Redox Chemistry & Electron Transport

Introduction to Oxidation-Reduction Chemistry

Oxidation-reduction (redox) reactions are the chemical engine of life. Cellular respiration, photosynthesis, and the detoxification of reactive oxygen species all depend on the controlled transfer of electrons between molecules. Mastering redox chemistry requires understanding oxidation states, electrode potentials, and the central role of electron carriers such as NAD⁺ and FAD.

Oxidation States and the Rules

The oxidation state (oxidation number) of an atom is a formal charge reflecting the number of electrons it has gained or lost relative to its elemental state. Rules:

1. Elemental atoms: oxidation state = 0 (e.g., O₂, Fe)
2. Monatomic ions: oxidation state = charge (Na⁺ = +1, Cl⁻ = −1)
3. Oxygen: usually −2 (except peroxides O₂²⁻ where it is −1)
4. Hydrogen: usually +1 (except metal hydrides where it is −1)
5. Sum of oxidation states in a neutral molecule = 0; in an ion = charge

Oxidation = loss of electrons (oxidation state increases); "LEO" (Lose Electrons, Oxidation)

Reduction = gain of electrons (oxidation state decreases); "GER" (Gain Electrons, Reduction)

Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain)

Half-Reactions and Balancing Redox Equations

Every redox reaction can be split into two half-reactions: an oxidation half-reaction (the reducing agent) and a reduction half-reaction (the oxidising agent).

Example — reaction of ferrous (Fe²⁺) with molecular oxygen in the ETC:

• Oxidation half-reaction: Fe²⁺ → Fe³⁺ + e⁻
• Reduction half-reaction: ½ O₂ + 2H⁺ + 2e⁻ → H₂O

To balance in acidic solution: balance O with H₂O, then H with H⁺, then charge with electrons.

Standard Reduction Potential (E°)

The tendency of a half-reaction to proceed as a reduction is expressed as its standard reduction potential (E°), measured in volts (V) relative to the standard hydrogen electrode (E° = 0 V by convention).

• Higher (more positive) E°: stronger oxidising agent; greater tendency to accept electrons
• Lower (more negative) E°: stronger reducing agent; greater tendency to donate electrons

Electrons flow spontaneously from lower E° (donor) to higher E° (acceptor).

The standard EMF of a cell:

ΔE°cell = E°cathode (reduction) − E°anode (oxidation)

A positive ΔE°cell indicates a spontaneous reaction. The relationship to free energy:

ΔG°' = −nFΔE°cell

Where n = number of electrons transferred, F = Faraday constant (96,485 C/mol). For every +0.1 V increase in ΔE°cell with n = 2: ΔG°' decreases by ~19.3 kJ/mol.

The Nernst Equation

At non-standard concentrations or temperatures:

E = E° − (RT/nF) × ln([Red]/[Ox])

At 25°C, this simplifies to: E = E° − (0.0592/n) × log([Red]/[Ox])

This is important for understanding how mitochondrial membrane potential and ion gradients drive biological electron transfer.

NAD⁺/NADH: The Primary Electron Carrier

Nicotinamide adenine dinucleotide (NAD⁺) is the most important electron carrier in catabolism. It accepts a hydride ion (H⁻ = 2 electrons + 1 proton) from substrates:

NAD⁺ + 2H⁺ + 2e⁻ → NADH + H⁺ E°' = −0.32 V

NADH is the reduced form and carries high-energy electrons. The NADH/NAD⁺ ratio is a key indicator of the cellular redox state. Key reactions generating NADH:

• Isocitrate dehydrogenase (Krebs cycle)
• α-Ketoglutarate dehydrogenase (Krebs cycle)
• Malate dehydrogenase (Krebs cycle)
• Pyruvate dehydrogenase (glycolysis → Krebs link)
• Glyceraldehyde-3-phosphate dehydrogenase (glycolysis)

Per glucose molecule: 10 NADH are generated (2 cytosolic + 8 mitochondrial) plus 2 FADH₂.

FAD/FADH₂: The Secondary Electron Carrier

Flavin adenine dinucleotide (FAD) accepts 2 electrons and 2 protons to form FADH₂. E°' = −0.06 V (less negative than NADH, so carries electrons at a higher, less energetically favourable potential). FAD is tightly bound to enzyme subunits as a prosthetic group.

Key reaction: Succinate dehydrogenase (Complex II): Succinate + FAD → Fumarate + FADH₂. FADH₂ enters the ETC at Complex II, bypassing Complex I, yielding fewer ATP molecules (~1.5 ATP vs ~2.5 ATP per NADH — the malate-aspartate shuttle determines the cytosolic NADH yield).

The Electron Transport Chain (ETC)

The ETC in the inner mitochondrial membrane consists of four large protein complexes and two mobile electron carriers (ubiquinone/coenzyme Q and cytochrome c):

Complex I (NADH-ubiquinone oxidoreductase): NADH → CoQ. Transfers 2 electrons from NADH to ubiquinone (CoQ). Pumps 4 H⁺ across the inner membrane. Contains flavin mononucleotide (FMN) and iron-sulphur (Fe-S) clusters as prosthetic groups.

Complex II (Succinate-ubiquinone oxidoreductase): FADH₂ → CoQ. Does NOT pump protons. Contains FAD and Fe-S clusters.

Complex III (Ubiquinol-cytochrome c oxidoreductase): CoQH₂ → cytochrome c. Uses the Q cycle to pump 4 H⁺ per 2 electrons. Contains cytochrome b and cytochrome c₁.

Complex IV (Cytochrome c oxidase): Cytochrome c → O₂. Reduces O₂ to H₂O (4 cyt c + 4H⁺ + O₂ → 2H₂O). Pumps 2 H⁺ per pair of electrons. Contains cytochrome a, a₃, and copper centres (CuA, CuB).

Per NADH: 10 H⁺ pumped (4+0+4+2 — note Complex II is excluded from the NADH pathway).

Per FADH₂: 6 H⁺ pumped (bypasses Complex I).

The Proton Motive Force and ATP Synthesis

The proton gradient across the inner membrane (high [H⁺] in intermembrane space, low in matrix) constitutes the proton motive force (PMF), expressed as:

Δp = ΔΨ − (2.303RT/F) × ΔpH

In mitochondria, ~70% is electrical (ΔΨ ≈ −180 mV, inside negative) and ~30% is chemical (ΔpH ≈ 0.5 units).

ATP synthase (Complex V / F₀F₁-ATPase) uses the downhill proton flux through its F₀ c-ring to drive rotation of the γ-subunit stalk, causing conformational changes in the three β-subunits of F₁ (Boyer's binding change mechanism). Approximately 8–10 H⁺ pass per 3 ATP synthesised. Combined with the 10 H⁺ pumped per NADH: ~30 ATP per molecule of glucose in total.

Uncouplers and ETC Inhibitors

Uncouplers dissipate the proton gradient without synthesising ATP (protonophores):

• 2,4-Dinitrophenol (DNP): lipid-soluble weak acid; carries H⁺ across membrane; causes hyperthermia
• Thermogenin/UCP1: protein uncoupler in brown adipose tissue; physiological thermogenesis

ETC inhibitors block specific complexes:

• Rotenone (Complex I): inhibits NADH:CoQ oxidoreductase → ↓ ATP, accumulation of NADH
• Malonate (Complex II): competitive inhibitor of succinate dehydrogenase
• Antimycin A (Complex III): blocks cytochrome bc₁ complex
• Cyanide, CO, azide (Complex IV): bind cytochrome a₃; prevent O₂ reduction; cells asphyxiate despite O₂ availability → histotoxic hypoxia
• Oligomycin (Complex V — ATP synthase): blocks H⁺ channel of F₀; ATP synthesis halts; PMF builds up and secondarily inhibits ETC

Clinical Relevance

Cyanide poisoning: binds ferric (Fe³⁺) form of cytochrome a₃ with high affinity; irreversibly inhibits Complex IV. O₂ cannot be reduced; ETC halts; ATP production falls to anaerobic glycolysis levels. Cells accumulate lactate (anaerobic) despite high O₂ saturation — "histotoxic hypoxia." Treatment: hydroxocobalamin (binds CN⁻), sodium thiosulphate (provides sulphur for rhodanese enzyme to convert CN⁻ to thiocyanate), or amyl nitrite (oxidises Hb to metHb which preferentially binds CN⁻).

Lactic acidosis and metformin: in rare cases, particularly with renal failure, metformin accumulates and mildly inhibits Complex I → ↑ NADH/NAD⁺ ratio → ↑ lactate production. Most clinically relevant in hypoxic states.

Mitochondrial diseases: mutations in mtDNA (13 ETC subunits encoded) or nuclear DNA (remainder) cause a spectrum of disorders (MELAS, MERRF, Leigh syndrome) with multi-organ involvement particularly of high-energy tissues (brain, muscle, heart).