BioIron 2019: Iron Essentials for Clinicians and Scientists BioIron: origin, chemical properties and evolution Kostas Pantopoulos Lady Davis Institute for Medical Research McGill University Iron: a highly abundant transition metal The elements in haiku by Mary Soon Lee, sciencemag.org Produced in the universe by fusion in high mass stars and scattered into space to accumulate in rocky planets like earth Most abundant element on earth (crust and inner core) 4th most abundant element on earth’s crust Electron configuration and oxidation states Fe2+: ferrous iron Fe3+: ferric iron Fe4+: ferryl iron (unstable intermediate) Coordination chemistry Iron readily forms complexes with organic molecules, and this has immense biological implications iron-sulfur clusters heme iron-oxo centers Iron cofactors can be utilized by proteins for: A wide range of electron transfer biochemical reactions within a broad electrochemical potential (Eo from -400 to +400 mV) Various other structural and metabolic functions Functions of proteins with iron cofactors Heme oxygen binding (hemoglobin, myoglobin) electron transfer reactions (cytochromes) catalysis (oxidases, peroxidases, NO synthases) hemoglobin Iron-sulfur (Fe-S) clusters electron transfer reactions (ferredoxins, Rieske proteins) catalysis (aconitases, other dehydratases) activation of substrate (cyclooxygenase, lipoxygenase, other oxidases) transcriptional regulation (bacterial transcription factors FNR, SoxR) structure stabilization (bacterial endonuclease III) Iron-oxo (Fe-O-Fe) centers catalysis (ribonucleotide reductase) oxygen binding (invertebrate hemerythrin) Oxidoreductases Oxidoreductases mediate electron transfer reactions that provide energy for life ~70% of oxidoreductases annotated in databases contain iron Oxidoreductases cluster into 4 distinct modular structures Raanan et al, PNAS, 2018 The first likely module was that of ferredoxin, a protein with an Fe-S cluster cofactor Ferredoxin and the Fe-S world hypothesis Ferredoxin is a ubiquitous ancient protein May have evolved by duplications of a tetra-peptide that binds an Fe-S cluster Eck and Daychoff, Science, 1966 May catalyze a putative carbon reduction cycle in a photosynthetic bacterium Evans et al, PNAS, 1966 Analogous early “chemistry of life” may have occurred on iron pyrite (FeS2) surfaces in deep hydrothermal vents to yield CO- and/or CO2-fixation Iron-sulfur world hypothesis for the origin of life by G. Wächterhäuser Fe-S clusters, the first catalysts in nature? Versatile chemistry based on both iron and sulfur, which can acquire oxidation states from -2 to +6 Complex biogenesis Lill, Nature, 2009 Most components of Fe-S cluster biogenesis machinery are essential for cell and organism viability Iron: a catalyst in chemistry, biology and beyond Cultural evolution iron age timeline Industrial revolution The Janus face of iron Switch between Fe2+ and Fe3+ under aerobic conditions provides the basis for iron’s toxicity Fenton chemistry 2+ 3+ - . (1) Fe + H2O2 Fe + OH + OH (Fenton reaction) 3+ .- 2+ (2) Fe + O2 Fe + O2 .- Fe - . (1+2) H2O2 + O2 OH + OH + O2 (Haber-Weiss reaction) . OH (hydroxyl radical) proteins nucleic acids oxidation lipids mutagenesis membrane lipid peroxidation Challenge for iron metabolism Satisfy the metabolic needs of cells and organisms for iron Minimize the risk of iron toxicity in the aerobic environment The problem of iron assimilation Iron is highly abundant, but … In aqueous solutions, Fe2+ is readily oxidized to Fe3+ 3+ At neutral pH, Fe forms essentially insoluble Fe(OH)3 Iron’s bioavailability is limited Strategies for iron acquisition Direct iron transport First step: Reduction of extracellular Fe3+ to Fe2+ by membrane-bound ferric reductases yeast Indirect iron uptake Many bacteria secrete siderophores that chelate extracellular Fe3+ Iron-loaded siderophores are internalized into the bacteria by binding to specific cell-surface receptors deferroxamine Iron assimilation in higher organisms Both concepts: Direct iron transport by trans-membrane transporters, following reduction to soluble Fe2+ and Receptor-mediated uptake of iron, that is “captured” in form of Fe3+ by an iron-binding molecule have been conserved in higher organisms Direct iron transport is critical for dietary iron absorption Reduction of Fe3+ to Fe2+ by ferric reductases (such as Dcytb), transport of Fe2+ across the apical membrane of enterocytes by DMT1 Export of internalized Fe2+ to plasma via ferroportin Re-oxidation of Fe2+ to Fe3+ by the ferroxidases hephaestin and ceruloplasmin, and binding of Fe3+ to transferrin, the plasma iron carrier Direct iron transport to tissues via Zip14 is operational under pathological conditions of iron overload Cellular iron uptake by receptor-mediated endocytosis Iron-loaded Tf binds to TfR1 on the cell surface and the complex undergoes endocytosis Fe3+ is released from transferrin in acidified endosome, reduced to Fe2+ by Steap3, and transported across the endosomal membrane by DMT1 Internalized iron is utilized for metabolic purposes and excess can be exported via ferroportin or stored in ferritin Ferritin, the iron storage protein Ferritin is composed of H- and L-chains containing a characteristic four-helical bundle domain H-ferritin has ferroxidase activity The 24 ferritin subunits fold to a shell-like structure that can store up to 4500 Fe3+ ions in a non-toxic, bioavailable form human H-ferritin murine holo-ferritin bacterioferritin Ferritin homologues are found in bacteria Evolution of ferritin Iron storage/detoxification is critical for cells and organisms Ferritin is the only iron storage protein and is widely conserved from archea to humans Ferritin belongs to the ancient superfamily of ferritin-like proteins, which contain the common four-helical bundle domain Andrews SC, BBA, 2010 The prototypic four-helical bundle domain likely originates from a rubrerythrin-like di-iron peroxidase Other ancient proteins of iron metabolism? Only the pathways for Fe-S biogenesis and heme biosynthesis are largely conserved across kingdoms and species Proteins involved in iron transport and regulation of iron metabolism are unique in higher eukaryotes Iron regulatory pathways have acquired unique characteristics across the evolutionary scale Mammalian iron metabolism Regulation at the cellular level via the IRE/IRP system Coordinate post-transcriptional regulation of cellular iron uptake, utilization, storage and efflux in response to iron supply and other signals Regulation at the cellular level via the hepcidin/ferroportin axis Hormonal regulation of iron efflux to bloodstream in response to iron stores, inflammatory stimuli, erythropoietic iron demand and other signals Fine-tuning via crosstalk of these fundamental pathways, and by additional complementary mechanisms Iron physiology in humans ~20-30 mg/day Fe iron homeostasis iron overload (hereditary or acquired) liver disease heart disease diabetes arthritis osteoporosis endocrinopathy iron-related disorders anemia growth defects iron deficiency cognitive impairment (mostly nutritional) immune defects heart disease Prevalence of iron-related disorders Iron deficiency the most common medical condition, affecting ~25% of the world’s population Hereditary hemochromatosis, a disease of iron overload the most common genetic disorder, with prevalence exceeding those of cystic fibrosis, muscular dystrophy and phenylketonuria combined Anemia of inflammation, a condition linked to aberrant iron traffic the most common anemia among hospitalized patients in industrial countries Iron deregulation in common disorders Metabolic syndrome (obesity, insulin resistance, dyslipidemia, hypertension, NAFLD, NASH) Viral hepatitis (chronic hepatitis C) Cancer Anemias with ineffective erythropoiesis (β-thalassemia, myelodysplastic syndromes, dyserythropoietic anemias) Cardiovascular diseases Neurodegenerative diseases (Parkinson’s, Alzheimer’s etc) The importance of hepcidin Hereditary hemochromatosis and anemia of inflammation are linked to hepcidin deficiency or excess, respectively Levels of hepcidin expression affect iron supplementation therapies to treat iron deficiency Hepcidin therapeutics Pharmacological targeting of the hepcidin/ferroportin axis has a high potential for treatment of iron-related disorders Why is hereditary hemochromatosis common? Hereditary hemochromatosis is a genetically heterogenous endocrine disorder caused by hepcidin deficiency due to mutations in genes of the hepcidin pathway The most common form is linked to mutations in the HFE gene, encoding an MHC class 1 protein Conceivably, the main C282Y HFE mutation was spread in central Europe at ~4000 BC, when humans switched from iron- rich meat-based diet to iron-poor agricultural dietary sources, and conferred advantages in iron assimilation The spread of the C282Y HFE mutation may also be attributed to protection against infection with intracellular pathogens Why is anemia of inflammation common? Inflammatory induction of hepcidin causes iron retention in tissues, which limits its availability for erythropoiesis The ensuing hypoferremia is thought to be protective against infection with extracellular pathogens (nutritional immunity) Eugene Weinberg 1922-2019 Failure to resolve inflammation prolongs hepcidin induction and iron restriction for erythropoiesis, causing anemia The battle for iron: Iron restriction as an innate immune defense Virulence of infectious organisms depends on their capacity to assimilate iron from the host The host strikes back by limiting iron availability