Lecture 19 - Electron Transport 3 Flashcards
What is complex 2 in the citric acid cycle
Succinate Dehydrogenase
What does Complex II do
Direct physical link to citric acid cycle
Oxidises succinate to fumarate, coupled to FAD/FADH2
Electron pair then used to reduce Q via Fe-S and a haem
What is complex III called
Ubiquinone-Cytochrome c Oxidoreductase
What does Complex III do
Reduces cyt c, while translocating 4H+
Dimeric complex: 2 x 11 subunits
CoQ uses Q cycle to convert 2e- process into two 1e- transfers
For every 2 electrons that pass through complex III, how many protons are released across the membrane
4
What is Complex IV called
Cytochrome c Oxidase
What does complex IV do
Cyt c oxidation
Then e.t. through one monomer of the homodimer, culminating in O2 reduction to form H2O
Four H+ are involved in the complex IV reactions:
2 H+ translocated into intermembrane space 2 H+ used to form H2O
What are the types of cytochromes and how are they classified?
Types a b and c, according to the type of haem
Which type haem cytochrome has a long hydrophobic tail
Type a
How are cytochrome c haem groups linked to the protein
through thiol groups from Cystine residues
Picture on phone
Summary of mitochondrial electron transport system
Complexes I-IV all have TM regions plus functional domains protruding into matrix
Complexes III and IV also have functional domains protruding into intermembrane space to interact with cyt c
NADH oxidation starting with complex I results in translocation of 10 H+
CoQ and FADH2 oxidation starting with complex II results in translocation of 6 H+
CoQ and cyt c transport 1e- at a time, so must make two trips to transfer 2e- from NADH or FADH2 to ½ O2 to form H2O
What is the overall reaction in the electron transport chain
NADH + H+ + ½ O2 NAD+ + H2O
What are the bioenergetics of the proton motive force eg.
NADH + H+ + ½ O2 NAD+ + H2O
Eo’(NAD+/NADH) = –0.32 V Eo’(O2/H2O) = 0.82 V DGo’ = –nFDEo’ = –nF[Eo’(acceptor) – Eo’(donor)] DGo’ = –2(96.5 kJ mol-1 V-1) [0.82 V – (–0.32 V)] DGo’ = –220 kJ mol–1 (of NADH oxidised)
How do you determine the free energy in the proton gradient
DG = RT ln (c2/c1) + ZFDy
where (c2/c1) = concentration ratio for the ion that moves Z = absolute value of its charge F = Faraday (96.5 kJ mol-1 V-1) Dy = electrical potential difference across membrane (V)
In actively respiring mitochondria,
DpH is about 0.75 pH units Dy is about –0.15 V DG = –0.74 kJ mol-1 + –14.48 kJ mol-1 = –15.21 kJ mol-1
10 H+ are available from each NADH, so DG = –152.1 kJ mol-1 per NADH
Bioenergetics of proton motive force efficienvy
- Most of the free energy available from the H+ gradient (the proton-motive force) in mitochondria is derived from Dy (–14.48 kJ mol-1) rather than DpH (–0.74 kJ mol-1)
- Efficiency of energy conversion by the electron transport system is about 70%:
152 kJ mol-1 / 220 kJ mol-1 ≃ 0.7
Energy difference of about 70 kJ mol-1 is lost as heat and contributes to thermogenesis
NB DG (as opposed to DGo’) of NADH oxidation is likely to be more negative than –220 kJ mol-1
- Efficiency based on theoretical vs actual number of ATP molecules made:estimated DG for ATP synthesis in mitochondrial matrix is –40 kJ mol-1
Hence theoretical yield per NADH oxidized is 152 kJ mol-1 / 40 kJ mol-1 ≃ 3.8 ATP
Actual yield per NADH oxidised is about 2.5 ATP, so efficiency is 2.5 / 3.8 ≃ 66%
SLIDE 48 - STRUCTURAL ORGANISATION OF ATP SYNTHASE COMPLEX
Bacterial ATP synthase complex - c ring has 10-15 cylinders, yeast ATP synthase complex has 10
What is the catalytic part of the ATP synthase complex
Alpha sub units which convert ADP+Pi into ATP
What are the functional units of ATP synthase
Stator - : a subunit with half-channels for H+ to enter and exit FO, plus stabilizing arm (b, d, h and OSCP)
Rotor - : c + g + d + e rotate as H+ enter and exit c-ring
Headpiece - : hexameric a3b3 unit responsible for ATP synthesis
SLIDE 49
Yeast ATP synthase structure
a3b3 hexamer contains the three catalytic sites, on outer edge of each b subunit
g subunit extends inside a3b3 hexamer and is connected to the c ring
SLIDE 50
What are the 3 basic principles to binding change mechanism of ATP synthesis
. g directly contacts all three b subunits, but each interaction is distinct, giving rise to three different b conformations
- ATP binding affinities of the three b subunits are T, L and O (tight, loose and open)
T: ATP bound
L: ADP and Pi bound
O: ATP is released
- H+ flow through FO causes rotation of g subunit counterclockwise during ATP synthesis (looking at F1 from matrix side).
With each 120° rotation, b subunits switch conformation sequentially L T O L
What is the Experimental proof that ATP synthase rotates, and that it rotates
in reverse under conditions that favour ATP hydrolysis rather than ATP synthesis
Actin filament is on the same side as the membrane
Beta subunits are attached to a solid surface which is on the same side as the mitochondrial matrix
Clockwise rotation of the y subunit as viewed from the matrix side
The g subunit rotates in 120º steps.
The fluorescence micrographs below show the position of the fluorescent actin polymer at 133 ms intervals.
As the actin polymer rotates, it makes a discrete jump about every 11 frames.
How does the F1 component act as a nanomotor driving ATP synthesis
in absence of electrochemical proton gradient
Magnetic bead is own the same side s the membrane, using a streptavidin linker
SLIDE 55 and 56
