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Electron transport chain

Electron transport chain






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    Electron transport chain Electron transport chain Presentation Transcript

    • M.Prasad Naidu MSc Medical Biochemistry, Ph.D.Research Scholar
    •  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.
    •  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.…
    •  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.
    •  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.
    •  5 distinct parts.  1.the outer membrane  2.the inner membrane  3.the inter membrane space  4.the cristae  5.the matrix.
    •  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.
    •  The interior ground substance.  Rich in enzymes responsible for TCA Cycle, oxidation of FA and the oxidation of amino acids.
    •  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.
    •  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.
    •  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, α-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.
    •  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.  Succinate+FAD- Fumarate + FADH2
    •  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.
    •  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
    •  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.
    •  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.
    •  Cyt are identified by their characteristic absorption spectra.  Ferricytochromes show diffuse and non-characteristic absorption spectra.  Ferrocytochromes exhibit characteristic absorption bands called α, β and γ–soret bands.  Cytochromes are characterized into different groups according to the light wavelength at which the alpha band shows its peak(α-abs.max.)
    •  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 @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
    •  Cyt.c1:like cyt.c – contains an ironprotoporphyrinIX complex-heme-c.  It has abs.maxima @554,524&418mµ  Incapable of combining w O2,CO,CN-  Cyt.b:also- 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.@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.
    •  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.
    •  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- 605,517&414;Cyt.a3600&445mµ.  Cyt.a does not react with O2,CO/CN- where as Cyt.a3 is autooxidizable and forms compounds with CO & CN- .
    •  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.
    •  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
    •  1. Monofunctional heme containing catalases.  2. Bifunctional heme containing catalase- peroxidases that are closely related to plant peroxidases.  3. Non-heme manganese containing catalases.
    •  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.
    •  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.
    •  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.
    •  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.
    •  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.
    •  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 diO2tpt 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.
    •  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.
    •  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
    •  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.
    •  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.