ETC 1

• Oxidative phosphorylation


  The process by which ATP is formed as a result of the transfer of electrons from NADH or FADH2 to O2 via an electron transport chain (ETC)

  
• NADH and FADH2


  • They are both produced by the breakdown of carbohydrates, fats and amino acids
  • ATP synthesis is driven by proton-motive force generated by the (ETC), referred to as the chemiosmotic hypothesis

  
• Fundamental concepts of oxidative phosphorylation

  • Electronegativity
  • Sources of reduced NADH and FADH2
  • Anatomy of the mitochondrion

  
• Reducing agents NADH & FADH2

  • ATP is used as free-energy currency by coupling its (spontaneous) dephosphorylation with a (nonspontaneous) biochemical reaction to give a net release of free energy
  • The nonspontaneous reaction of joining ADP to inorganic phosphate to make ATP is coupled to the oxidation reaction of NADH or FADH2
• The oxidation reaction for NADH has a larger, but negative, free energy change than the positive free energy change required for the formation of ATP from ADP and phosphate
• This set of coupled reactions is referred to as oxidative phosphorylation
• Mitochondrion


  • Consists of inner and outer membranes
  • Inner membrane has inward-facing fold-like projections known as cristae that vastly increase the surface area of the membrane to maximize the amount of energy production
  • Protein complexes involved in the electron transport chain are studded along this membrane
  • Inner membrane envelops the matrix, which houses mitochondrial DNA, ribosomes, and a multitude of enzymes and metabolites
  • Space between the inner and outer membrane is known as the intermembrane space; this is the site of hydrogen ion deposition for the protein complexes in the electron transport chain
  • Increased hydrogen ion (H+ ion) concentration (and effectual decreased pH) generate a membrane potential across the inner mitochondrial membrane

  
• Coupling the oxidative-phosphorylation reactions

  1. Electrons are transferred from NADH, through a series of electron carriers, to O2

  2. Transfer of electrons by these carriers generates a proton (H+) gradient across the inner mitochondrial membrane

  3. When H+ spontaneously diffuses back across the inner mitochondrial membrane, ATP is synthesized

  
• Proton motive force


  The energy obtained from the electrochemical gradient created by several of the electron carriers

  
• Chemiosmosis


  1. Proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix

  2. This diffusion of protons is facilitated by the transmembrane enzyme ATP synthase

  3. As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP

  
• ATP synthase


  • The 3 active β sites take turns catalysing ATP synthesis
  • A subunit starts in the β-ADP conformation that binds ADP and Pi
  • The conformation changes to the β-ATP form that tightly binds and stabilises ATP
  • The conformation then changes to β-empty, which has low affinity for ATP
  • The newly synthesised ATP leaves the enzyme
  • Another round of catalysis begins when this subunit again assumes the β-ADP form and binds ADP and Pi

  
• • Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked
  • Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient

  
• Cytochromes


  • Proteins with iron-containing prosthetic group
  • Mitochondria has classes a, b and c; distinguished by differences in their light absorption spectra
  • Iron exists in association with inorganic S or with the S atoms of Cys residues
  • All iron-sulfur proteins participate in one electron transfer; one iron atom is reduced or oxidized
• Complex I (NADH dehydrogenase)

  • The exergonic transfer of a hydride ion from NADH to FMN, then via a series of Fe-S centers to the Fe-S protein N-2
  • Electrons then transfer from N-2 to Q to form QH2 which diffuses into the lipid bilayer
  • The endergonic expulsion of four protons (per pair of electrons) from the matrix to the intermembrane space

  
• Complex II (Succinate dehydrogenase)

  • The only membrane-bound enzyme of the TCA cycle
  • Electrons pass from succinate to FAD, then through Fe-S centres to Q
• Complex III (Cytochrome C reductase)

  • Couples the transfer of electrons from ubiquinol (QH2) to cytochrome c to proton expulsion to the intermembrane space
  • QH2 is oxidized to Q and two molecules of cytochrome c are reduced
  • Cytochrome c is a soluble protein in the intermembrane space; its haem accepts an electron from Complex III and donates it to the binuclear Cu center of Complex IV

  
• Complex IV (Cytochrome C oxidase)


  • Carries electrons from cytochrome c to molecular O2, reducing it to H2O, via CuA, heme a and the FeCu centre
  • Four electrons, four substrate protons are used to give two molecules of H2O
  • Simultaneously, four protons are pumped from the matrix to the intermembrane space

  
• Ubiquinone (Coenzyme Q)


  • A benzoquinone; lipid-soluble, small, hydrophobic; resides in the membrane
  • Accepts one electron to form the semiquinone radical, QH or two electrons to form ubiquinol, QH2
  • Acts at the junction between two electron donors (Complexes I and II) and one electron acceptor (Complex III)
  • Couples electron flow to proton movement