Lecture 18 - Electron Transport & Oxidative Phosphorylation

Question 1

What are the 2 ways Electrons are transferred?

Answer 1

1. Through series of oxidation and reduction reactions in the cell
   - reactions can occur in different cellular compartments
2. By specific carriers
   - carry electrons from redox reaction in one location to redox reaction in another
   - NAD+
   - FAD
   - Coenzyme Q

Question 2

Examples of Electron Carriers

Answer 2

NAD+ and NADP
- water soluble electron carriers that transfer two electrons as a hydride ion (H-)
FMN and FAD
- strongly bound to proteins
- can transfer one or two electrons

Question 3

What are the 3 other types of Electron carriers?

Answer 3

These are all mostly bound to inner mitochondrial membrane
Ubiquinone - hydrophobic benzoquinone
Cytochromes - iron present in haem
Iron-sulphur proteins - iron not in haem but as a complex of sulphur and iron atoms

Question 4

Electron Transport Chain

Answer 4

Flow of electrons generated from oxidation of carbs, fatty acids and proteins finally converge and carried by NADH and FADH2 to reduce oxygen to H2O
Occurs in inner mitochondrial membrane

Question 5

What are the steps of the Electron Transport Chain

Answer 5

• NADH from matrix reaches complex 1 NADH Dehydrogenase
• NADH is oxidised and electron passed to this complex
• proton (H+) is also transferred from NADH into intermembrane space
• electron transferred to ubiquinone
• complex 2 succinate dehydrogenase receives electron from FADH2 and passes it to ubiquinone
• ubiquinone transfers electrons to complex 3 Cytochrome C ubiquinone oxidoreductase
• protons moved from matrix to intermembrane space
• complex 3 transfers electrons to Cytochrome C
• Cytochrome C transfers electrons to complex 4 Cytochrome oxidase
• electrons transferred to terminal electron acceptor O2
• this coupled with protons present makes water

Question 6

Reduction Potentials

Answer 6

Way of tracking number of electrons stored or transferred
Determines free energy of a redox reaction
Measured in Volts

Question 7

Chemiosmotic Hypothesis

Answer 7

Explains how electron transfer and oxidation is coupled to ATP synthesis
Phosphorylation of ADP to ATP is driven by electron transfer to oxygen - hence
‘oxidative phosphorylation’
Free energy of electron transport (exergonic) is coupled to endergonic flow of protons
This essentially conserves some free energy of oxidation of metabolites as a transmembrane electrochemical gradient
The transmembrane flow of protons down their concentration gradient through specific channels provides the free energy necessary for the synthesis of ATP

Question 8

Exergonic and Endergonic aspects of the Electron Transport Chain

Answer 8

Exergonic - e- flow along chain

Endergonic - H+ transport matrix to intermembrane space

Question 9

Uncoupling

Answer 9

Some proteins and chemicals can uncouple electron transport and ATP synthesis
- allows protons to leak through membrane and bypass ATP synthetase protein
E.g. FCCP
- removes chemical H+ component of proton motive force
Valinomycin
- removes electrical component of proton motive force

Question 10

Regulation of ATP Production

Answer 10

High [ATP] decreases
- glycolysis 
- acetyl CoA oxidation via TCA 
- oxidative phosphorylation
High [NADH]/[NAD+]
- inhibits pyruvate dehydrogenase 
Low [ATP] increases
- glycolysis
- acetyl CoA oxidation via TCA
- oxidative phosphorylation