Lec17/18 - Oxidative Phosphorylation Flashcards
What actually is oxidative phosphorylation?
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
What are the two main stages of oxidative phosphorylation?
- Electron Transport Chain - oxidation energy used to transport protons and establish a H+ gradient
- ATP synthase uses the free energy of the proton gradient to produce ATP
What is the final fate of the electrons in the Electron Transport Chain?
They combine with O2 to form water
Where in the cell is the electron transport chain?
In the inner membrane of the mitochondria
Name the 5 complexes in the ETC; state the role of the first 4, and the last 1
- NADH-ubiquinone oxidoreductase
- Succinate-ubiquinone oxidoreductase
- Ubiquinol-cytochrome c reductase
- Cytochrome c oxidase
- ATP synthase
First 4 = electron transport
5 = ATP synthesis
Describe how ubiquinone can be reduced
Ubiquinone + e- -> Semiquinone (free radical)
Semiquinone + 2H+ + e- -> UbiquinOL (fully reduced form)
Describe what happens at Complex I of the ETC (in terms of electrons)
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+
Describe what happens at Complex I of the ETC (in terms of protons)
Complex I transfers 4H+ from matrix to intermembrane space PER 2e- transferred, and captures 2H+ from matrix to form QH2
Describe what happens at Complex II of the ETC
Complex II (Succinate-Ubiquinol Oxidoreductase) does not translocate protons, but supplies electrons from succinate via FADH2 to ubiquinone, reducing it to ubiquinol (QH2)
What does Ubiquinol (QH2) do after being reduced at complexes I and II?
It diffuses freely in the membrane to reach complex III for the next step
Describe what happens at Complex III of the ETC
Electrons are transferred from QH2 to two molecules of Cytochrome C
Meanwhile, 4H+ are translocated (two from matrix, two from QH2)
What is Cytochrome C and what is its role in the ETC
It is a small, soluble, alpha-helical haem protein which acts as the electron carrier between complexes III and IV
Describe how QH2 (ubiquinol) can be oxidised
It is oxidised 1e- at a time, 2e- transferred to CytC via Complex III; 2H+ transferred to intermembrane space via Complex III
Describe what happens at Complex IV of the ETC
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)
Approximately how many H+ are needed to synthesise each ATP on average
3H+
Name the two parts of the Complex V (ATP Synthase) structure
F0 = stalk (proton channel)
F1 = knob (catalytic subunits)
Describe the structure of the F1 component of Complex V (ATP Synthase) in the ETC
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)
What is the role of each subunit in F1 of Complex V?
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
State the three possible conformations for each beta subunit in Complex V
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
Summarise in a sentence how Complex V synthesises ATP
Flow of H+ drives F0/ye rotation; forced cyclical conformational changes in ß-subunits are what actually catalyse ATP synthesis
Describe how ATP, ADP and Pi cross the inner mitochondrial membrane
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)
In total, how many H+ must cross the mitochondrial inner membrane per ATP synthesised?
4 (3 translocated by ATP Synthase; 1 needed for Pi symport)
What is the P:O ratio?
Molecules of ADP phosphorylated to ATP / atoms of Oxygen reduced
What are the respective P:O ratios for NADH and FADH2?
NADH: 10H+ transported, P/O = 2.5ATP/O
FADH2: 6H+ transported, P/O = 1.5 ATP/O
