential substrate to O2, is mediated by protein-bound redox cofactors. In mitochondria, hemes—together with flavin, iron–sulfur, and copper cofactors—mediate this multi-electron transfer. Hemes, in three different forms, are used as a protein-bound prosthetic group in succinate dehydrogenase (complex II), in bc 1 complex (complex III) and in cytochrome c oxidase (complex IV). The exact function of heme b in complex II is still unclear, and lags behind in operational detail that is available for the hemes of complex III and IV. The two b hemes of complex III participate in the unique bifurcation of electron flow from the oxidation of ubiquinol, while heme c of the cytochrome c subunit, Cyt1, transfers these electrons to the peripheral cytochrome c. The unique heme a 3, with CuB, form a catalytic site in complex IV that binds and reduces molecular oxygen. In addition to providing catalytic and electron transfer operations, hemes also serve a critical role in the assembly of these respiratory complexes, which is just beginning to be understood. In the absence of heme, the assembly of complex II is impaired, especially in mammalian cells. In complex III, a covalent attachment of the heme to apo-Cyt1 is a prerequisite for the complete assembly of bc 1, whereas in complex IV, heme a is required for the proper folding of the Cox 1 subunit and subsequent assembly. In this review, we provide further details of the aforementioned processes with respect to the hemes of the mitochondrial respiratory complexes. This article is part of a Special Issue entitled: Cell Biology of Metals
Keywords: Heme; Succinate dehydrogenase; Cytochrome c oxidase; Ubiquinol cytochrome c oxidoreductase; Mitochondria; Respiration;

The appearance of heme, an organic ring surrounding an iron atom, in evolution forever changed the efficiency with which organisms were able to generate energy, utilize gasses and catalyze numerous reactions. Because of this, heme has become a near ubiquitous compound among living organisms. In this review we have attempted to assess the current state of heme synthesis and trafficking with a goal of identifying crucial missing information, and propose hypotheses related to trafficking that may generate discussion and research. The possibilities of spatially organized supramolecular enzyme complexes and organelle structures that facilitate efficient heme synthesis and subsequent trafficking are discussed and evaluated. Recently identified players in heme transport and trafficking are reviewed and placed in an organismal context. Additionally, older, well established data are reexamined in light of more recent studies on cellular organization and data available from newer model organisms. This article is part of a Special Issue entitled: Cell Biology of Metals.► Enzymology and regulation of heme biosynthesis. ► Supramolecular protein assemblies in heme synthesis. ► Heme transport and trafficking. ► Intracellular and intercellular heme transport. ► Heme-binding proteins.
Keywords: Heme; Tetrapyrrole; Porphyrin; Iron; Trafficking;

Understanding selenoprotein function and regulation through the use of rodent models by Marina V. Kasaikina; Dolph L. Hatfield; Vadim N. Gladyshev (1633-1642).
Selenium (Se) is an essential micronutrient. Its biological functions are associated with selenoproteins, which contain this trace element in the form of the 21st amino acid, selenocysteine. Genetic defects in selenocysteine insertion into proteins are associated with severe health issues. The consequences of selenoprotein deficiency are more variable, with several selenoproteins being essential, and several showing no clear phenotypes. Much of these functional studies benefited from the use of rodent models and diets employing variable levels of Se. This review summarizes the data obtained with these models, focusing on mouse models with targeted expression of individual selenoproteins and removal of individual, subsets or all selenoproteins in a systemic or organ-specific manner. This article is part of a Special Issue entitled: Cell Biology of Metals.► Se regulates pathways through incorporation into selenoproteins in the form of Sec. ► Recent findings in selenoprotein biosynthesis and functions are summarized. ► Overview of available knockout mouse models relevant to Se biology is provided. ► Mouse models with targeted expression of selenoproteins are described. ► Limitations of using animal models and insights into human health are discussed.
Keywords: Selenium; Selenocysteine; Selenoprotein; Mouse model;