4.3 Electron Transport Chain & Oxidative Phosphorylation

Overview & Thermodynamics of Oxidative Phosphorylation

Oxidative Phosphorylation is the final common pathway of cellular respiration in eukaryotes. It couples the exergonic transfer of electrons from reduced coenzymes ($\text{NADH}$ and $\text{FADH}_2$) to molecular oxygen ($\text{O}_2$) with the endergonic synthesis of ATP from ADP and inorganic phosphate ($\text{P}_i$).

This process takes place within the inner mitochondrial membrane (IMM), which is uniquely impermeable to small ions (including protons, $\text{H}^+$). Electrons flow through four membrane-bound protein complexes (Complexes I–IV) in order of increasing standard reduction potential ($E^{\circ\prime}$):

$\text{NADH } (E^{\circ\prime} = -0.32\text{ V}) \rightarrow \text{Complex I} \rightarrow \text{CoQ} \rightarrow \text{Complex III} \rightarrow \text{Cyt } c \rightarrow \text{Complex IV} \rightarrow \text{O}_2 \quad (E^{\circ\prime} = +0.816\text{ V})$

The total reduction potential difference for NADH oxidation by $\text{O}_2$ is $\Delta E^{\circ\prime} = +1.14\text{ V}$. The overall thermodynamic free energy release is calculated using:

$\Delta G^{\circ\prime} = -n F \Delta E^{\circ\prime} = -2 \times (96.485\text{ kJ/V}\cdot\text{mol}) \times (1.14\text{ V}) = -220\text{ kJ/mol}$

This large release of free energy drives the pumping of protons from the matrix into the intermembrane space.

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Inner Mitochondrial Membrane Respiratory Complexes

 Intermembrane Space (IMS):  [ High H+ Concentration / Positively Charged ]
 ═════════════════════════════════════════════════════════════════════════════
  IMM:     Complex I       Complex II       Complex III       Complex IV
           (4 H+ Pumped)   (0 H+ Pumped)    (4 H+ Pumped)     (2 H+ Pumped)
 ═════════════════════════════════════════════════════════════════════════════
 Matrix:   [ Low H+ Concentration / Negatively Charged ]

Complex I: NADH-Ubiquinone Oxidoreductase

• Function: Accepts two electrons from matrix NADH, transferring them to flavin mononucleotide (FMN), through a series of iron-sulfur ($[\text{Fe-S}]$) clusters, to mobile carrier Coenzyme Q (Ubiquinone).
• Proton Pumping: Translocates $4\text{ H}^+$ from the matrix into the intermembrane space (IMS) per electron pair.
• Inhibitors: Rotenone (pesticide) and Amytal (barbiturate).

Complex II: Succinate-Ubiquinone Oxidoreductase

• Function: Accepts electrons from succinate via bound $\text{FADH}_2$ (Step 6 of TCA cycle) and passes them through $[\text{Fe-S}]$ centers to Coenzyme Q.
• Proton Pumping: Translocates $0\text{ H}^+$ across the membrane. Because electrons enter downstream of Complex I, $\text{FADH}_2$ yields less ATP than NADH.
• Inhibitor: Malonate (competitive inhibitor).

Mobile Carrier: Coenzyme Q (Ubiquinone / Ubiquinol)

Coenzyme Q is a small, highly lipophilic benzoquinone with a long isoprenoid tail residing entirely inside the IMM lipid bilayer. It accepts electrons from Complex I, Complex II, and glycerol-3-phosphate dehydrogenase, converting from Ubiquinone (Q) to reduced Ubiquinol ($ ext{QH}_2$), which diffuses laterally to Complex III.

Complex III: Ubiquinone-Cytochrome c Oxidoreductase

• Function: Transfers electrons from $\text{QH}_2$ to mobile carrier Cytochrome c via the Q-Cycle mechanism.
• Proton Pumping: Translocates $4\text{ H}^+$ into the IMS per pair of electrons.
• Inhibitor: Antimycin A.

Mobile Carrier: Cytochrome c

Cytochrome c is a small, water-soluble peripheral protein located in the intermembrane space. Containing a heme prosthetic group (iron cycles between $\text{Fe}^{3+}$ and $\text{Fe}^{2+}$), Cytochrome c carries one electron at a time from Complex III to Complex IV.

Complex IV: Cytochrome c Oxidase

• Function: Receives 4 electrons from 4 reduced Cytochrome c molecules to reduce one molecule of $\text{O}_2$ into two molecules of $\text{H}_2\text{O}$ using heme $a$, heme $a_3$, and copper centers ($\text{Cu}_A$ and $\text{Cu}_B$): $4\text{ Cyt } c (\text{Fe}^{2+}) + 4\text{ H}^+_{\text{matrix}} + \text{O}_2 \rightarrow 4\text{ Cyt } c (\text{Fe}^{3+}) + 2\text{ H}_2\text{O}$
• Proton Pumping: Translocates $2\text{ H}^+$ into the IMS per electron pair ($4\text{ H}^+$ per $\text{O}_2$).
• Inhibitors: Cyanide ($\text{CN}^-$), Carbon Monoxide ($\text{CO}$), and Sodium Azide ($\text{N}_3^-$) bind tightly to the $\text{Fe}^{3+}$ in heme $a_3$, completely arresting respiration.

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Summary Table: Complexes I–IV & Mobile Carriers

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Mitchell's Chemiosmotic Hypothesis & Proton-Motive Force

Proposed by Peter Mitchell in 1961, the Chemiosmotic Hypothesis states that energy released by electron transport is conserved by pumping $\text{H}^+$ from the matrix into the intermembrane space, creating an electrochemical proton gradient across the inner mitochondrial membrane termed the Proton-Motive Force ($\Delta p$).

The proton-motive force consists of two thermodynamic components:

1. Electrical Potential ($\Delta \Psi$): Matrix is negatively charged ($\approx -160\text{ to }-180\text{ mV}$) relative to IMS.
2. Chemical pH Gradient ($\Delta \text{pH}$): Matrix is alkaline ($\text{pH } \approx 7.8$) relative to IMS ($\text{pH } \approx 7.0$).

$\Delta p = \Delta \Psi - \left( \frac{2.3 RT}{F} \right) \Delta \text{pH} \quad (\approx 180 - 220\text{ mV})$

Protons cannot spontaneously re-enter the matrix through the hydrophobic lipid bilayer; they can only return through the transmembrane channel of ATP Synthase.

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Structural Mechanism of ATP Synthase ($F_o F_1$ Complex)

ATP Synthase (Complex V) consists of two major functional domains:

• $F_o$ Domain (Membrane-bound rotor): Transmembrane channel composed of a hydrophobic $c$-subunit ring attached to a stationary $a$-subunit. Protons enter an inlet half-channel in the $a$-subunit, protonate a conserved aspartate residue on a $c$-subunit, causing the $c$-ring to rotate 360 degrees as protons pass into the matrix.
• $F_1$ Domain (Catalytic headpiece): Matrix-projecting complex with composition $\alpha_3 \beta_3 \gamma \delta \epsilon$. The rotating $\gamma$-subunit central stalk connects $F_o$ to $F_1$, driving conformational changes in the three catalytic $\beta$-subunits (Boyer's Binding Change Mechanism):
  1. Open (O) Conformation: Low affinity for nucleotides; releases synthesized ATP.
  2. Loose (L) Conformation: Binds ADP and $\text{P}_i$ loosely.
  3. Tight (T) Conformation: Condenses ADP and $\text{P}_i$ to synthesize ATP.

                                  IMS (+)
                             │   H+   │   H+
                             ▼        ▼
                    ┌──────────────────────────┐
                    │  Fo Subunit (c-ring)     │  <-- Rotor rotates as H+ flows
                    └────────────┬─────────────┘
                                 │ (gamma-stalk)
                    ┌────────────▼─────────────┐
                    │  F1 Subunit (alpha3beta3)│  <-- Catalyzes ADP + Pi --> ATP
                    └──────────────────────────┘
                               Matrix (-)

Stoichiometry & P/O Ratios

• $3\text{ H}^+$ passing through $F_o$ drive 120-degree rotation, synthesizing $1\text{ ATP}$.
• $1\text{ H}^+$ is consumed by the Phosphate Transporter ($\text{H}^+/\text{P}_i$ symporter) to import inorganic phosphate into the matrix alongside ADP import by Adenine Nucleotide Translocase (ANT).
• Total requirement = $4\text{ H}^+$ per cytosolic ATP generated.
• P/O Ratio for NADH: $10\text{ H}^+ \text{ pumped} / 4\text{ H}^+ \text{ per ATP} = \mathbf{2.5\text{ ATP}}$
• P/O Ratio for $\text{FADH}_2$: $6\text{ H}^+ \text{ pumped} / 4\text{ H}^+ \text{ per ATP} = \mathbf{1.5\text{ ATP}}$

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Cytosolic NADH Shuttles: Malate-Aspartate vs. Glycerol-3-Phosphate

Because the inner mitochondrial membrane is completely impermeable to $\text{NADH}$ and $\text{NAD}^+$, the $2\text{ NADH}$ generated by cytosolic glycolysis must transfer their reducing equivalents into the matrix using specialized shuttle systems:

1. Malate-Aspartate Shuttle (Liver, Heart, Kidney)

• Cytosolic NADH reduces oxaloacetate to Malate via cytosolic Malate Dehydrogenase.
• Malate enters the matrix via the malate-$\alpha$-ketoglutarate antiporter.
• Matrix Malate Dehydrogenase reoxidizes Malate to oxaloacetate, generating matrix NADH.
• Oxaloacetate is transaminated to Aspartate to exit back into the cytosol.
• Yield: Produces matrix NADH $\rightarrow 2.5\text{ ATP}$ per cytosolic NADH $\rightarrow$ Net yield: 32 ATP per glucose.

2. Glycerol-3-Phosphate Shuttle (Skeletal Muscle, Brain)

• Cytosolic NADH reduces DHAP to Glycerol-3-Phosphate via cytosolic Glycerol-3-Phosphate Dehydrogenase.
• Glycerol-3-Phosphate is reoxidized back to DHAP by an outer-surface IMM-bound mitochondrial isoform, transferring electrons directly to FAD to form $\text{FADH}_2$ inside the membrane.
• $\text{FADH}_2$ transfers electrons to Coenzyme Q, bypassing Complex I.
• Yield: Produces membrane $\text{FADH}_2 \rightarrow 1.5\text{ ATP}$ per cytosolic NADH $\rightarrow$ Net yield: 30 ATP per glucose.

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Uncouplers vs. Respiratory Chain Inhibitors

AAMC MCAT High-Yield Distinction: The MCAT frequently tests the difference between respiratory chain inhibitors and uncoupling agents.

Respiratory Chain Inhibitors

Inhibitors (e.g., Cyanide, Rotenone, Antimycin A, Oligomycin) block electron transport or ATP synthase directly.

• Electron flow stops $\rightarrow$ proton pumping ceases $\rightarrow$ $\Delta p$ cannot be maintained $\rightarrow$ Oxygen consumption drops to zero and ATP synthesis stops.

Uncoupling Agents (Uncouplers)

Uncouplers (e.g., 2,4-Dinitrophenol [DNP], Thermogenin / UCP-1) collapse the proton gradient by providing an alternate channel for protons to re-enter the matrix without passing through ATP Synthase.

• Mechanism: DNP is a lipophilic weak acid that binds protons in the IMS, diffuses across the IMM, and releases protons into the alkaline matrix.
• Effects: Proton gradient collapses ($\Delta p \rightarrow 0$) $\rightarrow$ ATP synthesis stops $\rightarrow$ backpressure on the ETC is removed $\rightarrow$ Electron transport and $\text{O}_2$ consumption increase to maximum rates $\rightarrow$ energy is dissipated entirely as HEAT (hyperthermia).