Principle of the electron transport chain.

1. Principle of the electron transport chain.

2. ElectronTransfer Consider transfer of 2 electrons from NADH to oxygen: a. ½ O2 + 2H+ + 2e- H2O E°' = +0.820 V b. NAD+ + 2H+ + 2e- NADH + H+ E°' = -0.320 V Subtracting reaction b from a:  c. ½ O2 + NADH + H+ H2O + NAD+ DE°'= +1.14 V DG°’ = - nFDEo' = – 2(96.494)(1.14) = -220 kJ mol-1 (-52.4 kcal mol-1)

4. Respiratory assembly • Located in plasmamembrane of bacteria inner mitochondrial membrane • Consists of 4 complexes (I to IV) immobilised multiproteins/cofactors 2 mobile electron shuttles Ubiquinone (co-enzyme Q) between I/II and III Cytochrome c between III and IV • Accepts electrons (and H+) from NADH and FADH2 generated at numerous oxydation steps • Donates • electrons to terminal acceptor O2 • electrons to S, NO3- (inorganic respiration), etc Bio-wire

5. Respiratory chain Propel electrons through the multi-enzyme complexes. Convert released energy to form a H+-gradient across the inner mitochondrial membrane. Establish a H+-cycle back across membrane. Use of the H+-cycle to drive ATP production. Power transmission by H+-gradient. Rotate flagellae to propel bacterium. Active transport of Ca2+ by mitochondria. Entry of some sugars aand amino acids in bacteria. Heat production (hibernation). H+ gradient is a central interconvertible currency of free energy in biological systems. Supply Electrons sink ADP + Pi H+ ATP

6. The generation of an H+ gradient across a membrane by electron transport reactions.

7. ElectronCarriers NAD+/NADH and FAD/FADH2 were introduced earlier. FMN (Flavin MonoNucleotide) is a prosthetic group of some flavoproteins. It is similar in structure to FAD (Flavin Adenine Dinucleotide), but lacking the adenine nucleotide. When free in solution, FMN (like FAD) can accept 2 e- + 2 H+ to form FMNH2.

8. Coenzyme Q (CoQ, Q or ubiquinone) is lipid-soluble. It dissolves in the hydrocarbon core of a membrane. Most often n = 10. The isoprene tail of Q10 is longer than the width of a lipid bilayer. CoQ has a quinone ring, which can be reduced to the quinol. Free CoQ can undergo a 2 e- oxidation/reduction: Q + 2 e- + 2 H+ QH2.

9. Iron-sulfur Centers Iron-sulfur centers transfer only one electron, even if they contain two or more iron atoms, because of the close proximity of the iron atoms. e.g., a 4-Fe center might cycle between redox states: Fe+++3,Fe++1(oxidized)+1 e-Fe+++2, Fe++2(reduced)

10. Iron-sulfur Centers (A) A single iron ion bound by 4 cysteine residues

11. Iron-sulfur Centers [2Fe-2S];

12. Iron-sulfur Centers [4Fe-4S]

13. Cytochromes Cytochromesabsorb light at characteristic wavelengths. Absorbance changes upon oxidation/reduction of the heme iron provide a basis for monitoring redox state. Some cytochromes are part of large integral membrane complexes, each consisting of several polypeptides and multiple electron carriers. Cytochrome c is a small, water-soluble protein with a single heme group. Cytochrome c reversibly binds to integral membrane electron transfer complexes from which it receives, or to which it donates, an electron.

14. Hemes in the 3 classes of cytochrome (a,b,c) differ in substituents on the porphyrin ring.Only heme c is covalently linked to the protein via thioether bonds to Cys residues.

15. Cytochrome c receives electrons from complex III moves along outer surface of inner membrane donates electrons to complex IV Drawn by Dr B J Catley

17. Method for determining the sequence of electron carriers. In the presence of an electron donor and O2, each inhibitor causes a characteristic pattern of oxidized/reduced carriers: those before the block become reduced (blue) and those after the block become oxidized (red).

19. Separation of functional complexes of the respiratory chain.

21. Path of electrons from NADH, succinate, fatty acyl CoA and glycerol-3-phosphate to ubiquinone.

22. NADH:ubiquinone oxidoreductase (Complex I).

23. Structure of Complex I determined by EM.