The Electron Transport Chain (ETC) - A Complete Guide

1. Overview and Location

• NAD+ → reduced to NADH
• FAD → reduced to FADH2

2. Mitochondrial Architecture

3. Components of the ETC

Complex I - NADH Dehydrogenase (NADH:CoQ Oxidoreductase)

• Accepts electrons from: NADH (from TCA cycle, beta-oxidation, etc.)
• Contains: Flavin mononucleotide (FMN) + multiple iron-sulfur (Fe-S) centers
• Reaction: NADH + H+ + FMN → NAD+ + FMNH2, then electrons passed via Fe-S centers to CoQ
• Protons pumped: 4 H+ from matrix to intermembrane space per NADH
• Inhibited by: Rotenone, amobarbital, metformin (at high doses)

Complex II - Succinate Dehydrogenase (Succinate:CoQ Oxidoreductase)

• Accepts electrons from: FADH2 (produced when succinate is oxidized to fumarate in the TCA cycle)
• Contains: FAD + Fe-S centers
• Reaction: Succinate → Fumarate; electrons flow FADH2 → Fe-S → CoQ
• Protons pumped: 0 - no H+ are pumped at Complex II because the energy released is insufficient
• Inhibited by: Malonate (competitive inhibitor of succinate dehydrogenase)
• Clinical note: SDHB/SDHC/SDHD mutations are associated with hereditary paragangliomas/pheochromocytomas

Coenzyme Q (CoQ / Ubiquinone)

• A lipid-soluble quinone with a long isoprenoid tail - not a fixed protein complex
• Freely diffuses within the lipid bilayer of the inner mitochondrial membrane
• Accepts electrons from Complex I, Complex II, and other mitochondrial dehydrogenases (glycerol-3-phosphate dehydrogenase, acyl-CoA dehydrogenase in beta-oxidation)
• Transfers electrons to Complex III
• Functions as the electron hub linking all flavoprotein dehydrogenases to the cytochromes

Complex III - Cytochrome bc1 Complex (CoQ:Cytochrome c Oxidoreductase)

• Accepts electrons from: Reduced CoQ (CoQH2 / ubiquinol)
• Contains: Cytochrome b, Fe-S center (Rieske protein), Cytochrome c1
• Mechanism: The Q cycle - two electrons from CoQH2 split; one goes forward to cytochrome c1, the other recycles back to oxidize another CoQH2
• Protons pumped: 4 H+ per pair of electrons (via the Q cycle mechanism)
• Inhibited by: Antimycin A, myxothiazol

Cytochrome c

• A small, water-soluble protein located in the intermembrane space, loosely bound to the outer face of the inner membrane
• Mobile carrier that transfers electrons one at a time from Complex III to Complex IV
• Contains a single heme group with Fe3+/Fe2+ cycling
• Clinically important: cytochrome c release from mitochondria into the cytoplasm triggers apoptosis (activates caspase-9 via the apoptosome)

Complex IV - Cytochrome c Oxidase (Cytochrome a + a3)

• Accepts electrons from: Cytochrome c
• Contains: CuA, Cytochrome a, CuB, Cytochrome a3; heme groups with iron
• Reaction: 4 Cyt c (reduced) + O2 + 4 H+ → 4 Cyt c (oxidized) + 2 H2O
• Protons pumped: 2 H+ per pair of electrons
• This is the only site in the ETC where O2 is reduced to H2O - hence the name "cytochrome c oxidase"
• Inhibited by: Cyanide (CN-), carbon monoxide (CO), azide, hydrogen sulfide (H2S) - all bind to Fe in cytochrome a3, blocking electron transfer to O2

4. The Proton Gradient and Chemiosmosis

1. Chemical gradient (ΔpH): Higher H+ concentration in intermembrane space than matrix
2. Electrical gradient (Δψ, membrane potential): Positive charge on the cytosolic side, negative on the matrix side

5. Complex V - ATP Synthase (F0F1-ATPase)

• H+ flows down its electrochemical gradient through the F0 c-ring, causing it to rotate
• The c-ring rotation is coupled to the γ subunit ("rotor"), which rotates inside the α3β3 hexamer
• This rotation changes the conformation of the three β subunits sequentially through three states: open (empty) → loose (ADP+Pi bound) → tight (ATP formed)
• One full rotation (driven by ~3 H+ in vertebrates) releases 1 ATP from the tight site

6. ATP Yield (P/O Ratio)

• Only ~30% of the energy available from NADH and FADH2 oxidation is captured as ATP
• The remainder is released as heat (making the ETC our primary source of body heat)
• The overall ΔG0' for NADH oxidation by O2 is approximately -53 kcal/mol

7. Coupling: Respiratory Control

• When ATP is consumed → ADP rises → protons flow back through ATP synthase → proton gradient decreases → ETC accelerates to restore Δp
• When ATP demand is low → gradient builds up → "proton back pressure" slows electron transport and O2 consumption

8. Cytoplasmic NADH and the Shuttle Systems

Malate-Aspartate Shuttle

• Used in liver, heart, kidney
• Cytoplasmic NADH reduces oxaloacetate → malate; malate enters the matrix → regenerates NADH in the matrix
• Yields 2.5 ATP per cytoplasmic NADH (same as mitochondrial NADH)

Glycerol-3-Phosphate Shuttle

• Used in skeletal muscle and brain
• Cytoplasmic NADH reduces DHAP → glycerol-3-phosphate; G3P is oxidized by FAD-linked dehydrogenase → enters ETC at CoQ
• Yields 1.5 ATP per cytoplasmic NADH (less efficient)

9. Inhibitors of the ETC - Key Pharmacology

10. Uncouplers

Physiologic Uncouplers

• UCP1 (Thermogenin): Found in brown adipose tissue (BAT); forms H+ channels in the inner membrane; activated by cold via catecholamines; drives non-shivering thermogenesis. In neonates, ~90% of respiratory energy in brown fat is used for heat generation.

Chemical Uncouplers

• 2,4-Dinitrophenol (DNP): A lipophilic ionophore that shuttles H+ across the inner membrane; was historically used for weight loss but is lethally toxic
• Aspirin/salicylates (overdose): Uncouple OXPHOS at toxic doses, explaining the fever in salicylate poisoning

11. Reactive Oxygen Species (ROS)

• Superoxide (O2-): Primary radical; formed by single-electron leakage to O2
• Hydrogen peroxide (H2O2): Formed by dismutation of O2-
• Hydroxyl radical (·OH): Most reactive; formed from H2O2 via Fenton reaction

• Superoxide dismutase (SOD): O2- → H2O2
• Catalase: H2O2 → H2O + O2
• Glutathione peroxidase: H2O2 + 2 GSH → 2 H2O + GSSG

12. Clinical Relevance

Ischemia/Hypoxia

• Loss of O2 → electron transport stops → no Δp → no ATP → inner membrane becomes permeable → mitochondrial swelling → irreversible cell injury and necrosis

Cyanide Poisoning

• Inhibits Complex IV; tissues cannot use O2 despite normal delivery
• Presents as "histotoxic hypoxia" - venous blood is bright red (O2 not consumed)
• Treatment: hydroxocobalamin or nitrites (generate methemoglobin which competes with cytochrome a3 for CN-)

OXPHOS Diseases (Mitochondrial Diseases)

• Caused by mutations in mitochondrial DNA (mtDNA) or nuclear DNA encoding ETC subunits
• mtDNA is maternally inherited and encodes 13 ETC/ATP synthase subunits, 22 tRNAs, 2 rRNAs
• Examples: MELAS (mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes), MERRF, Leigh syndrome, Leber hereditary optic neuropathy (LHON)
• Iron deficiency anemia reduces Fe availability for Fe-S centers and cytochromes → impairs ETC → fatigue

Doxorubicin Cardiotoxicity

• The anthracycline doxorubicin binds cardiolipin in the inner mitochondrial membrane; inhibits succinate oxidation, inactivates cytochrome oxidase, interacts with CoQ, inhibits ATP synthase, and increases ROS production - leading to cumulative dose-dependent cardiomyopathy

13. Summary Flow Diagram

Glucose/Fatty Acids/Amino Acids
          ↓
    TCA Cycle / β-oxidation
          ↓
     NADH + FADH2
          ↓
  Complex I ──────────────────────► 4H+ pumped
  (NADH dehydrogenase)
          ↓
       CoQ (ubiquinone)  ←── Complex II (no H+ pumped)
          ↓
  Complex III ─────────────────────► 4H+ pumped
  (cytochrome bc1)
          ↓
      Cytochrome c
          ↓
  Complex IV ──────────────────────► 2H+ pumped
  (cytochrome c oxidase)
          ↓
    O2 + 4H+ → 2H2O

H+ gradient (Δp) drives ATP Synthase (Complex V)
ADP + Pi ──ATP Synthase──► ATP
        (~2.5 ATP per NADH; ~1.5 ATP per FADH2)