Oxidative Phosphorylation

Oxidative Phosphorylation

Learning Outcomes

  • After this lecture, you should be able to:

    • Describe the chemical nature and sequential arrangement of the electron carriers in the mitochondrial inner membrane.

    • Indicate how the electron carriers are organized in 'complexes' that conserve the energy from the reoxidation of NADH and FADH2 by pumping protons across the membrane to create a proton and an electrochemical gradient.

    • Describe how the energy of the proton gradient is used for the synthesis of ATP from ADP by the membrane-associated protein, the F0, F1- ATP synthase.

    • Indicate how the rate of electron transport is controlled by the levels of ADP and ATP (described as respiratory control, or ‘tightly-coupled’ oxidative phosphorylation).

    • Explain the different effects that inhibitors and uncouplers have on the electron transport chain.

Summary of Mitochondrial Events

  • Focus on the sequence of events leading to oxidative phosphorylation.

  • C. The process begins with glucose leading to the TCA cycle which produces reduced electron carriers, NADH and FADH2, essential for the electron transport chain (ETC).

Hydrogen Atoms and Electron Carriers

  • **Transfer of Hydrogen:

    • A hydrogen atom consists of one proton (H+) and one electron (e-). When transferring a hydrogen atom, both the proton and an electron are transferred.

    • The process can be summarized as follows:

    • Transfer of 2 H = transfer of 2 H+ and 2 e-

    • NAD+ is reduced to NADH + H+, releasing H+ to the solution.

    • NADH and FADH2 are known as electron carriers rather than hydrogen carriers due to their role in the transfer of electrons during oxidation. 

Oxidation and Reduction

  • Reduction and Oxidation:

    • This process involves the reduction of NAD+ to NADH + H+, capturing electrons in an enzymatic reaction.

Structure of Mitochondria

  • Briefly discusses the organelle responsible for oxidative phosphorylation, containing structures necessary for the electron transport process.

Electron Transport Chain (ETC)

  • Function of the ETC:

    • Electrons are transferred from NADH and FADH2 through protein complexes. The final acceptor of these electrons is O2, which forms H2O.

    • Key reactions in the ETC include:

    • NADH+H++rac12O<em>2ightarrowH</em>2ONADH + H^+ + rac{1}{2}O<em>2 ightarrow H</em>2O

    • H+ ions are extruded to create a proton gradient.

Components of the Electron Transport Chain

  • Proteins or protein complexes are embedded in the inner mitochondrial membrane. Each of these components has:

    • Prosthetic Groups:

    • FMN, FAD, heme groups, and Fe-S clusters function as electron carriers.

    • Ubiquinone (Q):

    • A non-protein-bound molecule, ubiquinone acts as a mobile electron carrier.

Role of Flavins and Cytochromes

  • Flavins act as hydrogen (therefore electron carriers):

    • FMN: Flavin mononucleotide.

    • FAD: Flavin adenine dinucleotide.

  • Cytochromes:

    • Proteins containing heme that transport electrons using Fe2+ ions. Variants include oxidized and reduced forms of cytochrome c, critical for ETC functionality.

    • cycles between fe3+ and fe2+ as it gains and passes on e-

Iron-Sulphur Proteins

  • Function:

    • Carry electrons on Fe2+ ions. Iron-sulfur clusters involve Fe-S proteins, which play essential roles in the electron transfer process.

    • anchored into proteins via cys 

Ubiquinone (Coenzyme Q)

  • Functions as an electron and hydrogen acceptor, freely moving within the inner mitochondrial membrane, facilitating electron transport between complexes I and II to complex III.

  • Its non-protein nature allows it to move freely, making it vital for efficient electron transport.

Redox Potential

  • Definition:

    • Redox potential, represented by E’0, quantifies a molecule's affinity for electrons. 

    • the higher the redox potential (+ve redox potential) is the more likely to accept an e- 

    • A more negative E'0 indicates a greater likelihood of electron donation. Conversely, a positive value indicates readiness for reduction.

  • Electrons move to acceptors with progressively higher E’0 values through quintessential steps involving each ETC complex:

    1. Complex IV → O2

    2. Q → Complex III

    3. Complex II → Q

    4. Complex III → Complex IV

    5. Complex I → Q

  • complex 2 doesnt pump protons, the difference in E’0 between 2 and 3 does not provide sufficient energy for the transfer of protons across the inner membrane  

Energy Transfer in the ETC

  • The transfer of electrons releases energy in small, usable 'packages.'

  • Comparison with burning sugar shows that:

    • Direct burning results in energy loss as heat, while cellular oxidation stores some energy in activated carriers.

Proton Gradient creation

  • Energy from electron transport is utilized to pump protons (H+) from the mitochondrial matrix to the intermembrane space. This establishes a gradient that is crucial for ATP synthesis.

    • The formation of this gradient leads to both a pH gradient and a voltage gradient across the membrane, essential for ATP generation.

ATP Synthase Mechanics

  • ATP Synthase Structure:

    • Comprised of two main components: F0 (membrane-embedded) and F1 (site of ATP synthesis).

    • Flowing protons through ATP synthase causes rotation, enabling ATP synthesis in F1 through energy conversion.

  • Energy Transduction:

    • The stalk connects F0 to F1, transmitting energy from FO rotation to facilitate conformational changes necessary for ATP production.

Coupling of ATP Synthesis to Electron Transport

  • This coupling is defined as oxidative phosphorylation (OXPHOS), where:

    • Oxidative: Each ETC component gets oxidized during electron transfer.

    • Phosphorylation: The phosphorylation of ADP using inorganic phosphate (Pi) leads to ATP formation.

ATP Yields in Oxidative Phosphorylation

  • Each NADH and FADH2 contributes to ATP production:

    • 1 NADH → 2.5 ATP

    • 1 FADH2 → 1.5 ATP

  • Control of electron transport occurs based on ADP and ATP levels (respiratory control), where increased ADP leads to heightened O2 uptake, and elevated ATP results in reduced O2 uptake.

Inhibitors and Uncouplers of Oxidative Phosphorylation

  • Inhibitors:

    • Examples include CN-, CO, and rotenone, which impede electron transfer, diminishing ATP synthesis and possibly causing damaging radicals.

  • Uncouplers:

    • Example: dinitrophenol (DNP), allows protons to bypass the ATP synthase, decreasing ATP synthesis and releasing energy as heat, leading to potential health risks.

Implications of DNP Usage

  • DNP proclaimed as a diet aid was linked to numerous fatalities and has been subsequently banned for human intake. It functions dangerously as a weight-loss agent by uncoupling oxidative phosphorylation.

  • inhibits metabolism 

Balance of ATP Production from Glucose Oxidation

  • Full oxidation of glucose yields:

    • Glycolysis: 2 ATP

    • TCA Cycle: 2 ATP (GTP)

    • NADH from Pyruvate Dehydrogenase: 5 ATP

    • NADH from TCA Cycle: 15 ATP

    • FADH2 from TCA Cycle: 3 ATP

    • NADH from Glycolysis: Approximately 5 or 3 ATP

    • Total possible yield: 32 or 30 ATP depending on the shuttle mechanism utilized to transfer NADH into mitochondria.

Recap of Learning Outcomes

  • Constant reiteration of learning goals emphasizes the importance of understanding the electron transport chain organization and functionality for ATP synthesis and regulatory mechanisms in oxidative phosphorylation.