Lec17/18 - Oxidative Phosphorylation

Question 1

What actually is oxidative phosphorylation?

Answer 1

A pathway linked to the Citric Acid Cycle that requires aerobic conditions, in which electrons are transferred from NADH and FADH2 to O2 and ATP is formed

Question 2

What are the two main stages of oxidative phosphorylation?

Answer 2

1. Electron Transport Chain - oxidation energy used to transport protons and establish a H+ gradient
2. ATP synthase uses the free energy of the proton gradient to produce ATP

Question 3

What is the final fate of the electrons in the Electron Transport Chain?

Answer 3

They combine with O2 to form water

Question 4

Where in the cell is the electron transport chain?

Answer 4

In the inner membrane of the mitochondria

Question 5

Name the 5 complexes in the ETC; state the role of the first 4, and the last 1

Answer 5

1. NADH-ubiquinone oxidoreductase
2. Succinate-ubiquinone oxidoreductase
3. Ubiquinol-cytochrome c reductase
4. Cytochrome c oxidase
5. ATP synthase

First 4 = electron transport
5 = ATP synthesis

Question 6

Describe how ubiquinone can be reduced

Answer 6

Ubiquinone + e- -> Semiquinone (free radical)

Semiquinone + 2H+ + e- -> UbiquinOL (fully reduced form)

Question 7

Describe what happens at Complex I of the ETC (in terms of electrons)

Answer 7

Electrons from NADH enter the respiratory chain 2-at-a-time via Complex I, which then transfers 2e- to ubiquinone, reducing it to ubiquinol (QH2)

NADH is oxidised to NAD+

Question 8

Describe what happens at Complex I of the ETC (in terms of protons)

Answer 8

Complex I transfers 4H+ from matrix to intermembrane space PER 2e- transferred, and captures 2H+ from matrix to form QH2

Question 9

Describe what happens at Complex II of the ETC

Answer 9

Complex II (Succinate-Ubiquinol Oxidoreductase) does not translocate protons, but supplies electrons from succinate via FADH2 to ubiquinone, reducing it to ubiquinol (QH2)

Question 10

What does Ubiquinol (QH2) do after being reduced at complexes I and II?

Answer 10

It diffuses freely in the membrane to reach complex III for the next step

Question 11

Describe what happens at Complex III of the ETC

Answer 11

Electrons are transferred from QH2 to two molecules of Cytochrome C

Meanwhile, 4H+ are translocated (two from matrix, two from QH2)

Question 12

What is Cytochrome C and what is its role in the ETC

Answer 12

It is a small, soluble, alpha-helical haem protein which acts as the electron carrier between complexes III and IV

Question 13

Describe how QH2 (ubiquinol) can be oxidised

Answer 13

It is oxidised 1e- at a time, 2e- transferred to CytC via Complex III; 2H+ transferred to intermembrane space via Complex III

Question 14

Describe what happens at Complex IV of the ETC

Answer 14

Complex IV/Cytochrome C Oxidase:
1. Receives electrons from CytC carrier, one at a time
2. Catalyses reduction of O2 to H2O (this free energy is used to translocate two more H+ into intermembrane space)

Question 15

Approximately how many H+ are needed to synthesise each ATP on average

Answer 15

3H+

Question 16

Name the two parts of the Complex V (ATP Synthase) structure

Answer 16

F0 = stalk (proton channel)
F1 = knob (catalytic subunits)

Question 17

Describe the structure of the F1 component of Complex V (ATP Synthase) in the ETC

Answer 17

3 alpha, 3 beta, 1 gamma, 1 delta, 1 epsilon subunit
(alpha and beta alternate like orange segments)
(gamma is main component of central axle, delta is in peripheral stalk)

Question 18

What is the role of each subunit in F1 of Complex V?

Answer 18

beta is for binding and catalysis
alpha is for structure only
delta locks the head in place, while gamma spins
epsilon connects axle to F0

Question 19

State the three possible conformations for each beta subunit in Complex V

Answer 19

Open state (O) = available to bind ADP + Pi
Loose state (L) = active site closes loosely on ADP + Pi
Tight state (T) = converts ADP + Pi into ATP

Question 20

Summarise in a sentence how Complex V synthesises ATP

Answer 20

Flow of H+ drives F0/ye rotation; forced cyclical conformational changes in ß-subunits are what actually catalyse ATP synthesis

Question 21

Describe how ATP, ADP and Pi cross the inner mitochondrial membrane

Answer 21

ATP/ADP cannot simply cross the membrane;

Adenine Nucleotide Translocase (ANT) allows coupled antiport exchange of ATP for ADP (ADP = IMS to Matrix, ATP = Matrix to IMS)

Pi enters through symport mechanism with H+ (both from IMS to matrix)

Question 22

In total, how many H+ must cross the mitochondrial inner membrane per ATP synthesised?

Answer 22

4 (3 translocated by ATP Synthase; 1 needed for Pi symport)

Question 23

What is the P:O ratio?

Answer 23

Molecules of ADP phosphorylated to ATP / atoms of Oxygen reduced

Question 24

What are the respective P:O ratios for NADH and FADH2?

Answer 24

NADH: 10H+ transported, P/O = 2.5ATP/O

FADH2: 6H+ transported, P/O = 1.5 ATP/O