ETC and Oxidative Phosphorylation

⚡ Bioenergetics: ETC & Oxidative Phosphorylation

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1. Biological Oxidation & Redox Potential

Biological oxidation is the process whereby living cells break down complex molecules (like glucose) to produce energy, fundamentally involving the transfer of electrons.

Standard Reduction Potential (E₀')

• E₀' measures a molecule's tendency to gain electrons (be reduced).
• The Golden Rule: Electrons spontaneously flow from carriers with a highly negative E₀' (e.g., NADH: -0.32 V) to carriers with a highly positive E₀' (e.g., Oxygen: +0.82 V).
• This downhill flow releases massive amounts of free energy (ΔG), which is harnessed to pump protons.

Nernst Equation & Gibbs Free Energy

ΔG°' = -nFΔE₀'

• n = number of electrons transferred.
• F = Faraday's constant (96.5 kJ/V·mol).
• A positive ΔE₀' yields a negative ΔG°' (spontaneous, exergonic reaction).

2. The Electron Transport Chain (ETC)

Located in the Inner Mitochondrial Membrane (IMM). It consists of four major multiprotein complexes that transfer electrons and pump protons (H⁺) from the matrix into the intermembrane space.

Complex	Name & Prosthetic Groups	H⁺ Pumped	Potent Inhibitors
I	NADH Dehydrogenase (FMN, Fe-S)	4 H⁺	Rotenone, Amytal, Piericidin A
II	Succinate Dehydrogenase (FAD, Fe-S). Also in TCA!	0 H⁺	Malonate, Carboxin
III	Cytochrome bc₁ Complex (Heme, Fe-S). Operates Q-Cycle.	4 H⁺	Antimycin A
IV	Cytochrome c Oxidase (Heme a, a₃, CuA, CuB)	2 H⁺	Cyanide (CN⁻), CO, Azide (N₃⁻)

3. Oxidative Phosphorylation & ATP Synthase

Chemiosmotic Theory (Peter Mitchell): The proton gradient generated by the ETC stores energy as a Proton Motive Force (PMF). As protons flow back into the matrix down their concentration and electrical gradients through Complex V (ATP Synthase), the energy is used to synthesize ATP.

Mechanics of the Nanomotor

• F₀ Domain: Integral membrane portion. Contains the c-ring. As protons bind to aspartate/glutamate residues on the c-subunits, the entire ring rotates physically within the membrane.
• F₁ Domain: Peripheral matrix portion. The rotating γ (gamma) stalk spins inside the stationary α₃β₃ hexamer.
• Binding Change Mechanism: The rotating γ stalk forces the catalytic β subunits to cycle through 3 conformations: O (Open) → releases ATP, L (Loose) → binds ADP+Pi, T (Tight) → forms ATP.

⚠️ Inhibitors vs. Uncouplers

• Inhibitors (Oligomycin): Binds directly to the F₀ domain of ATP synthase, blocking the proton channel. Both ATP synthesis and ETC halt (because the gradient builds up too high).
• Uncouplers (DNP, Thermogenin/UCP1): Highly lipid-soluble molecules that carry protons back across the IMM, bypassing ATP synthase. Result: ETC runs in overdrive, oxygen consumption skyrockets, NO ATP is made, and energy is released entirely as HEAT (Non-shivering thermogenesis in brown adipose tissue).

4. Cytosolic Shuttles (NADH Transport)

The IMM is completely impermeable to the NADH generated during glycolysis in the cytosol. To harvest its electrons, cells use specific shuttle systems to pass the reducing equivalents into the matrix.

Malate-Aspartate Shuttle

• Location: Heart, Liver, Kidney.
• Mechanism: Cytosolic NADH reduces oxaloacetate to Malate. Malate crosses the IMM. Inside, Malate is oxidized back to oxaloacetate, yielding a matrix NADH.
• Entry Point: Complex I.
• ATP Yield: ~2.5 ATP per cytosolic NADH. (Yields 32 ATP total per glucose).

Glycerol-3-Phosphate Shuttle

• Location: Skeletal Muscle, Brain (needs rapid energy).
• Mechanism: Cytosolic NADH reduces DHAP to G3P. The mitochondrial G3P dehydrogenase then transfers the electrons directly to FAD inside the IMM.
• Entry Point: Coenzyme Q (bypasses Complex I).
• ATP Yield: ~1.5 ATP per cytosolic NADH. (Yields 30 ATP total per glucose).