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Objectives
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  1. Objectives • To know about the structural and biochemical organizations of a mitochondrion • To understand the electrochemical reactions through which the chemical energy in food can be converted to chemical energy in ATP • To realize how the structural organizations of mitochondria have allowed the above electrochemical reactions to be carried out effectively

  2. Energy Conversion (1): Mitochondria • Cellular respiration • Flow of electrons from reduced coenzymes to an electron acceptor; generation of ATP • NADH and FADH2 from glycolysis, TCA cycle, b-oxidations, etc. • Ultimate electron acceptor is oxygen; reduced form as water (aerobic respiration); takes place with mitochondria in eukaryotic cells

  3. The Energy Powerhouse • Discrete sausage-shaped structures; the second largest organelle in most animal cells • A double-membrane organelle; outer membrane separated from inner membrane by intermembrane space • Outer membrane • Not a significant permeability barrier for ions and small molecules; transmembrane proteins (porins)

  4. Intermembrane space • Continuous with the cytosol • Inner membrane • A permeability barrier to most solutes • Locale of the protein complexes of electron transport and ATP synthesis • Distinctive foldings (cristae); increase surface area to accommodate more the protein complexes

  5. Matrix • Semi-fluid enclosed by inner membrane; • Enzymes for mitochondrial functions • A circular DNA molecule; coding for its own rRNAs, tRNAs, and a number of polypeptide subunits of inner-membrane proteins (genetic competence)

  6. Electron Transport System (ETS) • Transfer of electrons from NADH and FADH2 is highly exergonic • Multistep process; a series of reversibly oxidizable electron carriers; total free energy difference is released in increments to prevent excessive amount being released as heat (energy conservation for ATP) • 4 different kinds of carriers::

  7. Flavoproteins • Membrane-bound proteins using either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) as prosthetic group • Transfer both electrons and protons

  8. Iron-Sulfur Proteins • Proteins containing iron-sulfur (Fe/S) centers; iron and sulfur atoms complexed with cysteine groups of the protein • Alternates between the Fe3+(ferric) and Fe2+(ferrous) • Do not pick up and release protons • Cytochromes (Cyt) • Contain iron; part of a porphyrin prosthetic group (heme)

  9. One-electron carriers; transfer electrons only: • Cyt b, c1, a and a3 are integral membrane proteins • Cyt c is relatively hydrophilic; loosely associated with inner face of membrane; not a part of the complexes; mobile electron carrier

  10. Cyt a and a3 • Copper - containing - cytochromes (bimetallic iron-copper (Fe/Cu) center) • Components of cytochrome c oxidase • Keeping an O2 molecule bound to the oxidase complex; completely picked up the four electrons and four protons

  11. Coenzyme Q (CoQ) • Ubiquinone (a benzene derivative); the only nonprotein component • Carries both protons and electrons • Not part of a respiratory complex; a collection point for electrons from FMN- and FAD-linked dehydrogenases • Active transport of protons across inner mitochondrial membrane

  12. The electron carriers function in a sequence determined by their relative reducing power (reduction potentials) • Two interconvertible molecules or ions by the loss or gain of electrons (redox pair) • With exceptions of CoQ and Cyt c, the electron carriers are organized into four large multiprotein complexes (respiratory complexes)

  13. Complex I • NADH-coenzyme Q oxidoreductase • Transfers electrons from NADH to coenzyme Q • Complex II • Succinate-coenzyme Q oxidoreductase • Transfers electrons derived from succinate oxidation in TCA

  14. Complex III • Coenzyme Q – cytochrome c oxidoreductase • Accepts electrons from coenzyme Q and passes them to cytochrome c • Complex IV • Cytochrome c oxidase • A terminal oxidase; capable of direct transfer of electrons to oxygen

  15. Respiratory Complex Electron Flow Number Name Number of Polypeptides Prosthetic Groups Accepted from Passed to Proton Transport? I NADH dehydrogenase (NADH-coenzyme Q oxidoreductase) 22-26 1 FMN 6-9 Fe/S centers NADH Coenzyme Q Yes II Succinate-coenzyme Q oxidoreductase (succinate dehydrogenase) 4-5 1 FAD 3 Fe/S centers Succinate (via enzyme-bound FAD) Coenzyme Q No III Coenzyme Q -cytochrome c oxidoreductase (cytochrome b-c1 complex) 8-10 2 cytochrome b 1 cytochrome c1 1 Fe/S center Coenzyme Q Cytochrome c Yes IV Cytochrome c oxidase 9 1 cytochrome a 1 cytochrome a3 2 Cu centers (as Fe/Cu centers with cytochrome a3) Cytochrome c Oxygen (O2) Yes Properties of the Mitochondrial Respiratory Complexes

  16. ATP Generation / Electron Transport • ATP generation: ADP + Pi ATP • Photophosphorylation • Substrate level phosphorylation • Glycolysis: 1,3-bisphosphoglycerate  3-phospho-glycerate; phosphoenolpyruvate  pyruvate • TCA: succinyl CoA  succinate • 4 ATP molecules/glucose: 2 from glycolysis + 2 from TCA

  17. Oxidative phosphorylation • 6 different oxidations (12 pairs of electrons): • Glycolysis: glyceraldehyde-3-phosphate 1,3-bisphosphoglycerate (+NADH) • Pyruvate  acetyl CoA (+NADH) • TCA: isocitrate -ketoglutarate (+NADH); -KG  succinyl CoA (+NADH); succinate  fumarate (+FADH2); malate  oxaloacetate (+NADH)

  18. Chemiosmotic Model • Electrochemical potential across a membrane; the link between electron transport and ATP formation • Exergonic transfer of electrons between and within respiratory complexes; unidirectional pumping of protons across the membrane where the transport system is localized

  19. The F0F1 Complex • A F-type ATPase; both ATPase and ATP synthase activities • Converts electrochemical energy (proton gradient) into potential chemical energy (ATP) • F1 complex • 3a and 3b polypeptides; 3 ab complexes(catalytic hexagon)

  20.  subunit: catalytic site for ATP synthesis/hydrolysis; a subunit: ATP/ADP-binding site • Both ATP synthase and ATPase activities • Proton translocation through F0 drives ATP synthesis by F1 • Stalk • Composes of ,  and  subunits • Allows rotation of F1 complex about F0 complex

  21. F0 complex • Consists of 1a, 2b and 9-12 c subunits • c subunits are organized in a circle; proton channel • As a proton translocator: channel through which protons flow (protonation and deprotonation of aspartate)

  22. Binding Change Model • To explain how exergonic flow of protons through F0 can drive endergonic phosphorylation of ADP to ATP • Electrochemical–to–mechanical–to–chemical transducer • Each of the three b subunits exists in 3 different conformations at any point in time:

  23. (O)pen:little affinity; ADP and Pi are free to enter (ATP is free to leave) the catalytic site • (L)oose: higher affinity; lose binding of ADP and Pi • (T)ight: Packing the ADP and Pi together tightly; facilitating the condensation • O  L  T:

  24. Flowing of protons flow through a channel in the a subunit of F0 • Rotation of the ring of c subunits; rotation of the attached g subunit • Asymmetry of the g subunit; different interactions with the three b subunits at any point in time • Each b subunit passes successively through the O, L, and T conformations as the g subunit rotates 360º