Lec06 oxidative p

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  • 1. Berg • Tymoczko • Stryer Biochemistry Sixth Edition Chapter 18:Oxidative Phosphorylation Copyright © 2007 by W. H. Freeman and Company
  • 2. Oxidative Phosphorylation and Mitochondria Transport Systems Mitochondria = power house of the cellglyco. TCA  NADH, FADH2 (energy rich mols)f.a.oxi.  each has a pair of e- (having a transfer pot.) 2 e- 02 Energy released! (used for ATP)Oxidative Phoshorylation: the process in which ATP isformed as electrons are transferred from NADH or FADH2to O2 by a series of electron carriers
  • 3. Some Features…1. Oxidative phosphorylation is carried out by respiratory assemblies that are located in the inner membrane. – TCA is in the matrix2. The oxidation of NADH  2.5 ATP3. FADH2  1.5 ATP – Oxidation and phosphorylation are COUPLED4. Respiratory assemblies contain numerous electron carriers – Such as cytochromes5. When electrons are transferred H+ are pumped out6. ATP is formed when H+ flow back to the mitochondria
  • 4. Some Features Continued… Thus oxidation and phosphorylation are coupled by a proton gradient across the inner mitochondria membrane – So, we produce ATP through this – Glycolysis and TCA cycle can continue only if NADH and FADH2 are somehow reoxidized to NAD+ and FAD
  • 5. Release of Free Energy During Electron Transport1. Electrons transferred electron donor (reductant)  electron acceptor (oxidant) They can be transferred – H- – H+ – Pure electrons2. When a compound loses its electrons becomes oxidant cyt b (Fe ++) + cyt c1 (Fe +++)  cyt b (Fe +++) + cyt c1 (Fe ++) red. X oxi. Y oxi. X’ red. Y’ Red. X and Oxi. X’ Redox Red. Y’ and Oxi. Y Pairs
  • 6. Release of Free Energy Continued…3. PAIRS differ in their tendency to lose electrons – It is a characteristic of a pair – Can be quantitatively specified by a constant… E0 (volts) – E0: standard reduction potential – The more negative E0, the higher the tendency of the reductant to lose electrons – The more positive E0, the higher the tendency of the oxidant to accept electrons – Electron transfer: more –E0 ---------- more +E04. Free energy decreases as electrons are transferred Go = -nF E0 where “n” is the number of electrons transferred, and F is Faraday’s constant (23, 062) E0 = E0 (electron accepting pair) – E0 (electron donating pair)
  • 7. What Are the Electron Carriers in mt? Most of the electron carriers in mitochondria are integral proteins There are four types of electron transfers 1. Direct transfer of electrons Fe+3  Fe+2 2. As a hydrogen atom H+ + electron 3. As a hydride ion :H- (has 2 electrons) 4. Direct combination of an organic reductant with O2
  • 8. Flow of electrons and protons thru the respiratory chain
  • 9. How Is This Order Found?1. NADH, UQ, cytb, cytc1, c, a, and a3 is the order – Their standard reduction potentials have been determined experimentally! – The order  increased E0 because electrons tend to flow from more negative E0 to more positive E02. Isolated mitochondria are incubated with a source of electrons but without O2 – a, a3 is oxidized first – c, c1, b are second, third, and fourth respectively – When the entire chain of carriers is reduced experimentally by providing an electron source but no O2 (electron acceptor) then O2 suddenly introduced into the system – The rate at which each electron carrier becomes oxidized shows the order in which the carriers function – The carrier nearest O2 is oxidized first, then second, third, etc.
  • 10. Action of Dehydrogenases Most of the electrons come from Electron acceptors NAD or FMN, FAD Reduced subs + NAD+  ox. sub + NADH + H+ Reduced subs + NADP+  ox. Sub + NADPH + H+ In addition to FAD and NAD, there are three other types of electron carrying groups – Ubiquinone – Iron containing proteins (cytochromes, Fe-S proteins) Ubiquinone = CoQ or = UQ – When it accepts 1 electron  UQH (semiquinone) – When it accepts 2 electrons  UQH2 (ubiquinal)
  • 11. Oxidationstates ofquinones
  • 12. Oxidation states of flavins.• The reduction of flavin mononucleotide (FMN) to FMNH2 proceeds through a semiquinone intermediate.
  • 13. Complex I  NADH dehydrogenase (NADH Q reductase)  Huge protein – 25 pp  FMN, Fe-S  I electron  UQ
  • 14. Complex II Succinate Q Recuctase (Succinate dhydrogenase) – Is the only membrane bound enzyme in the TCA cycle – Contains  FAD, Fe-S II  electrons  UQ Cytochrome: an electron transferring protein that contains a heme prosthetic group!
  • 15. Complex III
  • 16. Complex III Cyt reductase (UQ-cyt c oxido reductase or cyt bc1 complex) – Contains cyt b, c1, Fe-S proteins and at least six other protein subunits UQ is 2e- carrier, cyts are 1e- carriers – This switch is done in a series of reactions (called Q cycle) Electron transfer in III seems to be complicated but it’s not Net reaction: – UQH2  UQ and cyt c is reduced – H+ is pumped out also
  • 17. Complex IV Cyto oxidase – Contains a, a3, and CuA, CuB The detail of this electron transfer in Complex IV is not known It also functions as a proton pump
  • 18. ATP Production in Mitochondria
  • 19. Chemiosmotic Theory
  • 20. Mitchel’s Theory The electrochemical potential difference resulting from the asymmetric distribution of the H+ is used to drive the mech. responsible for the formation of ATP
  • 21. Chemiosmotic Theory Continued… G = RT ln(C2/C1) + ZF [ + ] [ + ]When H+ is pumped against electrochemical gradient G=+ When protons flow back inside, this G becomes available to do the work!!
  • 22. Oxidation and ATP synthesisare coupled
  • 23. ATP Synthase
  • 24. Uncoupled Mitochondria in Brown Fat Produces Heat This is done by DNP or other uncouplers They carry protons across the inner mitochondria membrane In the presence of DNP, electron transport is normal but ATP is not formed Proton-motive force is gone or disrupted Uncoupling is also seen in brown adipose tissue It is useful to maintain BT in hibernating animals, newborns, and mammals adapted to the cold It has lots of mitochondria IMM  thermogenin (uncoupling protein) Thermogenin generates heat by short-circuiting the mitochondrial proton battery
  • 25. Shuttle Systems Required for cytosolic NADH oxidation NADH dehydrogenase IMM can accept electrons only from NADH in the matrix We also make cytosolic NADH by glycolysis They also have to be reoxidized to NAD+ IMM is not permeable to cytosolic NADH – We therefore need shuttle systems Electrons are transferred from NADH to Complex III (not I), providing only enough energy to make 2 ATP (G-3-P shuttle) It is active in muscle (insect flight) and brain Net reaction: – NADH + H+ + E- FAD  NAD+ + E-FADH2 (cytosolic) (mitochondrial) (cyto) (mito) So, 2ATP is formed UQ
  • 26. Malate-aspartate Shuttle Heart Liver Cytoplasmic NADH is brought to mitochondria by this shuttle This shuttle works only if NADH/NAD+ increase in the cytosol (then mitochondria) No energy consumed No ATP lost
  • 27. Regulation of ATP ProducingPathways Coordinately regulated – Glycolysis – TCA – FA oxidation – a.a. oxidation – Oxidative phosphorylation Interlocking regulatory mech. ATP, ADP controls all of them Acetyl CoA and and citrate
  • 28. Regulation of Oxidative Phosphorylation Intracellular [ADP] If no ADP  no ATP – The dependence of the rate of O2 consumption on the [ADP] (Pi acceptor) is called “acceptor control” acceptor control ratio = ADP-induced O2 consumption O2 consumption without ADP Mass action ratio: ATP is high normally [ADP][Pi] So, system is fully phosphorylated.ATP used, ratio decreases, rate of oxidative phosphorylationincreases.
  • 29. Tumor Cells Regulation is gone in catabolic processes Glycolysis is faster than TCA They use more Glc, but cannot oxidize pyruvate Pyruvate  lactate (PH decreases in tm.)
  • 30. Mutations in Mitochondrial Genes Mutations in mitochondrial genes cause human disease. DNA has 37 genes (16, 569 bp), 13 of them encode respiratory chain proteins. LHON- Leber’s Hereditary Opti-neuropathy – CNS problems – Loss of vision – Inherited from women. – A single base change ND4 Arg  His (Complex 1) – Result: defective electron transfer from NADH to UQ. Succinate  UQ okay, but NADH  UQ not.
  • 31. 3 Stages ofCatabolism
  • 32. Summary Electron flow results in pumping out H+ and the generation of membrane potential! ATP is made when protons flow back to the matrix! F0F1 complex Proton motive force, PH gradient, membrane potentialThe flow of two electrons through each of threeproton-pumping complexes generates a gradientsufficient to synthesize one mole of ATP!
  • 33. The proton gradient is an interconvertible form of free energy Proton gradients are a central interconvertible currency of free energy in biological systems. • Active transport of Ca • Rotation of bacterial flagella • Transfer of e from NADP+ to NADPH • Generate heat in hybernation