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BI7.1-2 | Integration of Metabolism and Biological Oxidation — Part 2
The Inner Mitochondrial Membrane — Architecture of Energy Conversion
The mitochondrion has two membranes, creating two compartments. The outer membrane is permeable to small molecules (via porins/VDAC channels). The inner membrane is highly impermeable — even protons (H+) cannot cross it freely. This impermeability is absolutely critical, because the entire mechanism of ATP synthesis depends on maintaining a proton gradient across this membrane.
Figure: The Inner Mitochondrial Membrane — Architecture of Energy Conversion
The inner membrane is folded into cristae (singular: crista), which massively increase the surface area available for the ETC complexes and ATP synthase. Cells with high energy demands (cardiac muscle, liver hepatocytes, renal tubular cells, skeletal muscle) have mitochondria with densely packed cristae. A single hepatocyte may contain 1,000-2,000 mitochondria.
The space between the two membranes is the intermembrane space (IMS) — this is where protons will be pumped to create the electrochemical gradient. The interior is the mitochondrial matrix — this is where the TCA cycle, beta-oxidation, and PDC operate.
The inner membrane contains: (1) the four ETC complexes (I, II, III, IV); (2) two mobile electron carriers — coenzyme Q (ubiquinone) in the lipid bilayer and cytochrome c on the outer face; (3) ATP synthase (Complex V); (4) specific transporters for metabolites (ADP/ATP translocase, phosphate carrier, malate-aspartate shuttle components).
The electron carriers in the ETC are arranged in order of increasing reduction potential — from NADH (E°' = −0.32 V, strong electron donor) to O2 (E°' = +0.82 V, strong electron acceptor). The total potential difference is 1.14 V, releasing ΔG°' = −220 kJ/mol — enough energy to drive the synthesis of approximately 2.5 ATP molecules per NADH (or 1.5 per FADH2).
Complex I (NADH:Ubiquinone Oxidoreductase) — The Giant Entry Point
Complex I is the largest complex of the ETC — a massive L-shaped protein containing 45 subunits (7 encoded by mitochondrial DNA, 38 by nuclear DNA), with a molecular weight of ~1,000 kDa.
Figure: Complex I (NADH:Ubiquinone Oxidoreductase) — The Giant Entry Point
What it does: Accepts electrons from NADH and transfers them to coenzyme Q (ubiquinone). The electron path within Complex I: NADH → FMN (flavin mononucleotide, the initial electron acceptor) → a chain of iron-sulphur (Fe-S) clusters (7 clusters forming an electron wire ~100 Å long) → coenzyme Q (reduced to ubiquinol, QH2).
Proton pumping: As electrons flow through Complex I, conformational changes pump 4 H+ from the matrix to the intermembrane space. This is the largest proton contribution of any single complex.
The reaction: NADH + H+ + Q + 4H+(matrix) → NAD+ + QH2 + 4H+(IMS)
Energy accounting: The free energy change is ΔG°' ≈ −69 kJ/mol, enough to pump those 4 protons against the concentration gradient.
Figure: Energy accounting
Clinical significance — Inhibitors of Complex I:
• Rotenone — a natural insecticide from the derris plant; blocks electron transfer from Fe-S clusters to CoQ. Used experimentally to create Parkinson's disease models in rats (suggesting a link between pesticide exposure and Parkinson's)
• Barbiturates (amobarbital) — also block Complex I; one mechanism of barbiturate toxicity in overdose
• Metformin — the first-line drug for type 2 diabetes; mildly inhibits Complex I in hepatocytes, reducing ATP, activating AMPK, which suppresses gluconeogenesis. This is beneficial in diabetes but explains why metformin overdose → lactic acidosis (cells shift to anaerobic glycolysis)
• MPTP — a contaminant in illicit synthetic heroin; metabolised to MPP+ which selectively destroys dopaminergic neurons in the substantia nigra by inhibiting Complex I → Parkinson's-like syndrome
Inhibitors of the Electron Transport Chain
| Complex | Inhibitor | Mechanism | Clinical Significance |
|---|---|---|---|
| Complex I | Rotenone | Blocks Fe-S → CoQ electron transfer | Natural insecticide; experimental Parkinson's model |
| Complex I | Barbiturates (amobarbital) | Blocks Complex I | Mechanism of toxicity in overdose |
| Complex I | Metformin | Mild Complex I inhibition in hepatocytes | First-line T2DM drug; lactic acidosis in overdose |
| Complex I | MPTP/MPP+ | Selective uptake by dopaminergic neurons → Complex I block | Contaminant in synthetic heroin → Parkinsonism |
| Complex II | Malonate | Competitive inhibitor (succinate analogue) | Experimental tool |
| Complex III | Antimycin A | Blocks Qi site → halts Q cycle → ROS generation | Antibiotic; major source of mitochondrial superoxide |
| Complex IV | Cyanide (CN-) | Binds Fe3+ of haem a3 → blocks O2 binding | Poisoning (bitter almonds, cassava, industrial) |
| Complex IV | Carbon monoxide (CO) | Binds Fe2+ of haem a3 → competes with O2 | Silent killer (charcoal heaters in Indian winters) |
| Complex IV | Hydrogen sulphide (H2S) | Binds binuclear centre | Sewer gas, industrial exposure |
Figure: Clinical significance — Inhibitors of Complex I
Complex II (Succinate:Ubiquinone Oxidoreductase) — The TCA Cycle Link
Complex II is unique in two ways: (1) it is the only ETC complex that is also a TCA cycle enzyme — it IS succinate dehydrogenase (the enzyme that converts succinate → fumarate in the TCA cycle); (2) it does NOT pump protons across the inner membrane.
Figure: Complex II (Succinate:Ubiquinone Oxidoreductase) — The TCA Cycle Link
What it does: Oxidises succinate → fumarate, transferring electrons to FAD (covalently bound, not free FADH2) → Fe-S clusters → coenzyme Q (reduced to QH2).
The reaction: Succinate + Q → Fumarate + QH2 (no proton pumping)
Why no proton pumping? The free energy released by the succinate → fumarate → CoQ electron transfer (ΔG°' ≈ −6 kJ/mol) is too small to pump protons across the membrane. This is why FADH2 yields fewer ATP than NADH — electrons from FADH2 (via Complex II) bypass Complex I's proton pump entirely.
Structure: Only 4 subunits (all nuclear-encoded) — the smallest ETC complex. Contains FAD, 3 Fe-S clusters, and a haem b group (whose function is debated, possibly prevents reactive oxygen species formation).
Other entry points to CoQ: Besides Complexes I and II, other flavoproteins also feed electrons to CoQ: (1) ETF:ubiquinone oxidoreductase — receives electrons from fatty acid beta-oxidation (via ETF = electron-transferring flavoprotein); (2) mitochondrial glycerol-3-phosphate dehydrogenase — part of the glycerol-3-phosphate shuttle (cytoplasmic NADH → mitochondrial FADH2 → CoQ). These bypass Complex I, so they also yield only 1.5 ATP per electron pair.
Figure: Other entry points to CoQ
Clinical significance: Mutations in Complex II subunits are associated with hereditary paragangliomas and pheochromocytomas (rare tumours of neural crest origin) — a fascinating link between mitochondrial dysfunction and cancer (pseudohypoxia pathway activating HIF-1α). The Warburg effect in cancer cells — preferring glycolysis even in the presence of oxygen — may involve Complex II dysfunction.
Figure: Clinical significance
Complex III (Cytochrome bc1 Complex) and the Q Cycle
Complex III receives electrons from ubiquinol (QH2) and passes them to cytochrome c, pumping protons in the process. It is a homodimer (two identical halves), each containing 11 subunits.
Figure: Complex III (Cytochrome bc1 Complex) and the Q Cycle
Key prosthetic groups: Haem bL and haem bH (two b-type cytochromes with different reduction potentials — L for low, H for high), a Rieske iron-sulphur protein (unique 2Fe-2S cluster with histidine ligands instead of the usual cysteine), and haem c1 (cytochrome c1).
Figure: Key prosthetic groups
The Q cycle — an elegant mechanism that maximises proton pumping: QH2 binds at the Qo site (outer, facing IMS). One electron goes to the Rieske Fe-S protein → cytochrome c1 → cytochrome c (released to IMS). The other electron goes to haem bL → haem bH → reduces a ubiquinone at the Qi site (inner, facing matrix) to a semiquinone radical. A second QH2 molecule repeats this at the Qo site, and the second electron at the Qi site fully reduces the semiquinone to QH2 (picking up 2H+ from the matrix).
Net result of one Q cycle: 2 QH2 oxidised at Qo → 2 cytochrome c reduced → 1 Q reduced at Qi → 4 H+ released to IMS (from the 2 QH2 at Qo) + 2 H+ consumed from matrix (to reduce Q at Qi). Net proton translocation: 4H+ to IMS per 2 electrons passed to cytochrome c (effectively 2H+ per electron, but usually counted as 4H+ per pair of electrons flowing through).
Figure: Net result of one Q cycle
Per NADH flowing through the full chain: Complex III contributes 4 H+ pumped (for the 2 electrons).
Clinical significance — Antimycin A: This antibiotic binds the Qi site of Complex III, blocking electron transfer from haem bH to ubiquinone. This halts the entire Q cycle, stopping electron flow. Electrons backed up at the Qo site can leak to O2, forming superoxide radical (O2•−) — Complex III is a major source of mitochondrial reactive oxygen species (ROS). This ROS generation is implicated in ageing and in ischaemia-reperfusion injury (e.g., after a heart attack, when blood flow is restored to ischaemic tissue, a burst of ROS from dysfunctional Complex III causes additional damage).
Figure: Clinical significance — Antimycin A
Complex IV (Cytochrome c Oxidase) — The Final Step: Oxygen Reduction
Complex IV is the terminal enzyme of the ETC — it transfers electrons from cytochrome c to molecular oxygen, reducing it to water. This is the reaction that makes aerobic life possible and the reason we breathe.
Figure: Complex IV (Cytochrome c Oxidase) — The Final Step: Oxygen Reduction
Structure: A dimer of 13 subunits each. The catalytic core has 3 subunits encoded by mitochondrial DNA (I, II, III). Key metal centres: CuA (binuclear copper centre in subunit II — receives electrons from cytochrome c), haem a (transfers electrons to the binuclear centre), and the binuclear centre containing haem a3 + CuB (where O2 binds and is reduced to H2O).
The reaction: 4 cytochrome c (reduced) + O2 + 8H+(matrix) → 4 cytochrome c (oxidised) + 2H2O + 4H+(IMS)
Proton pumping: Complex IV pumps 4 H+ per O2 reduced (= 2 H+ per pair of electrons, or 1 H+ per electron). Additionally, 4 H+ from the matrix are consumed in making 2 H2O (these are "chemical" or "scalar" protons, distinct from the "vectorial" pumped protons). So for every O2 reduced: 4 pumped H+ + 4 chemical H+ = 8H+ total removed from the matrix.
Per NADH: 2 electrons flow through Complex IV, pumping 2 H+ and consuming 2 H+ for water formation.
Oxygen binding and reduction: O2 binds at the haem a3-CuB binuclear centre. The enzyme has evolved to hold onto all intermediate oxygen species (peroxide, superoxide) until the 4-electron reduction is complete — this prevents release of dangerous partially reduced oxygen species. This is remarkable engineering: the enzyme reduces a dangerously reactive molecule (O2) safely, without releasing toxic intermediates.
Figure: Oxygen binding and reduction
Clinical significance — Inhibitors of Complex IV:
• Cyanide (CN−) — binds to Fe3+ of haem a3, blocking O2 binding. Sources in India: bitter almonds (amygdalin), cassava (linamarin — relevant in tribal populations who eat improperly processed cassava), industrial exposure (electroplating, gold extraction), and deliberate poisoning. Treatment: hydroxocobalamin (binds CN−), sodium nitrite + sodium thiosulphate (classic antidote kit), or 100% oxygen.
• Carbon monoxide (CO) — binds to Fe2+ of haem a3 (also haemoglobin with 200× greater affinity than O2). Common in India from coal/charcoal heaters in closed rooms during winter, and from house fires. CO produces a cherry-red skin colour. Treatment: 100% oxygen, hyperbaric oxygen if available.
• Hydrogen sulphide (H2S) — blocks Complex IV similarly to cyanide. Encountered in sewer workers ("sewer gas") and in the Bhopal-region sulphur spring exposures.
• Azide (N3−) — blocks Complex IV; a laboratory chemical.
Figure: Clinical significance — Inhibitors of Complex IV
SELF-CHECK
A 45-year-old electroplating worker is brought to the emergency department with sudden collapse, seizures, and a characteristic bitter almond smell on his breath. ABG shows metabolic acidosis with elevated lactate. Venous blood appears 'arterialised' (bright red). Which of the following best explains the mechanism of his toxicity?
A. Rotenone-like blockade of Complex I, preventing NADH oxidation
B. Uncoupling of oxidative phosphorylation, dissipating the proton gradient as heat
C. Cyanide binding to haem a3 (Fe3+) in Complex IV, preventing electron transfer to O2, causing histotoxic hypoxia and forcing cells to rely entirely on anaerobic glycolysis
D. Inhibition of ATP synthase (Complex V), blocking ADP phosphorylation while the ETC continues
Reveal Answer
Answer: C. Cyanide binding to haem a3 (Fe3+) in Complex IV, preventing electron transfer to O2, causing histotoxic hypoxia and forcing cells to rely entirely on anaerobic glycolysis
The bitter almond smell, electroplating exposure, sudden collapse, and metabolic acidosis with bright red venous blood (cells cannot extract O2, so venous blood remains oxygenated) are classic for cyanide poisoning. Cyanide (CN−) binds to the ferric iron (Fe3+) in haem a3 of Complex IV, preventing the final transfer of electrons to O2. Despite abundant O2, cells cannot use it (histotoxic hypoxia). All oxidative metabolism stops → cells switch to anaerobic glycolysis → massive lactate production → metabolic acidosis. Option A is rotenone/metformin. Option B is uncoupling (e.g., DNP, thermogenin). Option D is oligomycin.