Electron Transport Flashcards
What is the ETC?
A series of four protein complexes that transport electrons to oxygen and pump protons to create a proton gradient
Electrons in NADH and FADH2 are stored at a very ____________ reduction potential.
Negative, they want to react and release them to a more positive reduction potential
T or F: NADH has a more negative reduction potential than FADH2.
True
What is the name of complex I? what inhibits complex I?
NADH DH, rotenone
What is the substrate for complex I?
NADH
Describe the mechanism of complex I.
Takes 2e- from NADH, moves through iron sulfur clusters & FMN & ultimately pumps 4 protons out, transfers e- to complex III via QH2
Does FMN or Fe-S clusters get the electrons first?
FMN (required since NADH donates 2e- and ferric iron only accepts 1e- at a time)
What is the name of complex II? What is it also a part of? Where is it located?
Succinate DH, TCA cycle, Inner mitochondria membrane
Where is complex II located?
Mitochondrial membrane
What is the substrate for complex II?
FADH2
What are four flavoproteins that reduce Coenzyme-Q in the ETC?
Complex I, Complex II, ETF-QO, sn-glycerophosphate DH
Does complex II contribute directly to the proton gradient?
No, it only contributes e- to the Q pool, NO PROTONS ARE PUMPED
Where do electrons from the glycerol shunt enter the ETC?
Complex II (FADH2)
Is Q lipid soluble?
Yes’ that the whole purpose of using it to shuttle electrons through the mitochondrial membrane
Describe the structure of complex III
Rieske Iron, and two b hemes, Cytochrome c1 on cytosolic side
What is unique about electron flow in complex III?
The electron pair from QH2 is split, one travels to the rieske iron center which has a very positive dE(290mv, recall that dG=-nJdE so very spont. Rxn), another e- travels to heme bL (low potential heme, -20mv), this happens because iron center can only accept 1 e- at a time
What happens to the e- that goes to the rieske iron center?
(526) Is passed on to cyt c1 and goes onto Complex IV
What happens to the e- that goes to bL?
Is passed on to bH, then passed to a fully oxidized UQ at the N side of CIII, forms semiquinone that is fully reduced upon addition of 1 more e- (next rxn. ) passes QH2 into the Q pool
Describe movement of protons in complex III.
Each time 1 e- is moved through cycle it releases 2H+ via initial binding, and picks up 2H+ from matrix via bL pathway, however since QH2 provides 2 e- each cycle of the Q cycle in complex III produces a net 4H+ into the cytosol and 2H+ out of the matrix
What binds at the N side of complex III and inhibits it? P side?
Antimycin, stigmatelin, respectively
Describe Complex IV (Cyt c oxidase)
Secretes 2H+ into cytosol, binds oxygen in active site and uses it as a terminal electron acceptor, contains 4 cyt c proteins
How does complex IV pump protons?
(531)Takes 4e- to reduce oxygen, 4 cyt c proteins bind e- on interior of protein, in order to compensate for this thermodynamically unfavorable placement of charge inside a hydrophobic area we also pump 4H+ into it
Describe the path of electrons through complex IV
Cyt cCopper Aheme aheme a3Copper B
Why are there two hemes in complex IV?
Charge compensation, you would think that e- would flow directly to heme, however since we need to compensate for the large charge on the interior we move protons via one heme and electrons via another
Describe the formation of reduced oxygen (water) from Complex IV
Two electrons donated via copper B (one for each metal) and forma peroxide bridge (Fe-O-O-Cu), following this two protons come in and cleave the oxygen bond; two more electrons are dumped in to finish the reduction to water
Do the protons that reduce oxygen follow the same routes as those pumped ?
No, different pathways
What portion of the ATP Synthase structure is embedded in the inner mitochondrial membrane?
F0 Subunit
What is the purpose of the Aspartate residue on the C-Subunit?
**This is the Key of proton transport**
When the aspartate residues of the two C-Subunits are in contact with the hydrophillic environment of the two half channels they give up their protons so they are in the charged aspartate form.
The key to proton transport is that in a proton rich environment such as the cytoplasm of the mitochondria a proton will enter a half channel of the A subunit and bind to aspartate.
The A subunit then rotates until the aspartic acid is in the proton poor environment of the matrix where the proton will then be released.
What powers the rotation of the C-Ring?
The transfer of protons from high concentration on the cytoplasmic side of the mitochondrial membrane to the proton poor region in the matrix of the mitochondria.
Describe the structure of the A subunit of ATP synthase.
The A subunit is composed of two half channels that allow for interaction of protons with the C-Subunit’s aspartic acid residue. It is located in the F-0 portion of ATP synthase.
What subunitis the rotation of the C subunit attached to, the crankshaft of ATP Synthase. (If it were a Rolls Royce Merlin it would be the prop reduction drive.)
The gamma subunit which will be rotated through the Alpha and Beta subunit much as a camshaft will drive the open and closing if intake and exhause valves.
What portion of ATP synthase holds the F-1 subunit stationary while the Gamma subunit rotates?
Alpha Beta and Delta subunits.
What does the energy created in the proton gradient directly contribute to in the ATP synthesis reaction.
Rotation of the Gamma subunit in the stationary F-1 complex (the camshaft) and this causes a conformational change in the alpha and beta subunits of the F-1 complex that is used to release newly synthesized ATP from the tight binding sites.
How many protons must be pumped to make one ATP molecule?
4
How many C-Subunits are there in human ATP synthatase?
12
Where is the ATP synthase located in relation to the cristae of the mitochondria? Does this contribute to its efficency?
They are located in rows on the tips of the cristae of the mitochondria. This greatly increases the efficency of the ATP synthase
Electrical gradient is much greater where the cristae make this sharp turn
How does ATP get transported out of the mitochondria?
ATP-ADP translocase
Is the flow of ATP coupled to ADP
Yes ADP enters the matrix only when ATP leaves
How much ATP is generated by NADH transported by the glycerol 3 phosphate shuttle?
Where is the Glycerol 3 phosphate shuttle used and what advantage does it offer?
1.5, This is because FADH2 is the electron acceptor on the mitochondrial Glycerol 3 phosphate dehydrogenase.
Located in the muscle and allows for a very high rate of oxidative phosphorylation.
Using FAD as the electron acceptor allows the mitochondria to take in electrons from NADH against the concentration gradient.
What is the primary NADH transport mechanism in the heart and liver?
What is the yield of NADH when this shuttle is used and why?
Malate aspartate shuttle
2.5 ATP/NADH because NADH is the electron acceptor instead of FAD like in the glycerol shuttle
During pyrivate ox, What are the 4 steps NADH is produced? 1 step that forms FADH2?
NADH=> 1. pydruvate dehydrogenase, 2. isocitrate dehydrogenase, 3. alpha KG dehydrogenase, 4. malate dehydrogenase
FADH2 => succinate hydrogenase
T/F NADH and FADH2 give up their protons and electrons to the ETC
True
What is the proton gradient used for in the ETC?
generate ATP by the ATP synthase
What is the final step in the ETC?
export ATP out of the Mt to cytosol
How is energy stored since ATP is not made directly from NADH or FADH2?
the reduction potential of NADH and FADH2 so their electrons will tend to move to more positive reduction potentials and release free energy as they move
What is the final electron acceptor?
oxygen
Which coenzyme gives more energy?
NADH because more free energy is released on the way down
The free energy of the electrons is conserved in a chemical and electricla gradient by what mechanism?
pumping the protons out of the matrix across the inner membrane
What is the NADH dehydrogenase complex? What protein is associated with this complex as it is an inhibitor?
Complex I and encoded in Mt genome
-receives pair of electrons to FMN to iron-sulfur complex then 2 ubiquinone site
How many electrons are passed for each NADH oxidized? How many protons per electron?
2 electrons / NADH
4 protons / 2 electrons
What 3 molecules are important in the complex I of the ETC?
- Iron sulfur center
- FMN
- ubiquinone
What is a cofactor that is tightly bound to the peripheral arm of complex I? Where is this molecule reduce ubiquione?
FMN
FMN reduces ubiquinone at 1. complex I, 2. Complex II, 3. electron transfer flavoprotein dehydrogenase, 4. NADH shuttle
***Why is FMN needed in complex I?***
NADH cannot donate its electrons directly to iron so FMN must be an intermediae that accepts 2 electrons and gives Fe-S one electron at a time
What can be found in complex II of the ETC?
succinate dehydrogenase, FAD and 3 Fe-S cluster
What is the only TCA cycle enzyme to be an integral membrane protein?
succinate dehydrogenase
Why can complex II not contribute to the proton gradient?
none of its subunits are coded in Mt geneome so it cannot pump protons across inner membrane
Describe the complex II including the subunits
heme is sandwiched between 2 hydrophobi membrane subunits that is liganded by 2 His residues
T/F Mutagenesis of His changes the side and causes the loss of heme, the ETC will be disrupted
False, the heme is not needed and complex II will still assemble and work well
Describe the flow of electrons in complex II
Succinate to FAD then to Fe-S cluster and to ubiquinone
Describe the mitochondrial sn-glycerophosphate dehydrogenase
part of the glycerophosphate shuttle for movign electrons from cytolisc NADH into electron transfer pathway
Describe how electrons are transferred from cytosolic NADH
electrons transferred to DHAP to form glycerol 3-P and back to membrane bound glycerol 3-P Dehydrogenase FAD coenzyme then to ubiquinone
Describe ubiquinone
lipid soluble electron carrier and exists in the Mt inner membrane and carriers electrons between complexes in ETC
What is a semiquinone?
it is a ubiquinone that has only accepted one electron=> thus when it has 2 electrons, it is fully reduced but when it has none then it is fully oxidized
- stabilized by binding to proteins sites but does not float around in membrane
- When exiting membrane=> must be fully reduced or fully oxidized
Complex III is the next electron acceptor and has important redox centers. What are they?
2 b type hemes => cytosolic side is the Rieshke Fe-S protein and the cytochrom c1
Which path is most preferred in complex III? Where does the electron go after that?
Rieske due to highest reduction potential
electron goes to rieske then to cytochrome c1 then passed on to cytochrome c then reduce oxygen in complex IV, cytochrome c oxidase
Describe the operation of complex III
Q cycle=> electrons from ubiquinone flow into 2 different pathways
remember, the electrons and protons are not going the same place in the Q cycle
- fuly reduced ubiquinone diffuses to cytosolic side to center P (oxidation center)
- 2 electrons split into different paths
a. Rieske Fe-S center (high reduction potential)
b. first b heme called BL (low reduction potential)
What is the major energy barrier to the Q cycle?
deprotonation of the loos ubquinone
What happens to the 2nd electron? What is formed by the electron transfer?
given to fully oxidized ubiquinone near matrix side at N center to become reduced.
semiquinone stays tightly bound to this site
Account for the net movement of 4 protons out of the cytosol and 2 in the matrix…
This occurs for every 2 ubiquinones oxidized and 2 protons released on cytosolic side at P center
Where does the majority of protons transfer come from? Why?
Rieske Fe-S protein due to largest drop in reduction potential
What could block the bc1 complex (complex III)?
antimycin binding to the N center at the matrix side of membrane and preventing electrons from reaching ubiquinones from b hemes
Stigmatellin binds to the P center and interferes with the Rieske protein and cytochrome b
Which complex reduces oxygen to water and is a proton pump?
complex IV with cytochrome oxidase
How do the electrons get to cytochrome oxidase? Describe the process
come on the carrier cytochrom c which can only deliver 1 electron at a time and takes 4 electrons to reduce oxygen so 4 cytochrom c proteins have to bind sequentiall to one docking site on periplasmic surface of protein
Describe the charge compensation of cytochrome c oxidase
4 electrons are takin in and 4 protons are taken up linking it to acting as a proton pump
What are the redox centers of complex IV?
2 copper centers called copper A and B
2 hemes of the a type
magnesium ion
zinc ion
What is the first redox center that gets an electron after leaving cytochrom c?
copper A
Describe the electron flow after reaching copper A
Copper A=> heme a=> heme a3 / copper B (binuclear center)
What happens to the proton assocated with copper B, when an electron moves from heme a to heme a3, copper B (binuclear center)? What is the result?
the proton is forced to leave which is a driving force for the proton pump
Since 4 electrons have to come in to reduce oxygen, what is the order of addition?
2 electrns enter heme a3, copper B site, one on each metal ion then oxygen binds between them
Fe-O-O-Cu => peroxy bridge
2 protons come in and 1 more electron to break the O-O bond with reduction to water and waters leave
