OxPhos 2 2024

Oxidative Phosphorylation Overview

• Instructor: Dr. Mark Skidmore

• Course Code: LSC-10064

• Location: Keele University

Major Metabolic Pathways

• Fats, Polysaccharides, Proteins

  • Stage I:

    • Fats: Fatty acids and glycerol

    • Polysaccharides: Other sugars

    • Proteins: Amino acids

  • Stage II:

    • Conversion to Acetyl CoA

    • Links to Citric Acid Cycle

      • Results in 2 CO2 emission

  • Stage III:

    • Electron Transport Chain (ETC)

      • Generates 8 electrons and utilizes O2

      • Produces H2O and ATP

Chemiosmotic Hypothesis

• Consists of 4 postulates explaining ATP synthesis via proton motive force.

Postulates of the Chemiosmotic Hypothesis

Postulate 1: H+ Pump

• Proton pumps located in the inner mitochondrial membrane

• Key complexes:

  • Complex I (NADH dehydrogenase)

  • Ubiquinone (UQ)

  • Cyt c (cytochrome c)

  • Complex IV (cytochrome c oxidase)

• Protons (H+) pumped into Inter-membrane Space generating a proton gradient.

Postulate 2: Membrane Impermeability

• The inner mitochondrial membrane is impermeable to protons (H+).

• Agents known as uncouplers can inhibit ATP synthesis by increasing membrane permeability to H+.

Postulate 3: ATP Synthase

• Contains two regions: Fo and F1

  • Fo: H+ pore and responsible for proton flow.

  • F1: Catalytic site for ATP synthesis (ADP + Pi → ATP).

Postulate 4: Inner Membrane Exchange Carriers

• Facilitates the coupling of anion entry to proton entry and cation exit to proton entry, allowing the establishment of an electrochemical gradient.

Motors, Shuttles, and the Protonmotive Force

• ATP synthase functions as a motor powered by proton flow.

• Shuttles transport metabolites between mitochondrial and cytoplasmic compartments.

• The protonmotive force links energy generation to consumption.

Protonmotive Force

• Also referred to as:

  • Proton gradient

  • H+ gradient

  • Electrochemical potential difference of hydrogen ions.

ATP Synthase Structure

• Present in the inner mitochondrial membrane

• Two regions:

  • Fo: Responsible for H+ translocation over the membrane

  • F1: Catalysis of ATP production

Mechanism of ATP Synthase

• ATP synthesis is influenced by the flow of H+ ions through ATP synthase.

• Significant international effort contributed to understanding the mechanics of ATP synthesis.

Contributions to ATP Synthase Understanding

Efraim Racker

• Focused on reconstitution of oxidative pathways from pure components.

Paul Boyer

• Developed the Binding Change Mechanism, implicating the protonmotive force in ATP release.

Sir John Walker

• Focused on high-resolution structures using X-ray crystallography.

Rotary Binding Change Mechanism for ATP Synthesis

• Describes the rotational aspect of F1 subunits in ATP production linked to the flow of protons through Fo.

Evolutionary Perspectives

• ATP Synthase exemplifies modular evolution across species.

• ATP synthesis mechanisms show structural/functionality conservation among different biological kingdoms.

ATP Synthase in Plants

• Presence in chloroplasts (CF1FO-ATP synthase) in thylakoid membranes and its role in photosynthesis.

Active Transport & Protonmotive Force

• Comprises electrical and chemical components, driving the transport of various ions and metabolites.

Electron Transport Chain and Ion Transport

• Mechanisms for ion and metabolite transport that couple to the protonmotive force regarding energy generation and consumption.

Mechanism of ATP/ADP Translocase

• Essential for ATP export and ADP/Pi import across the inner mitochondrial membrane.

NADH Oxidation and Transport Shuttles

• Two primary shuttles for cytoplasmic NADH:

  • Glycerol 3-Phosphate shuttle

  • Malate-Aspartate shuttle

• Each employs specific enzymes for effective transport across membranes.

Discussion on ATP Yield

• Theoretical vs practical yield differences in ATP from glucose oxidation.

• Reasons for discrepancies in ATP yield per glucose oxidized, including transport costs and efficiencies.

Conclusions of Oxidative Phosphorylation

• Requires extensive transport mechanisms in mitochondria.

• Complex regulation of transporter proteins affecting ATP synthesis and energy yields.

Final Remarks

• Importance of understanding the costs of ATP export and transport processes contributing to overall energy yield.