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Metals in Redox Biology Ariana Farrand, Linda Pudelko, Songita Choudhury, Amirata
Saei Dibavar, Arun Kumar Selvam
May 17th, 2017 Course: „Redox regulaGon, oxidaGve stress and
selenoproteins“
QuesGons 1. Which metals are the major contributors to hydroxyl
radical formaGon in cells? 2. What are the intracellular concentraGons of these
metals in mammals? 3. Show the reacGon by which metals catalyze the
formaGon of hydroxyl radicals 4. Examples of how mammalian cells import/export metals 5. How are import/export systems used to protect against
stress? 6. Examples of a metalloprotein involved in oxygen and
redox sensing à descripGon of the mechanism of response
Hydroxyl radical
Source: BRIGHTWATER Website
DNA
• OxidaGon of DNA bases • MutaGon & DNA damage
• Cellular senescence, apoptosis or carcinogenesis
Lipids • Lipid peroxidaGon • Membrane damage
Proteins • Amino acid oxidaGon • (De-‐) AcGvaGon of enzymes or signaling molecules
Ø Dangerous to any biological system Ø Long list of associated diseases affecGng any part of the body
GeneraGon of hydroxyl radical in vivo
Ø Excessive exposure to ionizing radiaGon Ø Breakdown of hydrogen peroxide via Fenton Reac*on
• Iron (Fe) and Copper (Cu) catalyze the producGon of hydroxyl radical from hydrogen peroxide
1. Which metals are the major contributors to hydroxyl radical formaGon in cells?
Ø Iron and Copper
Metal homeostasis Ø Metals involved in various biological processes
• i.e.: enzyme reacGons, signal transducGons, electron transfer, oxygen transport à electron donor and acceptor
Ø TransiGon metal ions: criGcal roles as electron transfer intermediates in redox reacGons
Ø Metals are taken up from diet • Metabolized // post-‐translaGonal modificaGon to form metalloproteins
Ø Excess metal accumulaGon and release in free reacGve form à toxic!
Tight regulaGon of metal uptake, metabolism, transport, assembly into metalloproteins and detoxicifaGon of outmost importance!
Ø Common oxidaGon states: • Ferrous (+2) ion, ferric (+3) ion
Ø Due to high toxicity, most iron is bound to proteins Ø labile iron: free iron bound to low-‐affinity complexes Ø Binding to proteins: limiing iron‘s ability to do harm, while
benefiing from its funcGon )
DistribuGon • FuncGonal iron (Haemoglobin, Myoglobin, Haem enzymes) • Transport iron (Transferrin) • Storage iron (FerriGn, Haemosiderin) Ø Oxygen transport and cellular respiraGon
Ø Common oxidaGon states: • Cuprous (+1) ion, Cupric ion (+2)
Ø IncorporaGon into variety of proteins and metalloenzymes for metabolic funcGons, i.e.: • Ceruloplasmin (ferroxidase I): iron transport • Cytochrome c oxidase: electron transport chain • Superoxide dismutase: anGoxidant • Lysyl oxidase: cross-‐linking of collagen and elasGn
Ø EssenGal funcGons in i.e.:
• Growth, developmen and maintenance of bones, connecGve Gssue, brain and heart
• FormaGon of red blood cells • AbsorpGon and uGlizaGon of iron
2. What are the intracellular concentraGons of these metals in mammals?
Iron Copper
Ø Total intracellular: 20 µM Ø Intracellular labilie iron < 1 µM Ø i.e.: Erythrocytes: ̴300 µM Total body iron Male: 3000 – 4000 mg Female: 2000 – 3000 mg
Daily uptake /loss (19 – 50 yrs) Male: 8 mg/1-‐2 mg Female: 18 mg/ 1-‐2 mg Distribu*on FuncGonal iron: 80 % Storage iron: 20 %
Ø Total intracellular: 0.8 – 10 µM Total body copper 80 – 100 mg
Daily uptake 1.3 mg/day (< 5 mg/day) Distribu*on • Liver (15 %) • Brain (10 %) • Heart and kidneys • Skeleton (20 %) • Serum (6 %)
Show the reacGon by which metals catalyze the formaGon of hydroxyl radicals
Provide an example of how mammalian cells import/export metal ions
Copper import Ø Reduced copper comes into cell via Ctr1 protein
Ø Transported to mitochondria via Cox17
Ø To Golgi via Atox1 Ø CCS directly interacts with SOD to insert Cu
Ø MT induced to bind excess free Cu
Provide an example of how mammalian cells import/export metal ions
Copper export Ø ATP7A (MNK) Ø ATP7B (WND) Ø Sequester in vesicles for
excreGon Ø Cuproprotein synthesis
The importance of metal transport
Balance is reached through uptake, storage and secreGon. A delicate balance of transport acGviGes is required in difference cellular compartments, because: § Transi,on metals are essen,al for the func,on of most proteins
involved in redox reac,ons § Several life processes involves toxic reagents that, when present in
abnormal amounts, damage proteins and nucleic acids For example, abnormal iron uptake is implicated in the most common hereditary disease hemochromatosis, along with neurological disorders such as Parkinson’s disease, Friedreich’s ataxia, etc.
ClassificaGon of metal transporters
The different transporters can be grouped into: § Those driven by the chemical energy of ATP § Those driven by electrochemical gradients of protons and
other ions Some of the systems are built up by couples of transporters, one of high affinity and low capacity and the other of low affinity and high capacity
Various protein-‐based components involved in metal trafficking
Lalla Aicha Ba et al. Metallomics, 2009, 1, 292–311.
Promiscuity of the transporters • The divalent caGon transporter DCT1, for instance, may
transport iron, zinc, manganese and copper, but also cobalt, cadmium, nickel and lead.
• Similarly, the phosphate transporter may be used by structurally similar vanadate and arsenate to gain entry into the cell, whilst the sulfate transporter is prone to being ‘abused’ by chromate, selenite and molybdate.
Components of metal ion trafficking
• Metal binding proteins e.g. calmodulin, ferriGn • Pre-‐formed metal binding sites in proteins and enzymes e.g. DCT1, SOD2 • Chaperones (metallochaperone and ion-‐inserGng proteins) e.g. CCS • The labile metal ion pool
Some detailed examples: Ca • SynergisGc binding of two to four labile, ‘free’ Ca2+ ions to the
four EF-‐hands of the Ca sensor calmodulin triggers a change in the structure from the inacGve to the acGve form.
• The later then binds to various proteins and enzymes and acGvates Ca2+ -‐ATPase pumps, which in turn lower intracellular Ca2+ levels.
• Once these levels have fallen below 10-‐7 to 10-‐8 M, Ca2+ ions begin to dissociate from the calcium–calmodulin complex. This returns calmodulin to its metal-‐free, inacGve form, and the proteins and enzymes acGvated by calmodulin switch-‐off.
• The Ca2+-‐ATPase pumps, in parGcular, are also turned off, which ensures that intracellular Ca2+ levels do not fall below a criGcal level required by the cell (around 1.0–5.0x10-‐8 M).
• If Ca2+ levels rise again, binding to calmodulin occurs and the regulatory feedback loop is triggered once more.
Iron • It is proposed that DCT1 is the main
port of iron entry in the duodenum. • In the blood stream another copper
protein, ceruloplasmin, oxidizes Fe2+ to Fe3+ and makes it amenable to bind apotransferrin.
• Iron enters the human cell as part of the iron-‐transport protein transferrin, which is taken up by endocytosis.
• Inside the cell, iron is either stored in ferriGn or escorted to appropriate apoproteins.
• The chaperone frataxin is parGcularly important. It provides iron for the assembly of iron/sulfur clusters in proteins.
• Excess of iron is removed from the cell by a set of proteins, including IREG-‐1 and the hephaesGn iron export complex.
Manganese • It appears that manganese enters the cell via DCT1 and
relies on MCF to cross the mitochondrial membrane, where it binds to SOD2.
Molybdenum • Unlike the other metal ions, molybdenum is trafficked not as a
caGon but as molybdate (MoO4 anion). • It almost exclusively ends up in the molybdenum cofactor
(Moco), which exhibits a trafficking system on its own.
Transport and inserGon of copper • Copper enters the cell via the high-‐
affinity CTR transporters or the low-‐affinity DCT1.
• Once inside the cell, copper is passed on to one of the chaperones, which escort the ion either to the Golgi (ATOX1), apo-‐SOD1 (CCS) or to the mitochondria (Cox17, Cox19).
• Upon reaching its desGnaGon, copper is either imported into the Golgi and released (using a P-‐type ATPase denoted as cP, including the Wilson and Menkes disease proteins), inserted into apo-‐SOD1 or incorporated into cytochrome c oxidase located in the mitochondria (using proteins such as Sco1, Sco2 and Cox11).
• ‘Most of the copper trafficked appears to be Cu+, yet redox processes and Cu2+ may also play an important part in copper trafficking.
Zinc Zn2+ enters the cell via the ZIP transporters and in part also by diffusion (there is a 500-‐fold excess of Zn2+ outside the cell) and possibly by the (apparently bi-‐direcGonally acGve) Na+/Zn2+ exchangers. Once inside the cell, Zn2+ is taken up by apoproteins or is sequestered by the thioneins, the apo-‐form of the metallothionein (MT) proteins. Either directly or via the MT proteins, Zn2+ is passed on to the zinc transport (ZnT) proteins which traffic zinc within the cell and also expel it from the cell. Zinc may also leave the cell via the Na+/Zn2+ exchangers. Intracellular concentraGons of labile Zn2+ are extraordinarily low. They are regulated by a complex feedback loop which involves the Zn2+ sensing transcripGon factor MTF-‐1, the MRE of DNA and de novo synthesis of thionein and ZnT.
Import/export systems
5. How are import/export systems used to protect against stress?
6. Examples of a metalloprotein involved in oxygen and redox sensing à descripGon of the mechanism of response
Import/Export Systems Ø Iron mediated oxidaGve stress is prevented by chelaGng free
iron, prevenGng the Fenton reacGon. Ø Desferrioxamine mesylate (DFO) is a “hexidentate“. It will bind all six
sites of iron once inside the cell to reduce redox acGvity. Ø Siderophores
Ø Copper homeostasis is maintained via metallochaperones. They must also be chelated to prevent the Fenton reacGon Ø Within hepatocytes, copper remains bound to metallothionenin
Valko et al., Curr Med Chem, 2005
Heme Oxygenase-‐1
• HMOX-‐1 as a protecGve mechanism against oxidaGve cellular stress.
• Free radicle generaGon by chemical or physical mean can increase the expression of HMOX1.
• GSH depleGon act as a signal for HMOX-‐1 transcripGonal acGvaGon.
HMOX1 AcGvaGon
HMOX1 acGvaGon by stress
• HMOX regulaGng by MAPK, transcriGon factor such as NRF2, AP-‐1 & HIF1.
• General marker of oxidaGve stress in cell culture models.
• Heme: a substrate for HMOX
Mechanism of acGon
• Free heme a potent catalyst of lipid peroxidaGon, and to promote oxidaGve damage to vascular endothelial cells
References • Chapter 1 : Overview of ReacGve Oxygen Species, in Singlet Oxygen: Applica,ons in Biosciences and Nanosciences, Volume 1,
2016, 1, pp. 1-‐21 • Pham-‐Huy LA, He H, Pham-‐Huy C. Free Radicals, AnGoxidants in Disease and Health. Interna,onal Journal of Biomedical
Science : IJBS. 2008;4(2):89-‐96. • James P. Kehrer & Lars-‐Oliver Klotz, Free radicals and related reacGve species as mediators of Gssue injury and disease:
implicaGons for Health. Toxicology. 2015; 45:9,765-‐798 • Waldvogel-‐Abramowski S, Waeber G, Gassner C, et al. Physiology of Iron Metabolism. Transfusion Medicine and
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(Ed.), ISBN: 978-‐953-‐51-‐0605-‐0, InTech, Available from: htp://www.intechopen.com/books/iron-‐metabolism/iron-‐metabolism-‐in-‐humans-‐an-‐overview
• WHO/FAO/IAEA, (1996), Trace Elements in Human NutriGon and Health. World Health OrganizaGon, Geneva) • Tolerable Upper Intake Levels For Vitamins And Minerals (PDF), European Food Safety Authority, 2006 • InsGtute of Medicine. Food and NutriGon Board.
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(Bremen: UNI-‐MED, 2008) • Stern, Bonnie Ransom; Solioz, Marc; Krewski, Daniel; Agget, Peter; Aw, Tar-‐Ching; Baker, Scot; Crump, Kenny; Dourson,
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• Lalla Aicha Ba, Mandy Doering, Torsten Burkholz and Claus Jacob (2009) Metal trafficking: from maintaining the metal homeostasis to future drug design. Metallomics,, 1, 292–311.
References • Chapter 4: Redox regulaGon of physiological processes, in Redox textbook
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