3. Oxidative Phosphorylation and the Electron Transport Chain (ETC)

The standard free energy released during the complete oxidation of glucose is conserved chemically in the reduced coenzymes NADH and FADH₂. During oxidative phosphorylation, these carriers surrender their high-energy electrons to a series of membrane-bound carriers in the inner mitochondrial membrane (IMM), with molecular oxygen (O₂) acting as the terminal electron acceptor, forming water.

Figure: electrons descend a redox tower of increasing reduction potential, from NADH/FADH₂ through Complex I/II, coenzyme Q, Complex III, cytochrome c, and Complex IV, releasing free energy at each downhill transfer that is captured as a proton gradient.

3.1 The Four Respiratory Complexes and Their Prosthetic Groups

The electron transport chain is organized into four large, multiprotein complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV associate as a supramolecular structural unit called the respirasome.

Figure: the four respiratory complexes embedded in the inner mitochondrial membrane, linked by the mobile carriers coenzyme Q and cytochrome c, with net proton-pumping stoichiometry at Complexes I, III, and IV.

3.2 Iron–Sulfur (Fe–S) Clusters: Coordination Chemistry

Iron–sulfur clusters are non-heme iron prosthetic groups that participate in single-electron transfer reactions. They consist of inorganic sulfide-linked iron atoms coordinated by cysteine or histidine residues of the parent protein.

Figure: the [2Fe–2S] rhombic cluster and the [4Fe–4S] cubane cluster, the two principal Fe–S geometries used throughout the respiratory chain.

3.3 Coenzyme Q (Ubiquinone) and Its Redox States

Coenzyme Q (CoQ), or ubiquinone, is a highly mobile, hydrophobic benzoquinone linked to a long isoprenoid tail (typically 10 isoprene units in human mitochondria, hence CoQ₁₀). It is the only electron carrier in the respiratory chain that is not a protein-bound prosthetic group. Being highly lipid-soluble, CoQ diffuses freely within the hydrophobic core of the inner mitochondrial membrane, acting as a metabolic bridge between Complex I/II and Complex III.

Figure: coenzyme Q cycles between fully oxidized ubiquinone (Q), the unstable ubisemiquinone radical (Q•⁻), and fully reduced ubiquinol (QH₂), carrying both electrons and protons.

3.4 The Q-Cycle in Complex III (Cytochrome bc₁ Complex)

To resolve the electronic mismatch between the two-electron donor ubiquinol (QH₂) and the one-electron acceptor cytochrome c, Complex III runs a cyclic pathway called the Q-cycle. Complex III has two distinct CoQ binding sites: the Q_p site on the P-face (intermembrane space side), and the Q_n site on the N-face (matrix side).

Figure: the two bifurcated half-cycles of the Q-cycle — each QH₂ oxidized at the Q_p site sends one electron to cytochrome c (via Rieske Fe–S and cyt c₁) and one electron across the membrane dielectric to the Q_n site via cytochromes b_L and b_H.

3.5 Complex IV (Cytochrome c Oxidase) and Binuclear Catalysis

Complex IV catalyzes the final transfer of electrons from reduced cytochrome c to molecular oxygen:

Complex IV contains two heme groups (a and a₃) and three copper ions organized into two functional copper centers:

Figure: electrons flow Cu_A → heme a → the binuclear heme a₃–Cu_B center, which stores four electrons before reducing O₂ to two molecules of water.

• Cu_A Center: a binuclear copper center with two copper ions linked by two bridging cysteine residues; it receives electrons one at a time from cytochrome c.
• Cu_B / Heme a₃ Center: a binuclear catalytic center where copper is coordinated by three histidine residues in close proximity to heme a₃.

3.6 Classification of Cytochromes

Cytochromes are heme-containing proteins classified into three major groups based on the structural properties of their heme groups and their visible-light absorption spectra.

Figure: a-type and b-type hemes bind their protein non-covalently via hydrophobic pocket interactions, while c-type hemes are covalently anchored through thioether bonds to cysteine residues.

3.7 Site-Specific Inhibitors of the Respiratory Chain

The pathway of electron transport has been mapped using site-specific inhibitors that bind to and block individual redox carriers.

Figure: Complex I inhibitors (rotenone, amobarbital, piericidin A) block electron transfer to ubiquinone; antimycin A blocks the Q-cycle at Complex III; cyanide, azide, and carbon monoxide block heme a₃ at Complex IV.

Analysis of Inhibitor Potency: Rotenone vs. Antimycin A

While both compounds completely block respiration when added to isolated mitochondria in the presence of NADH, Antimycin A is significantly more potent and lethal than Rotenone. If Complex I is blocked by rotenone, the chain can still receive electrons bypass-style from succinate (via Complex II) or glycerol-3-phosphate, allowing electron flow and ATP synthesis to continue at Complexes III and IV. Antimycin A, by contrast, blocks Complex III — a downstream convergence point for electrons entering from both Complex I and Complex II — so it completely abolishes all electron transfer to oxygen.

3.8 Cyanide-Resistant Respiration (Alternative Oxidase Pathway)

Many plants, fungi, and some protozoa possess an alternative electron transport pathway that bypasses the classic Complexes III and IV entirely.

Figure: the alternative oxidase (AOX) pathway diverts electrons from the ubiquinone pool directly to O₂, bypassing Complexes III and IV and their associated proton pumping.

4. The Electrochemical Proton Gradient and Proton Motive Force (pmf)

Active proton pumping across the inner mitochondrial membrane establishes an electrochemical proton gradient with two physical components: a chemical pH gradient (ΔpH) — the intermembrane space becomes acidic while the matrix becomes alkaline — and an electrical membrane potential (Δψ), since pumping positive charges out makes the intermembrane space positive and the matrix negative. Together these exert a proton motive force (pmf) that drives protons back into the matrix through ATP synthase.

4.1 Quantitative Calculation of the pmf

Figure: the proton motive force combines the electrical membrane potential (Δψ) and the chemical pH gradient (ΔpH) generated by proton pumping into the intermembrane space.

4.2 Comparative Gradient Contributions: Mitochondria vs. Chloroplasts

While both organelles use a proton motive force to synthesize ATP, they distribute the energy of the electrochemical gradient differently across Δψ and ΔpH.

Figure: mitochondrial pmf is dominated by the electrical membrane potential, whereas chloroplast pmf is dominated almost entirely by the chemical pH gradient across the thylakoid membrane.

4.3 Experimental Determination of Δψ and ΔpH

Because mitochondria and thylakoids are too small for direct microelectrode insertion, biochemists use indirect molecular probes to measure these parameters.

Figure: valinomycin renders the inner membrane transiently K⁺-permeable; the resulting equilibrium distribution of radioactive K⁺ is used with the Nernst equation to calculate Δψ.