Electron transport chain

1
Electron transport chain

• M.Prasad Naidu
• MSc Medical Biochemistry,
• Ph.D.Research Scholar

2
INTRODUCTION

• ETC is the 4th and final stage of aerobic
  respiration.
• Through ETC, the E needed for the cellular
  activities is released in the form of ATP.
• ETC is an O2 dependent process which occurs in
  the inner mitochondrial membrane.

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What is ETC ?

• The energy rich carbohydrates (Glu), FA and AAs
  undergo a series of metabolic reactions and
  finally get oxidized to CO2 and H20.
• The reducing equivalents from various metabolic
  intermediates are transferred to NAD and FAD to
  produce NADH and FADH2.
• The latter two reduced coenzymes pass through the
  ETC or respiratory chain and finally reduce O2 to
  H20.
• The passage of electrons through the ETC is
  associated with the loss of free energy.
• A part of this free E is utilized to generate ATP
  from ADP and Pi.

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DEFINITION

• This is the final common pathway in aerobic cells
  by which electrons derived from various
  substrates are transferred to oxygen.
• ETC is series of highly organized
  oxidation-reduction enzymes.

5
Location

• ETC is localized in Mitochondria.
• MC are the centres for metabolic oxidative
  reactions to generate reduced coenzymes (NADH
  and FADH2) which in turn, are utilized in ETC to
  liberate E in the form of ATP.
• Hence, MC is regarded as Power House of the Cell.

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Mitochondrial Organization

• 5 distinct parts.
• 1.the outer membrane
• 2.the inner membrane
• 3.the inter membrane space
• 4.the cristae
• 5.the matrix.

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Inner mitochondrial membrane

• ETC and ATP synthesizing system are located on
  IMM.
• IMM is rich in proteins.
• It is impermeable to ions(H,K,Na) and small
  molecules (ADP, ATP).
• IMM is highly folded to form Cristae.
• The surface area of the IMM is greatly increased
  due to Cristae.
• The IMM Possesses specialized particles (
  that look like lollipops ), the phosphorylating
  subunits which are the centres for ATP
  production.

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Mitochondrial matrix

• The interior ground substance.
• Rich in enzymes responsible for TCA Cycle,
  oxidation of FA and the oxidation of amino acids.

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Structural Organization of ETC

• The IMM can be disrupted into 5 distinct enzyme
  complexes, denoted as Complex I, II, III, IV and
  V
• The complex I-IV are carriers of electrons while
  V is responsible for ATP synthesis.
• Besides these enzyme complexes, there are certain
  mobile e- carriers in ETC.
• These include NADH, Coenzyme Q, Cytochrome C and
  Oxygen.
• The complexes (I-IV) and the mobile carriers are
  collectively involved in the transport of e-
  which ultimately combine with O2 to produce H2O.
• Most of the O2 supplied to the Body is utilized
  by MC for ETC.

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Components of the ETC

• Complex I(NADH-CoQ reductase), Complex
  II(Succinate Co.Q reductase), Complex III(CoQ-Cyt
  C reductase) Complex IV(Cyt.oxidase) Complex
  V(ATP synthetase)
• There are 5 distinct carriers that participate in
  the ETC.Viz
• 1.Nicotinamide nucleotides
• 2.Flavo proteins
• 3.Iron-Sulfur proteins
• 4.Coenzyme Q
• 5.Cytochromes.
• These carriers are sequentially arranged and are
  responsible for the transfer of e- from a given
  substrate to ultimately combine with proton and
  O2 to form H2O.

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Nicotinamide Nucleotides

• Of the 2 coenzymes NAD and NADP derived from
  the vit. Niacin, NAD is more actively involved
  in the ETC.
• NAD is reduced to NADH H by dehydrogenases
  with the removal of 2H atoms from the substrate.
• The substrates include Gly-3-P, Pyruvate,
  isocitrate, a-KG, and malate.
• NADPH H produced by NADP -dependent
  dehydrogenase is not usually a substrate for ETC.
• NADPH is more effectively utilized for anabolic
  reactions
• Eg FA synthesis, Cholesterol synthesis.

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Flavoproteins

• The enzyme NADH dehydrogenase (NADH-CoQ
  reductase) is a flavo protein.
• FMN is the prosthetic group.
• FMN accepts 2e- and a proton to form FMNH2.
• NADH dehydrogenase is a complex enzyme closely
  associated with non-heme iron proteins (NHI) or
  iron-sulfur proteins.
• NADH H FMN--?NAD FMNH2
• SDH(Succinate-Co.Q reductase) is an enzyme found
  in the IMM.
• It is also a flavoprotein with FAD as the
  coenzyme.
• SDH can accept 2 H atoms(2H 2e- ) from
  succinate.
• SuccinateFAD-? Fumarate FADH2

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Iron Sulfur proteins

• A group of quinones has been found to be present
  in MC namely FeS, Fe2S2, Fe4S4 and Fe3S4 etc.,
• FeS proteins exist in the oxidized(Fe3) or
  reduced (Fe2).
• About 6 FeS proteins connected with ETC have been
  identified.
• The machanism of FeS proteins in ETC is not
  clearly understood.
• One FeS participates in the transfer of electrons
  from FMN to Co.Q
• Other FeS proteins associated with cyt.b and
  cyt.c1 participate in the transport of electrons.
• Vit E, D and plastoquinones also involved in ETC.

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FeS Proteins

• FeS It has a single Fe coordinated to the side
  chain-SH groups of 4 Cys.residues.
• Fe2S2 It contains 2 iron atoms, 2 inorganic
  sulfides and 4 SH groups. Each iron is linked to
  2-SH and 2-sulfur groups.
• Fe4S4 It consists of 4 iron atoms and 4 cys-SH
  groups and 4 inorganic sulfides. Each iron
  remains linked to 1-SH, 3-inorganic sulfides
  while each sulfide is coordinated to 3 iron
  atoms.
• Fe3S4 It consists of 3 Fe, 4- SH and 4inorganic
  sulfides.
• Each FeS protein transfers only one e- at a time.
• The enzymes may have one or more of the
  combinations

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Coenzyme Q

• Also called Ubiquinone since it is ubiquitous in
  living system.
• It is a quinone derivative with a variable
  isoprenoid side chain.
• The mammalian tissues possess a quinone with 10
  isoprenoid units which is known as coenzyme
  Q10.(CoQ10)
• CoQ is a lipophilic e- carrier.
• CoQ is not found in Mitochondria
• Vit k performs similar function as CoQ in these
  organisms.

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Cytochromes

• Cytochromes are conjugated proteins.
• Contains Heme group.
• The heme group of cyt differ from that Hb and Mb.
• The Iron of Heme in cyt is alternately
  oxidized(Fe3) and reduced(Fe2), which is
  essential for the transport of e- in ETC.
• This is in contrast to the Hb and Mb iron which
  remains in the Fe2 state.

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Cytochromes

• Cyt are identified by their characteristic
  absorption spectra.
• Ferricytochromes show diffuse and
  non-characteristic absorption spectra.
• Ferrocytochromes exhibit characteristic
  absorption bands called a, ß and ?soret bands.
• Cytochromes are characterized into different
  groups according to the light wavelength at which
  the alpha band shows its peak(a-abs.max.)

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Types of cytochromes

• cyt.c- Since it is largely available , it is the
  best studied of the cyts.
• It is a central member of ETC with an
  intermediate redox potential)
• Water soluble-loosely bound to IMM-easy to
  extract.
• Shows characteristic absorption spectra in the
  reduced form at 550,521 and 416mµ
• Oxidized form _at_530mµ and 400mµ
• The iron content of cyt.c. is 0.38
• Heme is attached with protein by means of 2
  thioester linkages involving sulfur of 2 cys and
  apoprotein.
• Cyt.c is incapable of combining with O2/CO.
• a protein with 1-PPchain 104aa (mw12400-13000)
• NADPH-Cyt.c.reductase can readily reduce Cyt.c

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Types of cytochromes

• Cyt.c1like cyt.c contains an
  ironprotoporphyrinIX complex-heme-c.
• It has abs.maxima _at_554,524418mµ
• Incapable of combining w O2,CO,CN-
• Cyt.balso- protoporphyrinIXcomplex-(heme-b). But
  the apoprotein is diff.
• Tightly bound to Flavo proteins and ubiquinones
  in the MC.
• The Ferrocyt.b has an abs.max._at_563mµ, 530mµ
  430mµ.
• It is thermostable not easily extractable.
• It also does not react with O2,CO or CN-
• Normally its oxidation requires the presence of
  Cyt.c,a, a3.

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Cytochrome a a3 (cyt.oxidase)

• Complex IV of the ETC.
• Both contain an identical type of iron porphyrin
  complex
• Inspite of identical hemes, cyt.a a3 differ in
  e-affinity bio.activity.This is bcos of their
  location of hemes
• One heme is located along with one Cu ion. This
  heme is called heme-a
• This Cyt.a functions as anaerobic oxidizing unit.

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Cytochrome a a3 (cyt.oxidase)

• The other heme is located along with the 2nd Cu
  ion and is called heme-a3 (functions as aerobic
  reducing unit).
• Cyt.a-?abs.max-605,517414Cyt.a3?600445mµ.
• Cyt.a does not react with O2,CO/CN- where as
  Cyt.a3 is autooxidizable and forms compounds with
  CO CN-.

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PEROXIDASES

• Heme containing enzymes.
• Found in bacteria, fungi and animals.
• On the basis of sequence similarity peroxidases
  are grouped into two super families.
• 1.fungal, plant bac.peroxidases
• 2.animals form the 2nd super family of
  peroxidases.
• P.ases use H2O2 as the e- acceptor to catalyze a
  no. of oxidative reactions.
• P.ases contain heme group and this heme group is
  responsible for carrying out the activity of
  peroxidases
• Heme consists of a protoporphyrin ring and a
  central iron atom in 3 oxidation state.
• A protoporphyrin is made up of 4 pyrrole rings
  linked by methine bridges.--- with diff. side
  chains.

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CATALASES

• Heme containing redox enzymes.
• Produced by all aerobic organisms ranging from
  bacteria to man.
• Converts H2O2 to H2O and mole.O.
• Utilizes H2O2 both as an e-acceptor and an e-
  donor.
• Catalase also catalyzes
• RCOOH HQOH ? ROH QO H2O where R is an
  alkyl or acyl group and HQOH is a 2e- donor.
• Most catalases exist as tetramers of 60-75KD.
• Each subunit contains an active heme group
  buried deep within the structure.
• The stable structure of catalases is resistant to
  PH, thermal denaturation and proteolysis.
• About heme

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Classes of catalases

• 1. Monofunctional heme containing catalases.
• 2. Bifunctional heme containing
  catalase-peroxidases that are closely related to
  plant peroxidases.
• 3. Non-heme manganese containing catalases.

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Haemocyanin (Blue blood)

• Are copper containing dioxygen carriers.
• Responsible for Di O2 transport in molluscs and
  Arthopods
• High mol.wts and multiple subunits.
• Each subunit has a mol.wt of 76000D
• Each subunit is made up of 3 domains.
• Di O2 is bound to the active site in 2nd domain.
• There are 2 Cu atoms with an oxidation state of
  1
• So a PP chain of Arthopod HeCN binds 1 O2
  molecule.
• The structure of Molluscan HeCN is diff. from
  that of Artho. in Wt, subunits structure and O2
  binding capacity.
• Mol.wt of HeCN is 290000D with 2 Cu atoms for
  every 50KD
• So 1 PP chain can bind 6-8 O2 molecules.

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Haemerythrin

• Is a biological Di O2 carrier.
• Responsible for Di O2 transport in marine
  invertebrates.
• The 4 diff.phyla of invertebrates are
  Sipuniculids, priapulids, Brachiopods annelid
  worm magelona
• HeEy is found as an oligomer.
• Blood contains an octameric form tissues
  contain trimeric or tetrameric
• Octameric HeEy consists of 8 subunits which are
  very similar to 40 structure to MyoHeEy.
• Although diff oligomers are known, all of them
  share a DiIron active site.
• The 2 iron atoms are 3.25-3.30Ao apart.
• The 2 Fe atoms are bound to 5 His.residues.

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Model synthetic complexes

• Study of model compound involves the structure
  determination, physical measurements and
  reactions of simple co-ordination compounds.
• A model can give only a partial view of real
  system and provide valuable evidence for the
  study of the real systems.

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BIO INORGANIC CHEMISTRY-I

• ESSENTIAL TRACE ELEMENTS
• Essential for life of animals, plants microbes.
• They include Na, K(alkali metals), Ca, Mg (alkali
  earth metals) transition metals
  (V,Cr,Mn,Fe,Co,Zn, Mo and Cd.)
• These elements are required for biological
  processes and are called essential elements.
• Trace metals----occurs low conc in animal and
  plant cells. They are a part of good nutrition.
• In high doses they may be toxic to the body or
  produce deficiencies in other trace metals.
• For Eg. High levels of Zn can result in the def.
  of Cu.

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Role of metal ions in bio.processes

• Regulatory axn is exercised by Na, K,Mg2
  Ca2 ions.
• As cellular regulators they are involved in nerve
  transmission,
• Maintanence of cell membrane permeability and
• Regulation of osmotic pressure
• Ca regulates muscle contraction, cell division
  and growth, enzyme activities.
• Also blood coagulation system
• Mg,Ca, and Zn ions have structural role.
• Ca is a component of bones, teeth and animal
  shells.
• Zn structural role in fingures
• Mg helps to stabilize 3D-structure of RNA DNA.

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Role of metal ions in bio.processes

• Metallo enzymes catalyze several biological
  reactions.
• Metal ions are at the active site of these
  enzymes.
• Imp.metal enzymes CP(Zn), Urease (Ni)
  vit.B12(Co)
• Metal ions play imp.role in diO2 tpt and storage.
• A diO2 carrier protein contains diO2 binding
  site. This active site is a complex of Fe/Cu.
• The 3 imp.DiO2 carriers are Hb, HeEy HeCN
• Hb- found in RBC----respiration---- the active
  site of Hb consists of iron- porphyrin( heme)
  group.
• HeEy- found in marine invertebrates. The O2
  binding site contains a pair of Fe atoms.
• HeCN- Cu containing diO2 carriers found in
  molluscs and arthopods.

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Role of metal ions in bio.processes

• Mb stores O2 in muscles. It contains a heme group
  
• Metal ions play an imp. Role in e- transfer
  agents include ferredoxin, rubredoxin and
  cytochromes.
• Ferredoxin and rebredoxin contain Fe-S sites.
• These sites are involved in e- transfer
• Cytochromes serve as e- carriers in both plants
  and animals.

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Role of zinc

• 2nd most abundant transition element in the hu.
  body.
• About 2 gms of Zn and requires a daily intake of
  (RDA) 8-13mg.
• Stimulates the activity of 100 enzymes.Eg CA,
  CP
• Plays structural role in proteins called zinc
  fingures.
• Also required in plants for leaf formation and
  synthesis of auxin.
• Zn ion is good lewis acid in biochemical systems.
• Zn2 can be 4,5,6- coordinate.
• Zn2 complexes show easily 4 to 5 coordinate
  interconversion. If the interconversion is fast,
  catalysis is also fast.
• Zn complexes are labile than Ni2/ Mg2 complexes

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Role of Calcium

• Ca has a structural role.
• Chief component of bones, teeth and animal
  shells.
• Imp. In cellular messenger system.
• Muscle contraction, secretion, ion transport,
  cell division and growth and blood clotting.
• Ca and P are imp for bone formation.
• 99 of Ca is stored in bones
• Ca is necessary for the growth of children.

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Role of Iron

• Humans contain about 4 gms of iron.
• Functions as the principal e- carrier in
  biological oxidation reduction reactions.
• Fe-S proteins are present in all forms of life.
• Fe-S sites occur in ferredoxins and rubredoxins.
• They are involved in intra protein and inter
  protein e- transfer.