Lecture 11: Electron Transport Chain (ETC)

Main function of ETC?

Extract electrons from NADH & FADH2 → transfer them to O2 → generates proton gradient → power ATP synthesis

Where is the ETC located?

Inner mitochondrial membrane; proton pumped from matrix to intermembrane space 

Why is ETC a multistep process?

Electron carriers increase in affinity stepwise → allows controlled transfer proton pumping, and less ROS

ΔG° of NADH → O₂ transfer?

-219 kJ/mol (NADH); -164 kJ/mol (FADH2) — favorable

Direction of electron flow?

NADH/FADH2 → FMN/Fe-S → CoQ → Cyt C → O2

Electron source and flow in complex I? 

NADH donates 2e- → FMN → Fe-S clusters → CoQ → QH2

How many protons are pumped at complex I?

4 H+

Key prosthetic groups in complex I?

FMN (2e-) and Fe-S cluster (1e- tansfers)

Electron source for Complex II? 

FADH2 from succinate dehydrogenase reaction 

Protons pumped at Complex II?

0 H+ (why FADH2 yields less ATP)

Electron source for complex III?

QH2 generated from Complex I and II

Mobile electron carrier produced? 

Cytochrome C (1e- carrier) 

What is the Q cycle?

2-cycle mechanism splitting QH2 electrons: 1 goes to Cyt, 1 regenerates QH2; pumps total of 4 H+

Final electron acceptor (in complex IV)?

O2 → reduced to H2O

How many electrons needed to reduce O2? 

4 electrons from 4 Cyt C

Protons pumped at Complex IV?

2H+

Why stepwise electron transfer?

Prevent ROS damage and control radical intermediates

What are respirasomes? 

Supercomplexes of ETC for efficient electron transfer and assembly of complexes 

2 shuttles for cytosolic NADH?

Malate-Aspartate Shuttle → keeps NADH identity. Glycerophosphate Shuttle → NADH → FADH2

Total H+ pumped per NADH?

10 H+ (Complex I: 4, III: 4, IV: 2)

Total H+ pumped per FADH2? 

6H+ (Complex III: 4, IV: 2) 

How does proton gradient make ATP?

Proton flow through ATP synthase induces conformational change → ATP synthesis

4 ETC complexes:

NADH enters at I, FADH2 at II

Proton pumping: 

I (4), III (4), IV (2) 

Key mobile carriers

QH2 and Cyt C

— affect total ATP yield.

Shuttles

Stepwise transfer prevents — buildup. 

ROS