M

Bioenergetics 2 — Electron Transport Chain & Proton Motive Force

Recap of Previous Lecture

  • Origins, architecture and compartments of mitochondria reviewed.
  • F(1)F(0)-ATPase and mitochondrial dynamins (mitofusins/microscaffolds) positioned at cristae tips; shape drives function.
  • Materials/ions must transit specific membranes (outer, inner, cristae junctions) via dedicated transporters.

Why an Electron-Transport Chain (ETC)?

  • Direct reaction of high-energy electrons with O_2 releases large, explosive free energy → biologically unusable.
  • Biology subdivides the reaction into many redox steps → small energy packets can be siphoned off and conserved.
  • Energy captured as an electrochemical (proton-motive) gradient, later used for:
    • ATP synthesis via molecular motors (ATP synthase)
    • Secondary transport (nutrient uptake, flagellar motors, etc.)

Chemiosmotic Theory (Peter Mitchell, Nobel 1978)

  • Universal principle for mitochondria, chloroplasts, bacteria.
  • High-energy electrons (respiration or photosynthesis) → proton gradient (Δp).
  • Gradient then powers ATP synthesis or other work.
  • Concept initially controversial; now central dogma of bioenergetics.

Electrochemical Gradient & Proton-Motive Force

  • Two components across inner mitochondrial membrane (IMM):
    • Chemical: ΔpH (difference in [H(^+)] ⇒ pH)
    • Electrical: Δψ (membrane potential, charge separation)
  • Overall free energy (proton-motive force, pmf) often expressed as millivolts:
    \Delta G = -nF\Delta E \qquad (\text{or replace } \Delta E \text{ by } \Delta\psi)
    where F = Faraday constant, n = charge (for protons, n=1).
  • Nernst-type formulation links concentration term and potential term.

Key Electron Carriers & Their Standard Potentials

  • Remember order by ΔE (more negative → donates first):
    • NAD(^+/)NADH: 2-e⁻ carrier, E^0 \approx -0.32\,\text{V}.
    • Flavins (FMN/FAD): 2-e⁻ carriers, capable of 1-e⁻ semiquinone, E^0 \approx -0.20\,\text{V}.
    • Ubiquinone (Q) ⇌ ubiquinol (QH(_2)): lipid-soluble 2-e⁻ shuttle in IMM, anchors by isoprenoid tail.
    • Iron–sulfur (Fe–S) clusters ([2Fe–2S], [4Fe–4S]): 1-e⁻, tunable E^0\approx -0.25\,\text{ to }+0.05\,\text{V} depending on protein.
    • Cytochromes (heme b, c, a): 1-e⁻; E^0 increases along series (b ≈ 0 mV, c ≈ +0.25 V, a ≈ +0.38 V).

Predicting Electron Flow

  • Electrons move spontaneously from carriers with more negative to more positive E^0 (opposite sign to ΔG).
  • Lining up carriers by potential effectively predicts their positioning in complexes I → IV.

Complex I — NADH:Ubiquinone Oxidoreductase

  • L-shaped megacomplex (≈1 MDa):
    • Peripheral arm in matrix: redox chemistry.
    • Membrane arm: proton pumping.
  • Electron path:
    1. NADH binds; hydride → FMN → series of Fe–S clusters.
    2. Final cluster (N2) 15 Å from Q-binding pocket; Q pulled ≈15 Å out of lipid into hydrophobic cavity.
  • Proton pumping: 4 H(^+)/2 e⁻ from matrix → IMS.
    • Long horizontal (transverse) helix spans membrane arm; acts like connecting rod on steam engine.
    • Redox-induced conformational wave moves through four antiporter-like subunits → alternating-access channels.
  • Quantum tunnelling: despite 13–22 Å gaps, e⁻ transfer remains fast because of quantum probability of electron presence across barriers.

Complex II (Succinate Dehydrogenase) — Brief Note

  • Part of TCA cycle: Succinate + FAD → Fumarate + FADH(_2).
  • Electrons pass through Fe–S centres directly to Q; no proton pumping and start at a higher (less negative) E^0.
  • Reason lecturer skipped detailed discussion: contributes electrons but not to Δp.

Complex III — Ubiquinol:Cytochrome c Oxidoreductase (bc(_1) Complex)

  • Challenge: QH(_2) (2 e⁻) → Cyt c (1 e⁻).
  • Solution: Q-cycle (two half-cycles)
    1. First QH(2) binds at Qo site (IMS side); releases 2 H(^+) to IMS. • e⁻(1) → Rieske Fe–S → Cyt c(1) → soluble Cyt c.
      • e⁻(2) → heme b(L) → heme b(H) → reduces a waiting Q at Qi site to semiquinone (•Q⁻).
    2. Second QH(2) repeats; semiquinone at Qi gains second e⁻ + 2 H(^+) from matrix → regenerates QH(2).
  • Net per 2 e⁻ (1 QH(_2) oxidised):
    • 4 H(^+) translocated (2 released, 2 taken from matrix).
    • 2 Cyt c (each 1 e⁻) reduced.
  • Rieske protein mobility (hinged domain) ensures proper routing of first electron versus second.

Cytochrome c — Mobile 1-e⁻ Shuttle

  • Small soluble protein (~12 kDa) in inter-membrane space.
  • Single heme c housed in surface cleft → entry/exit path for e⁻.
  • Surface rich in positive charge; electrostatically docks onto cardiolipin-rich, negatively charged IMM patches near complexes III & IV.
  • Diffuses laterally (2-D) along membrane surface → fast hand-off.
  • Also famous for apoptosis signalling when released to cytosol (ethical/philosophical bio relevance).

Analogies & Visual Aids Mentioned

  • Steam-engine wheels ≈ transverse helix coupling four proton pumps (Complex I).
  • Video simulations & Nature Reviews recommended for molecular motion insight.
  • Quantum tunnelling highlighted as counter-intuitive but essential.

Connections & Applications

  • Same chemiosmotic logic in chloroplast thylakoids, bacterial plasma membranes, archaeal rhodopsin pumps.
  • Δψ across neuronal membranes (action potentials) similarly described by Nernst/Goldman equations.
  • Proton-motive force not only for ATP; can drive nutrient symports/antiports or flagellar rotation.

Equations & Constants Re-stated

  • Free energy / potential:
    \Delta G = -nF\Delta E
  • Proton-motive force (pmf):
    \Delta p = \Delta\psi - (2.303\,RT/F)\,\Delta pH
    (At 37 °C, 59 mV per pH unit.)
  • Faraday constant F = 96{,}485\,\text{C·mol}^{-1}.

Study Tips & Resources (as per lecturer)

  • Replace numerical complex names with reaction names ("NADH:Q oxidoreductase", etc.).
  • Sketch cartoon diagrams showing:
    • redox centres (with ΔE order),
    • Q-cycle paths,
    • proton pumping stoichiometry.
  • Review Nature Reviews articles & embedded animations.
  • Reflect on why Complex II lacks pumping and what this means for ATP yield (hint: FADH(_2) oxidation yields fewer protons than NADH oxidation).

To Be Covered Next Lecture (Teaser)

  • Complex IV (Cytochrome c oxidase): O(2$$ reduction to H(2)O, coupled proton pumping.
  • Full integration with ATP synthase and overall P/O ratios.