Oxidative Phosphorylation explained

• Oxidative phosphorylation involves redox reactions using the electron transport chain and phosphorylation, producing ATP via ATP synthase.
• Oxidative part is needed to make the phosphorylation part.
• Peter Mitchell explained the process (chemiosmotic theory).
• Electrons move from more negative to more positive reduction potentials.
• Energy difference in electrons movement helps pump H+ from the matrix to the intermembranous space, against their gradient.
• Proton motive force (PMF) connects electron movement and ATP synthesis.
• High proton concentration outside the mitochondria membrane creates pressure, but protons can only enter through ATP synthase.
• Generation of ATP is aided by O2 reduction.
• Oxidative phosphorylation has three aspects:
  • Electron flow through membrane-bound carriers.
  • Free energy (exergonic) is coupled to transport protons across a proton-impermeable membrane.
  • Transmembrane proton flow down the concentration gradient provides free energy for ATP synthesis.

Electron Transfer Types

• Direct electron transfer, as in Fe+3 to Fe+2 reduction.
• Transfer as a hydrogen atom, like in FADH2.
• Transfer as a hydride ion (H-), like in NAD+.
• Stages include: Digestion; Acetyl-CoA, TCA, OxPhos

Electron-Carrying Molecules

Ubiquinone

• Also called coenzyme Q or Q.
• Lipid-soluble benzoquinone with a long isoprenoid side chain.
• Cyclic diene structure has 2 keto (carbonyl) groups, which contribute to Reduction reaction and produce ubiquinol (Alcohol).
• It’s small and hydrophobic (freely diffusible).
• Works as electron carrier in the IMM.
• Accepts 1 or 2 electrons as hydrogen atoms sequentially.
• Hydrocarbon chain makes it hydrophobic enabling it to pass the membrane; mix structure found in membrane accepting electrons.
• Junction between 2 electron donor and 1 electron acceptor.

Cytochromes

• Heme groups act as electron carriers.
• Proteins with strong visible light absorption (Fe-containing heme prosthetic groups).
• Classification based on light absorption and mode of binding (a, b, c).
• Mitochondria have three cytochrome classes (a, b, c).
• Heme ΔE varies.
• Each reduced cytochrome (Fe+2) has 3 absorption bands in the visible range.
  • α band: near 600 nm in type a; near 560 nm in type b; near 550 nm in type c.
  • Cytochromes named by α band wavelength: Cytochrome b562, Cytochrome C550, Cytochrome C551.
• Heme carries one electron.
• ΔEº' depends on the protein.
• Cytochromes a, b & c are transmembrane (c is the exception).

Iron-Sulfur Clusters (Fe-S)

• Have structures inside proteins.
• Pick up and deliver electrons, like FAD, FMN, and metals.

Oxidative Phosphorylation Requirements

• Electron source (NADH and FADH2).
• ETC of proteins, electron-carrying molecules.
• Final electron acceptor (oxygen).
• ATP synthase.
• Intact IMM.
• Inner mitochondrial membrane is impermeable to charged molecules, including H+.
• Impermeability is due to cardiolipin that prevents passing any molecule. It's 22% in inner membrane, also there is no Cholesterol.
• Outer membrane is similar to cell membrane with 3% cardiolipin and 45% cholesterol.

Mitochondria

• OMM: permeable to small molecules (MW<5,000) and ions via porins (transmembrane channels).
• IMM: impermeable to H+; specific transporters.
• IMM contains the respiratory chain and ATP synthase.
• Matrix: contains pyruvate dehydrogenase complex, TCA cycle enzymes, fatty acid β-oxidation pathway, and amino acid oxidation pathways.
• Matrix contains all fuel oxidation pathways except glycolysis (cytosol).
• The Krebs cycle occurs in the mitochondrial matrix, producing NADH and FADH2.
• NADH donates 2 electrons to complex one, becomes NAD+, and is catalyzed by NADH Dehydrogenase.
• Electrons move to FMN, then Fe-S in complex one, and reach Ubiquinone.
• Enzymes function as oxidoreductases because they contain molecules that accept and donate electrons (FMN, Fe-S clusters, Copper, etc.).
• Electrons moving from complex 1 to ubiquinone produce ubiquinol.
• Other names for complex 1: NADH-Q oxidoreductase & NADH dehydrogenase
• Electrons move to complex 3 via Complex 1 through NADH (more used) and Complex 2 through FADH2
• Complex 3 receives electrons from 2 sources via ubiquinone.
• FADH2 is a product of the Krebs cycle, which consists of 8 steps & 8 enzymes. 7 of them in the matrix and one suspended in the inner mitochondrial membrane.
• In the sixth step, succinate becomes fumarate (by succinate dehydrogenase/succinate CoQ Oxidoreductase), producing FADH2.
• 2 electrons from NADH and 2 electrons from FADH2 arrive at complex 3.
• Then, via cytochrome c, they go to complex 4 receiving 1 electron at a time. Complex 4 contains 2 Cu atoms and 2 heme types (Hem a and Hem a3).
• NADH: Complex I → Coenzyme Q → complex III → Cytochrome c → complex IV→ O2
• FADH2: Complex II → Coenzyme Q → complex III → Cytochrome c → complex IV→ O2