Lecture 17 and 18 - Oxidative phosphorylation

Question 1

Electron transport chain complexes

Answer 1

Complexes I-IV - electron transport chain:

I - NADH-ubiquinone oxidoreductase
II - Succinate-ubiquinone oxidoreductase
III - Ubiquinol-cytochrome C reductase
IV - Cytochrome C oxidase

Question 2

ATP synthase complex

Answer 2

Complexes V - ATP synthase

Question 3

Which electron transporters pass electrons to which complex

Answer 3

NADH - complex I
FADH₂ - complex II

Question 4

Electron transport complex I: what is it, how big is it, and what does it do?

Answer 4

NADH-ubiquinone oxidoreductase (NADH dehydrogenase)

Largest complex - ~40 subunits

Oxidises 2NADH into 2NAD⁺, reduces Q into QH₂, and translocates 4H⁺ into the intermembrane space per 2e⁻ passed into the chain

Question 5

Electron transport complex II: what is it, what are its key features, and what does it do?

Answer 5

Succinate-ubiquinone oxidoreductase (succinate dehydrogenase)

• Only enzyme common to both the citric acid cycle and respiratory chain
• Does not contribute to proton gradient (the protons from FADH₂ are given to Q to form QH₂)

Oxidises FADH₂ into FAD⁺

Question 6

Electron transport complex III: what is it and what does it do?

Answer 6

Ubiquinol-cytochrome C oxidoreductase (cytochrome C reductase)

Transfers electrons from ubiquinol to cytochrome C and causes 4H⁺ to be translocated (2 from QH₂ and 2H⁺ from the matrix)

Question 7

Electron transport complex IV: what is it, where are its electrons obtained from, how are the electrons moved and what does it do?

Answer 7

Cytochrome C oxidase

Receives electrons from Cytochrome C carrier, one at a time

Contains iron atoms (in haem groups) and copper
atoms which are both reduced and oxidized as
electrons flow to oxygen

Catalyzes the reduction of O₂ to 2H₂O (using 4H⁺ captured from the matrix) and translocates two more H⁺

Question 8

ATP synthesis complex V: what is its structure and what does it do?

Answer 8

Composed of a “knob-and-stalk” structure
F₁ (knob) - catalytic subunits
F₀ (stalk) - proton channel

Uses, on average, 3H⁺ to passively produce/synthesise of ATP

Question 9

Cytochrome C: what is it, what is its structure, and what are its key features?

Answer 9

The electron carrier between electron transport complexes III-V

α-helical haem-protein with an iron atom (Fe²⁺/Fe³⁺) in the centre (which isn’t used to bind oxygen)

Small, highly soluble protein

Question 10

The F₁ subunit of the ATP synthase complex: what subunits does it have and what is each subunit’s role?

Answer 10

F₁ has 3α, 3β, 1γ, 1δ, and 1ε subunits

α and β alternate
γ is the main component of the central axle
ε is connected to the γ subunit
δ is in the peripheral stalk
Each β subunit has an active site for ATP synthesis

Question 11

β subunits of the ATP synthase complex: what three different conformations can they take and how can they form ATP?

Answer 11

Open (O) - Available to bind to ADP + Pᵢ
Loose (L) - Active site closes loosely on ADP + Pᵢ
Tight (T) - Converts ADP + Pᵢ into ATP

The flow of protons drives F₀/γε rotation and forces cyclic conformational changes into each β subunit

Question 12

Transport of ATP, ADP and Pᵢ across the inner mitochondrial membrane: how does transport occur?

Answer 12

ATP and ADP cannot cross the inner mitochondrial membrane usually

Adenine nucleotide translocase (ANT): coupled exchange of ATP for ADP (antiport)
Pᵢ enters through a symport mechanism with H⁺

Question 13

Transport of ATP, ADP and Pᵢ across the inner mitochondrial membrane: how many protons are used and how many ATP molecules are synthesised per NADH/FADH₂?

Answer 13

3H⁺ is used by ATP synthase for each ATP produced and one H⁺ is needed for the transport of Pᵢ across the inner mitochondrial membrane so overall: ~4 H⁺ per ATP is synthesized

NADH = 10 H⁺ transported, P/O = 2.5 ATP/O
FADH₂ = 6 H⁺ transported, P/O = 1.5 ATP/O

P = ADP phosphorylation
O = oxygen reduced