CHEMISTRY OF LIFE IRON

Ioav Cabantchik

Sept 2015

The Alexander Silberman Institute of Life Sciences The Hebrew University of Jerusalem

“Life is nothing but an electron looking for a place to rest”

Albert Szent-Györgyi , Nobel Prize 1937

denitrifiers

photoferrotrophs

aerobes

AGENDA • • • •

Iron as transition element Electronic configuration of oxidation states Distribution in our planet and the biosphere Bioforms: chemical role of redox potential – – – – –

Spectrum of ligands Fe-S- cluster proteins Heme proteins Labile iron in protein active centers Coping with O2 and ROI (reactive O intermediates)

• Bioforms: coping with Fe limited availability • Metal management: in ”sanity” and in “crisis”

ID placed at the middle of the transition element

implies that Fe can assume various oxidation states

Atomic Number

26Fe

1s2 2s2p6 3s2p6 d64s2

electronic configuration of Fe0

Atomic Number

26Fe

1s2 2s2p6 3s2p6 d64s2

OXIDATION STATES: 

theoretical: from -2 to +6

PREVAILING



Fe++ iron(II)



Fe+++ iron(III)





Iron (II) forms coordination complexes with H2O and other ligands

Fe

Iron(III) forms coordination complexes with H2O and other ligands

Prussian Blue (PB) (a complex of hydrated ferric ferrocyanide ) was introduced in Histology by the German pathologist Max Perls (1843–1881) as stain for tissue iron deposits as ferritin or hemosiderin (Perls stain). PB was the first synthetic dye used in the textile industry, as medicine (for decontamination from metals-primarily radioactive ones) and in paintings by famous artists.

The Great Wave of Kanagawa by Hokusai (painted with PB),

Starry Night by Vincent van Gogh's (PB and ceruleans).

Two women sitting at a bar', ( P Picasso's blue period)

IRON IN OUR PLANET

COMPOSITION and DISTRIBUTION

Ca (1.1) Al (1.1) S (1.9) Ni (2.4) Mg (13)

(2.1) Na (2.3) K (2.4) Ca (4.0) Mg (6.0) Fe (8.0) Al

IRON IN OUR PLANET

Si (15) (28) Si O (30)

WHOLE EARTH (%) Fe (35) most abundant element

EARTH’S CRUST (%) 2nd most abundant metal

(46) O

IRON IN THE BIOSPHERE: mostly in insoluble forms:

Log abundance ratio (ocean/crust)

sea

planet crust

iron(III)-salts or mixed iron (II-III) oxides that are hardly water soluble and extractable

Fe

oxides

Group in periodic table

Fe(III)H2O solubility product Ksp=10-39 M solubilitypH 7= 10-18 M

and yet organisms manage to acquire the lifeessential metal despite its poor solubility Log [element] in human body

Iron extraction sea-earth

Fe

107 fold concentration

Log [element] in sea water

and accumulate it inside or outside their boundaries

Purpur bacterium

Leptothrix*

microaerobic sulfide oxidizer

autotrophes auxotrophes heterotrophes anaerobes aerobes

homo sapiens

3-4 g

Fe oxide *Comamonadaceaebacteria (often co-infects with Trichomonas vaginalis)

Iron is essential for • cell replication • DNA synthesis • energy production (ATP synthesis) • O2 transport • Neurotransmitter synthesis • Hormone synthesis • Blood vessel dilation • Alcohol breakdown (liver)

and much more… (30% of enzymatic reactions are metal-dependent)

The widespread involvement of iron in bio-processes is associated with Fe(II)/Fe(III) capacity for donation/acceptance of e- s to/from a wide range of groups (e.g. reduction involves donation of e– s from donors DH2 that are oxidized in the process: DH2+AD+AH2) Iron Introligands lect 1

This REDOX property is largely dictated by the redox potential E which is critically dependent on the e configuration and geometry of the metal ligands. In aqueous solutions Fe redox potentials (E) are restricted to: -400 to +820 mV relative to standard H electrodes (H+/H2O2/OH-): • at low E : H+ will be reduced to H2 ; • at high E: OH- will be oxidized to O2 more negative E  greater the tendency to donate e- ; e.g Fe-S ferredoxins and coenzymes NAD(H) & NADP(H).

Bio-iron complexes

Fe

E Redox potential mV

Fe(III)/Fe(II) EDTA-[Fe(III)/Fe(II)] Citrate-[Fe(III)/Fe(II)] ADP-[Fe(III)/Fe(II)] O2/O2Fe sulfur centers heme Fe(III)/(II) NAD/NADH, NADP/NADPH Transferrin: TfFe(III)/Fe(II) Ferritin FTFe(III)/Fe(II)

+ 770 + 120 + 100 + 100 - 330 - 600 to +350 - 400 to +400 -320 - 400 - 190

S Fe

In Fe-OXIDIZING PROTEOBACTERIA various iron salts, serve as e- donors / acceptors due to their favourable redox potentials

Fe involvement in chemical and microbiological biochemical reactions

http://www.nature.com/nrmicro/journal/v12/n12/full/nrmicro3347.html

Iron proteins

protein ligands comprise S, N or O groups that form niches finely tuned to favor a selected function Intro lect 1

Fe

The Fe(III) binding site in transferrin

2 tyr , 1 his and 1 asp residues provide N and O coordinating ligands that together with HCO3 comprise the Fe binding niche of Tf that is essentially non-labile in physiological solutions

Fe3 +

METALLOPROTEINS AND METALLOENZYMES • ISC proteins (ISPs) have evolved into 2 classes structurally similar but with different electrochemical potentials: high potential ISPs and bacterial ferredoxins (Fds). Differences in redox potentials seem to be related to types of cluster hydration, a property that is highly susceptible to protein/protein and protein/DNA interactions, i.e. =physiological modulation Iron Introligands lect 1

[Fe4S4]2 E0=-300mV to -400mV

[Fe4S4]2 E0=-400mV

METALLOPROTEINS AND METALLOENZYMES

Iron sulfur cluster (ISC) proteins (ISP)

2Fe-2S ISC ligated by 2 cys and 2 his residues is characteristic of the Rieske and Rieske-type proteins

Cubic 4Fe-4S ligated by 4 cys in aconitase (mitochondrial) and cytosolic (high cell Fe) (at low Fe the ISC is 3Fe-4S= IRP1)

1. Rieske proteins Components of e- chain (bc complexes of the respiratory chain and b6f photosynthetic complex). Endowed with high redox potentials (pH dependent). 2. Rieske-type proteins : Components of dioxygenase & detoxification systems. Endowed with low or negative redox potential (not pH sensitive in the physiological range). 1 and 2 differ structurally: Rieske protein has a S-S bond and various H bonds between NH or OH groups and S or Sγ of 2Fe-2S clusters (that affect their redox potential)

ISP as

e-

carriers

Redox potential - 600 to +350 mV

The stereochemistry of the coordination center controls the chemical reactivity of the metal center. Iron proteins containing Fe4S4 iron-sulfur clusters (ISCs) are ubiquitous in nature and catalyze one-e- transfer processes.

Metalloproteins/enzymes as activators of O2 cytochrome oxidase, peroxidases, cyt P-450 the geometry (e.g. porphyrin) is of 5 fixed ligands plus one (the 6th) axial, reserved for O2 or H2O2, or to a substrate (via Fe-C bond).

Binding of O2 causes Fe to move in/out of heme’s plane deoxyHb (T) O2 

O=O)

LS Fe(III)

The axial ligands control the spin state of the iron

LS Fe(II)

d5 d6

oxyHb

oxyHb (R)

O=O)

HS

Fe(II) & O2 are paramagnetic but HbO2 is diamagnetic

(PHD) PROLYL HYDROXYLASE

O2 sensing via HIF-hydroxylation role of labile iron Fe+2

hypoxia

HYDROXYLATION OF A PROLYL RESIDUE of HIF by THE ENZYME PROLYL HYDROXYLASE (PHD) causes VHL to associate with HIF and BOTH undergo proteosomal degradation after ubiquitination enhancer  erythropoietin  erythropoiesis

PHD involved in COLLAGEN SYNTHESIS uses Fe3+ reduced by ascorbate to get activated and produce OH-prolyl residues that stabilize the collagen triple helix by forming interstrand H-bonds.

Fe+2

Fe+3

role of labile iron

Mechanism of action of ribonucleotide reductase RRNase (role of labile iron in generating TYR radical )

reduction

Complete (fault-free) transfer of 4 e-s from a reductant to O2 is carried out by Cytochrome C Oxidase (complex IV) that transfers 4 e-s (from cyt c (Fe) via Cu) :

O2 + 4H+ + 4e-

cox

 from e- chain (cyt c)

2H2O

Ferritins (FTs): iron storage proteins found in bacterial, plant and animal cells. They form hollow, spherical particles that harbor 2000 to 4500 Fe+++ 8-12 nm in diameter, with several channels that appear to mediate iron transport to and from the interior. All FTs are composed of 24 subunits (heavy H-with ferroxidase activity and light L) that associate to form a spherical particle-different H/L proportions in different cells. Highly regulated expression in cytosol of all mammalian cells but in mitochondria of only few cell types (sperm, brain).

A brief description of iron and oxygen partnership in living systems

although “life might be nothing but an electron looking for a place to rest”, the routes starting from Fe++ are not risk-free en route to O2 

RO

Fe

partners O2 • 2nd most abundant gas in the atmosphere

• 2nd most abundant metal on earth • essential to all living organisms (1 exception) • appears as Fe(II), Fe(III) and Fe(IV): • Fe(II) (aqueous solubility 10-1M). • Electro-negativity: 16 • Redox potential: -0.447 V • Rapidly oxidizes in O2 and in aq. solutions it induces formation of reactive O (ROS). • Fe(III) (solubility 10-17 M), forms oxyhydro polymers (can be made soluble with chelators) • Fe(IV) is formed in highly oxidizing conditions. • Iron has 6 coordination sites. • To be catalytically active, Fe(II) must be able to donate e-,but also have at least 1 coordination site occupied by a readily dissociable ligand (e.g. H2O). The redox potential is highly sensitive to the ligand (+0.35V to -0.45V)

• Soluble in membranes, partly in aq. solutions (problems of diffusion for long distance distribution of the gas). Electronegativity: 35 Redox potential: +0.816 (O2) • The biradical  O2 dictates the slow speed of O-bond formation (10-5 M-1.sec -1), which depends largely on spin inversion. • Radiation (UV-vis, ionizing) can raise one eto a hight orbital and metals such as Fe or Cu enhance O2 reactivity. • Adding an e- to O2 generates superoxide anion O2-. ; by lifting the spin restriction, the addition of another e- generates another ROI (reactive O intermediate) H2O2 • O2 +e---> O2-. O2-. + e---> H2O2 • Further e- addition result in ROS formation H2O2 +e---> HO· + OH- ( +H+ > H2O) • 2HO· +2e---> 2 OH-( +2H+ > 2H2O)

SOURCES OF ROIs-REACTIVE O INTERMEDIATES (O2-, H2O2( • • • • • •

e- CHAIN: (1% of O2 CONSUMED) NADPH-oxidases Lipoxygenases cyclooxygenases Hb-MHb oxidation MAOxidases

associated functions Cell respiration Ischemia-reoxygenation Inflammation Infection Lung injury Smoking

labile forms of Fe catalyze ROI conversion to ROS REACTIVE O SPECIES Fe(II)/Fe(III)

O2 , H2O2

OH

via the Fenton reaction •Fe2+ + H2O2 ----> Fe3+ + ·OH + OH•Fe3+ + H2O2 ----> Fe2+ + ·OOH + H O2- + Fe3+ ----> Fe2+ + H2O2

Fe(II) + H2O2 

Fe(III) +

·OH

+ OH

The .OH radicals formed by Fenton reaction can engage in variety of chemical reactions ·OH

.OH

+ C6H6  (OH)C6H6

.OH

addition

H abstraction

+ CH3OH  CH2OH + H2O

.OH

+ [Fe(CN)6]4  [Fe(CN)6]3- + OHe- transfer

.OH

+ .OH  H2O2

radical interaction

CELL ANTIOXIDANT DEFENSE FERRITIN (sequestration)

FERROPORTIN (efflux)

labile forms of Fe (II/III) catalyze ROI conversion to ROS

Coping with ROI & ROS

O2 , H2O2

SOD GPX superoxide Glutathione dismutase peroxidase (GSH) (Cu-Zn, Fe-Mn) catalase, TRX

OH uric acid, bilirubin, radical scavengers

CELL ANTIOXIDANT DEFENSE

key issues in biological systems • Solubility product of Fe(III) in water Ksp=10-39 M (solubility ~10-18 at pH 7.0). Difficult to keep iron(III) in solution unless it is complexed with water-soluble agents (anions or chelators) that determine the bioavailability of the metal (as Fe+2 or Fe+3) .



• The redox potentials of iron complexes determine: a. the bioavailability of Fe+3 in terms of their ability to reduce the Fe+3 to Fe+2 and render it transportable b. the potential toxicity of iron due to labile Fe propensity for ROS formation by reacting with partial products of O2 reduction

IRON MANAGEMENT 1. generate iron acquisition mechanisms of high specificity 2. vary iron acquisition sensibly (by sensing iron status) 3. take no risks (sensibility): use prophylactics or repressing measures

Control the expression of genes associated with iron acquisition and free radical annihilation (SOD)

 anti-oxidant response

Control the protein levels associated with cell iron acquisition (TfR) and storage (ferritin) or egress (FPN))

Questions about bioiron in our planet scientific and teleological 1. why is our planet so rich in iron? the answer is in the SUN thermonuclear fusion-fission properties of elements. 2. what is iron’s role in the abiogenesis of first organic molecules? reduction of CO2, NO3- and HPO43-. 3. why is iron so poorly bioavailable in crust and oceans ? historically: UV and di-oxygen led to insoluble Fe(III) forms. 4. why is iron the prevalent transition element in life processes? Abundance in the primordial “soup” , widest span of redox potential that can be narrowed by chemical ligands 5. Can we envision alternative Fe-free scenarios for the generation and/or maintenance of life?