Electron Transport Chain

How is ATP production powered?

• Proton gradient → proton motive force across membrane (electrochemical gradient)

• ATP Synthase: use nrg stored in gradient to make ATP

Complex I (NADH Dehydrogenase)

• pumps 4 H into intermembrane space

• NADH → NAD+ (donates 2 e-) (cap: 2 e-, 1 H+)

• which is accepted by FMN (flavin mononucleotide) → FMNH2

• e- move through Fe S clusters (1 e- carrier cycling b/w Fe 2+ / Fe 3+) → NADH -→ FMN —> CoQ red to CoQH2

Coenzyme Q

• lipid soluable (bc long "fatty" isoprenoid tail) + mobile e- carrier

• interior of membrane + diffuses freely

• From Complex I: Receives 2 e- from NADH (via FMN + Fe-S clusters)

• From Complex II: Receives 2 e- from Succinate (via FAD + Fe-S clusters).

• Once Q is”full” (QH2): to Complex III: delivers e- to the Cyt-B (ox to Q)

Ubiquione (Q) Redox States

• Q (Ubiquinone): fully oxidized, "empty."

• Q- (Semiquinone): radical intermed. "half-full" (1 e-) + unstable—If it leaks e- to Oxygen, creates Superoxide (ROS)

• QH2 (Ubiquinol): fully reduced. "Full" (2 e- + 2 H)

Complex II-Succinate dehydrogenase

• Succinate → FAD: Succinate is oxidized to Fumarate + FAD is reduced to FADH2.

• FADH2 → Fe-S clusters: 3 different Fe-S clusters act as a wire to move e- toward the membrane.

• Fe-S clusters → Heme b: A heme group acts as an intermed (control e- flow to prevent ROS)

• Heme b → Q: Q picks up the 2 e- and 2 H from matrix to become QH2

• NO PROTON PUMP (skips proton pumping of Complex I, e- entering via Complex II (from FADH2) result in less ATP (1.5 ATP) vs. NADH (2.5 ATP)

• Smallest complex

• FAD in Complex II is covalently bound, FADH2 enters at lower nrg level

Complex III

• use Fe-S cluster transfer e- from CoQH2 to heme forming cyt c

• dimer in inner membrane: each monomer has a cyt b subunit lining an inner "cavern."

• The "cavern" allows 2 Q’s to bind simultaneously at distinct sites:

• Qo Site: (P-side/Intermembrane space):QH2 is oxidized (gives up e-)

• Qi Site: (N-side/Matrix):Q or Q- is reduced (accepts e-)

• 3 hemes (bH, bL + c1).

  • All protoporphyrin IX (same structures)

  • diff reduction potentials (Eo) bc of unique microenvironments, allowing a specific "downhill" path for e-

• Rieske 2Fe-2S Cluster: Fe-S protein essential for the "upstairs" path to Cyt c.

  • Coordinated to His instead of the Cys

  • Result: higher Eo (higher affinity for e-), allowing it to pull e- from QH2 effectively

• Cyt c: Last destination for e- in complex

• - mobile, water-soluble protein that stays on the P-side (intermembrane space)

• PUMPS 4 PROTONS

Complex IV: cytochrome c oxidase

• Role: uses cytochromes + Cu2+ to transfer e- in form of H- from cyt c → O2 → H2O

• contains: 2 heme groups (a + a3), 3 copper atoms (CuA/CuA + CuB)

• Specific E- Path:

  1. Cytochrome c binding: Occurs at the intermembrane space (P-side).

  2. Di-copper center (CuA): The first internal recipient.

  3. Heme a: Intermediate carrier

  4. Heme a3: final + leading to the oxygen-binding site

• Pump 4 protons physically moved to the intermembrane space.

• 4 protons removed from the matrix to form water.

• Net Result: 8 H cleared from the matrix (N-side)

• Lethal Inhibitors:

  • Cyanide (CN-) & Azide (N3-): Bind Fe3+ in heme a3, stopping e- transfer to O2.

  • Carbon Monoxide (CO): Competes with O2 for binding at Fe2+ in Heme a3

Complex IV mechanism

O2​ + 4cyt c(red)​ + 8H (matrix) + ​→ 2H2​O + 4cyt c(ox) +4 H+ (intermem space)

1. First 2 e- arrive (from 2 CytCred)

• 2 reduced cyt c each donate 1 e⁻ → total 2 e⁻

• e- move to:

  • 1 e⁻ → heme a₃ (Fe)

  • 1 e⁻ → CuB

✅ Fe–Cu center is now reduced and ready to bind O₂

2. O₂ binds b/w Fe (heme a₃) + CuB accepts 2 e- → forms a peroxide bridge (O₂²⁻)

3. Next 2 e- arrive (2 more CytCred): 2 e- further reduce the bound oxygen intermed (dangerous intermediate (like O₂•⁻ / peroxide) is fully held in place at the Fe–Cu center (never released)

4. 2 protons from the matrix (H⁺) are added: Breaks the O–O bond + forms Hemea3–OH and Cub –OH

5. Obtain 2 more protons from matrix + ox. Hemea3 + Cub into original state + release 2 H2O

Complex III transfer: THE Q CYCLE

• QH2​ + 2cyt c(oxidized)​ + 2H (n-matrix)+​→Q + 2 cyt c(reduced)​+ 4H + (p: ntermembranespace)

• QH2 carries 2 e-, but Cyt c can only accept 1

• FIRST HALF: First QH₂:

  • Releases 2 H⁺ into intermembrane space

  • 1 e⁻ → cyt c

  • 1 e⁻ → Q → forms semiquinone (Q•⁻) (held in the “cavern”)

  • Second QH₂:

    • Releases 2 more H⁺ into intermembrane space

    • 1 e⁻ → another cyt c

    • 1 e⁻ → reduces Q•⁻ → QH₂ (fully reduced) using 2 H⁺ from matrix

• Overall: Converts 2-e- carrier → 1-e transfers

• Produces 2 cyt c (each carrying 1 e⁻)

• Proton movement:

  • 4 H⁺ released into intermembrane space

  • 2 H⁺ taken up from matrix

• drives ATP synthesis

OVERALL EQUATIONS FOR ETC

2 overall paths for e- in ETC:

Complexes I,III,IV 2 NADH + O2 + 22H+(n) → 2 NAD+ + 2H2O + 20H+ (p side)

Complexes II,III,IV 2 succinate + O2 + 12H+(n) → 2 fumarate + 2H2O + 12H+ (p side)