Electron Transport Chain Flashcards
What provides more energy than fermentation?
Oxidation of glucose or FA
Oxygen is the terminal acceptor for what? Why?
- Electrons carried by NADH and FADH2
- You want a substance with the lowest reducing potential
High energy electrons will end up as what?
FADH2 or NADH
Name 3 properties of O2 that favour energy extraction
- O2 abundant (~21% of air)
- O2 diffuses through membranes
- O2 very reactive and easily accepts e-
Why did cells evolve detoxification enzymes? Name some.
- Oxygen is reactive and makes toxic intermediates, can modify DNA, lipids and proteins, etc.
- Catalase, superoxide dismutase
What is brown fat in babies?
Babies have a larger surface mass > volume
Higher concentration of mitochondria
- Catalyzes a futile cycle - generates hydrolyzed ATP to generate heat (thermoregulation)
Why can oxygen diffuse through membranes?
Since it is nonpolar
Describe what happens generally in the ETC.
- high energy in electrons extracted for carbon sources (glucose, fatty acid) on NADH and FADH2
- transported to oxygen via the transportation down an electron chain
What is generated through the ETC?
Electrical energy is converted to chemical energy
How is the ETC organized? Where?
As a chain on the inner mitochondrial membrane
How does the reduction potential change down the ETC?
Down the chain = decrease in reduction potential
What is the ΔE′o for transfer of electrons from NADH to oxygen ?
+1.14 V
Where does the ETC pump protons to? What does it generate?
- From matrix into inter membrane space
- To create a proton gradient (acts like a battery)
How many molecules of ATP does 2 electrons generate?
2e- generates 5 molecules of ATP
What stops protons from moving out of their porous space?
Because they’re charged (+)
Donnan effect, they want to go to equilibrium (do not diffuse out of mitochondria)
Name the 4 complexes of the ETC.
1) NADH dehydrogenase
2) Succinate dehydrogenase
3) Cytochrome b1 complex
4) Cytochrome oxidase
Where are the complexes of the ETC located?
Located on the inner mitochondrial membrane?
Where is NADH located?
In mitochondrial matrix
Are the complexes of the ETC in contact?
Not intimately in contact, you need mechanisms that’ll allow the movement of electrons
Name 3 key components that are used to carry electrons through the ETC.
Quinones, iron-sulfur proteins, cytochromes
What is another name for quinones?
Coenzyme-2
What is the structure of a quinone?
Aromatic ring + 2 carbonyl + isoprenoid tail
How is a quinone anchored to the membrane?
Via the isoprenoid tail
What is a quinone?
electron carrier that is anchored to
the inner membrane
Where is the isoprenoid tail of a quinone found?
found on the INNER membrane of mitochondria
How does a quinone act in the membrane? Where does it sit?
mobile in the membrane and thought to sit in
the hydrophobic core
A quinone can transfer electrons electrons from which complexes? How?
- Complex 1 to 3
- Complex 2 to 3
- Can flow in the membrane between 2 complexes
Quinones accept electrons from what?
NADH or FADH2
Quinones can undergo a 2-step reduction, what does that do?
Allows quinones to carry either:
1) Dihydroquinone: carry 2e- and 2p+
2) Semiquinone: carry 1e- and 1p+
They are happy either way
What is the structure of an iron-sulfur center?
prosthetic groups containing 2, 3 , 4 or 8 iron atoms
How is an iron-sulfur center bound to a protein?
via chelation with cysteine residues
What is the structure of an iron-sulfur center with 4 Fe?
tetrahedral structure that resemble a cube.
How many electrons can an iron-sulfur centre transfer?
one electron at a time
Electron transfers in iron-sulfur proteins are coupled to what?
Red-ox reaction
2Fe3+ + 1e-↔ Fe3+ Fe2+
What is the structure of cytochromes?
Proteins that have a Heme (prosthetic) group at their centre that is covalently attached
What does Heme contain? What is it chelated by?
Contains Fe3+ that is chelated by 4 N groups
What will iron flip between?
Between Fe3+ and Fe2+
How many electrons can cytochromes transfer? How?
- 1 electron at a time using the single iron center
- Fe3+ + 1e- -> Fe2+
Name 2 ways that the three cytochromes differ.
1) The types of substituents attached to the porphyrin ring
2) The reducing potential influenced by the backbone protein
How is the heme group in cytochrome C attached?
Covalently to the protein backbone via cysteine residues
Where does NADH enter the ETC chain?
At the tip of the boot (complex 1)
The energy recovered from electrons carried by NADH depends on what?
Complexes 1,3,4
How do electrons pass from one complex to another in the ETC chain?
Using the Q cycle and soluble cytochrome c proteins
Where is cytochrome C put?
Inner membrane space
How many protons (H+) are pumped into the intermembrane space in complex 1?
Four H+
From matrix into the intermembrane space
What happens during ETC complex 1?
-electrons from NADH → FMN forming FMNH2
-electrons FMNH2 → 2 Fe-S cluster 1 electron
at a time
-electrons from 2 Fe-S → UQ (coenzyme Q)
Can cytochromes carry protons? What happens?
NO, can carry electrons but not protons
SO protons are pumped into the intermembrane space
How many molecules of cytochrome c are required for each UQH2?
Two molecules
How many electrons does the conversion of ubiquinone to dihydroquinone are required to electrons reduction of Fe3+ to Fe2+?
1 electron
What catalyzes the oxidation of UQH2? How?
- CoQH2-cytochrome c oxidoreductase
- by passing electrons to cytochrome c (intermembrane space)
What is the Q cycle&
Electrons come in the form of dihydroquinone, passes electrons one at a time to cytochrome
- Q cycle bcos cytochrome c can only carry 1 electron at a time, but dihydroquinone has 2 electrons
How many protons does complex III pump out for every 2 electrons into the intermembrane space?
4 protons for every 2 electrons
What catalyzes the transfer of electrons from cytochrome c to oxygen?
Cytochrome c oxidase
What is required for complex IV?
4 cytochrome c (4 electrons) + 4 protons
For every 4 reduced cytochrome c during complex IV, how many protons are pumped out?
4 protons
How many protons would be pumped out for two molecules of NADH?
2 NADH = pumps out 20 protons
The electrons from the coenzyme FADH2 are fed into the ETC through what?
- Complex II
- Also uses the Q cycle
What enzyme is primarily contained in complex II? What is used to move electrons?
- Succinate dehydrogenase
- 2 Fe-S proteins to move elctrons
How does complex II capture FADH2 electrons?
Generated by oxidation of succinate to fumarate?
Where are the electrons from complex II passed to?
Passed to coenzyme Q which transfer electrons to complex III
Where is the succinate dehydrogenase inserted in complex II?
Inserted into the inner mitochondrial membrane leaflet facing the matrix
How many molecules of ATP do NADH and FADH2 produce for every electron pair transferred to oxygen?
NADH: 2.5 molecules
FADH2: 1.5 molecules
Name inhibitors of the ETC.
Rotenone, antimyacin, CN-, N3-, CO, MPP+
How does rotenone inhibit the ETC?
Binds to complex 1 and blocks flow of electrons through ETC –> cells now run under anaerobic conditions
How does antimyacin inhibit the ETC?
Blocks cytochrome b to cytochrome c SO complex 1 still functions
- Stops and goes back up
- Electrons can’t go down the chain, diffuses out of the membrane and binds with O –> forms dangerous species
How do CN-, N3- and CO inhibit the ETC?
React with cytochrome a and a3, go after metal centers and block the transmissions of electrons
How does MPP+ inhibit ETC?
- Attacks complex 1 but only goes after the substantia niagra (in brain)
- Cases of synthetic heroin have caused this
- Monoamine oxidase -> MPTP, which binds to complex 1
- Is said to be linked to Parkinson’s disease.
What hypothesis is behind the energy release by the movement of electron down the electron transport chain generates used to drive the synthesis of ATP ?
chemiosmotic hypothesis
What is the first principle of the chemiosmotic hypothesis?
Electron transport chains are organized in the inner mitochondrial membrane and movement of electrons down the chain and final donation to oxygen and the extrusion of proteins into the intermembrane space
What is the second principle of the chemiosmotic hypothesis?
Movement of electrons generates a an electrical potential (ψ) and proton gradient
(ΔpH) across the inner membrane (this electrochemical gradient is also known as the
proton motive force
What is the third principle of the chemiosmotic hypothesis?
The flow of protons through the ATP synthase into the mitochondrial matrix drives synthesis of ATP
Define the proton motive force.
The proton motive force (Δp) is the sum of the electrical and chemical components
of the transmembrane proton gradient.
What is the equation for proton gradient?
ΔpH = pHout - pHin
What is the equation for electrical gradient?
Δψ = ψout – ψin
The proton motive force across the inner mitochondrial membrane is described by which equation?
Δp = Δψ - zΔpH
z = conversion factor 61.8 mV at 37 0C)
What evidence has been proved to support the chemiosmotic theory?
1) Substrate oxidation by respiring mitochondria or bacteria (in the presence of oxygen) expel protons and drop pH in intermembrane space (Δ pH ~ 0.75 units)
2) ATP synthesis is halted by disruption of inner membrane integrity
3) Uncouplers (small molecules ie dinitrophenol or gramacidin) that dissipate proton gradient stop ATP synthesis
Electron micrographs showed that the
Matrix side of the inner mitochondrial
membrane was loaded with lollipop like
protein structures. What are these structures?
ATP synthase
Name the 2 major components of ATP synthase.
1) F1
2) F0
What does F1 in ATP synthase contain? What does it do? How many subunits and nucleotide binding sites?
- ATP synthesis activity
- (5 subunits 3α , 3β, γ, δ, ε)
- Has three nucleotide binding sites
What does F0 contain? How many subunits?
- Transmembrane channel for protons
- 3 subunits (a, 2b, 12c)
How does ATP synthase resemble an electrical motor?
- rotor (rotating component)
- stator (stationary component)
What is the rotation of the motor driven by in ATP synthase?
Proton motive force (Δp)
The synthesis of 1 ATP by synthase requires how many H+? What is it needed for?
- 3 H+ (for each 120o rotation)
- 1H+ to drive the H2PO4- import into the matrix
Which way does the c ring rotate for ATP synthesis? For ATP hydrolysis?
ATP synthesis: counterclockwise
ATP hydrolysis: clockwise
If you pair two molecules with different reduction potentials, which will be reduced and which will be oxidized?
The molecule with the higher potential will be reduced, while the other molecule becomes oxidized
What are the electron donors and acceptors in the ETC?
Donor: NADH
Acceptor: Oxygen
What is the net effect of Complex I?
Passing high-energy electrons from NADH to CoQ to form CoQH2
Describe the steps in Complex I.
1) NADH transfers its electrons over to FMN, thereby becoming oxidized to NAD+ as FMN is reduced to FMNH2
2) Flavoprotein becomes reoxidized, while the iron-sulfur subunit is reduced
3) The reduced iron-sulfur subunit donates the electrons it received from FMNH2 to coenzyme Q, which becomes CoQH2
- Four protons are moved to the intermembrane space
- NADH + H+ + CoQ -> NAD+ + CoQH2
What does complex II obtain its electrons from?
Succinate
What is succinate oxidized to when it interacts with FAD?
Fumarate
Since FAD is covalently bonded to complex II, what is it converted to when succinate is oxidized?
FADH2
Describe the steps in Complex II.
1) Succinate is oxidized to fumarate, and FAD bound covalently to complex II is converted to FADH2
2) FADH2 gets reoxidized to FAD as it reduces an iron-sulfur protein
3) Iron-sulfur protein reoxidizes as CoQ is reduced
- 0 protons are moved to the intermembrane space
- succinate + CoQ + 2H+ -> fumarate + CoQH2
What is the net effect of Complex II?
Passing high-energy electrons from succinate to CoQ to form CoQH2
What is the net effect of Complex III?
Facilitates the transfer of electrons from CoQ to cytochrome c.
What happens in the Q cycle?
Two molecules are shuttled from a molecule of ubiquinol (CoQH2) near the intermembrane space to a molecule of uniquinone (CoQ) near the mitochondrial matrix
What is the net effect of Complex IV?
Transfer of electrons from cytochrome c to oxygen, the final electron acceptor
What happens in Complex IV?
Through a series of redox reactions, cytochrome oxidase gets oxidized as oxygen becomes reduced and forms water
Name two changes that contribute to the electro-chemical gradient when (H+) increases in the intermembrane space.
1) pH drops in the intermembrane space
2) Voltage difference between the intermembrane space and matrix increases due to proton pumping
Why is there a variability in the net ATP yield per glucose (30-32)?
Because efficiency of aerobic respiration varies between cells caused by the fact that cytosolic NADH formed through glycolysis cannot directly cross into the mitochondrial matrix (needs shuttle mechanisms)
What does a shuttle mechanism do?
Transfers the high-energy electrons of NADH to a carrier that can cross the inner mitochondrial membrane.
How much ATP is produced from the shuttle mechanisms?
either 1.5 or 2.5 ATP depending on which mechanisms NADH participates in
How many molecules of ATP does the glycerol-3-phosphate shuttle generate? Where does it transfer its electrons to?
1.5 ATP for every molecule of cytosolic NADH
Complex III
How many molecules of ATP does the malate-aspartate shuttle generate? Where does it transfer its electrons to?
2.5 ATP for every molecule of cytosolic NADH
Complex I
Describe how the glycerol-3-phosphate shuttle functions.
1) G-3-P dehydrogenase oxidizes cytosolic NADH to NAD+ while forming G-3-P from DHAP (dihydroxyacetone phosphate)
2) Mitochondrial FAD is the oxidizing agent, and ends up being reduced to FADH2
3) FADH2 transfers its electrons to ETC via complex II
Describe how the malate-aspartate shuttle functions.
1) Cytosolic oxaloacetate is reduced to malate by cytosolic malate dehydrogenase so it can pass through the inner mitochondrial membrane
2) Accompanying this reduction is the oxidation of cytosolic NADH to NAD+
3) Once malate crosses into the matrix, mitochondrial malate dehydrogenase reverses the reaction to form mitochondrial NADH
4) NADH in the matrix can pass along its electrons to the ETC via Complex I
What is F0?
- The proton motive force interacts with the portion of ATP synthase that spans the membrane
- F0 functions as an ion channel, so protons travel through F0 along their gradient back into the matrix
What is F1?
- Utilizes the energy released from this electrochemical gradient to phosphorylate ADP to ATP
Electrons come in what form in complex III? The electrons are passed to what?
- Come in the form of a dihydroquinone
- Passed e- one at a time to cytochrome c
Why is there a Q cycle in Complex III?
Because cytochrome c can only carry one electron at a time but dihydroquinone has 2 electrons
How many protons are pumped into the intermembrane space in each of the complexes?
Complex I: 4 protons
Complex II: 0 protons
Complex III: 4 protons
Complex IV: 2 protons
How many electrons do quinones, iron sulfurs and cytochromes carry?
Quinones: 1 or 2 e-
Iron sulfurs and cytochromes: 1 e-
How does oxygen get into the mitochondria?
O diffuses in and will bind to complex IV
How do electrons bind to O?
e- pass through copper center then to cytocrhome a and then to cytochrome a3-Cub then to O2
What do you need before binding oxygen to complex IV?
at least 2 electrons
Where does O sit in complex IV?
Sit between reduced iron and reduced copper
What is the name of Complex 1?
NADH dehydrogenase
What is the name of Complex 2?
FADH2 dehydrogenase
What is the name of Complex 3?
Cytochrome b1 complex
What is the name of Complex 4?
Cytochrome oxidase
Where is the synthesized ATP moved to?
Intermembrane space by ADP/ATP translocator, then diffuse to cytosol through outer membrane porin
How is Pi required for ATP synthesis imported?
By phophate translocate (symporter uses proton gradient as energy)
What is ATP synthase inhibited and activated by?
Inhibited: High levels of ATP
Activated: High levels of ADP and Pi
