Energy Generation Chemiosmotic hypothesis and the generation of ATP Flashcards
(20 cards)
What does the main part of the electron transport chain consist of?
4 large protein complexes embedded in the inner mitochondria, they are transmembrane proteins:
Complex I= NADH- CoQ Reductase Complex
Complex II= Succinate-CoQ Reductase Complex
Complex III= COQH2-Cytochrome c Reductase Complex
Complex IV= Cytochrome c Oxidase Complex
Also need 2 small electron carriers that are needed to link the large complexes=
CoQ= Coenzyme Q (Ubiquinone)
Cytochrome C
What happens during the electron transfer chain?
Electrons flow from NADH to oxygen through the complexes
1) NADH passes 2 electrons to NADH-CoQ Reductase, reduces it, generating NAD+
2) NADH-CoQ passes 2 electrons to CoQ= Reduces CoQ= Generates ubinquinol
3) CoQ ALSO REDUCES Complex II= Electrons passed to CoQ
4) CoQ then reduces Complex III= Electrons passed between them
5) Complex III then reduces Cyt C
6) Cyt C reduces Complex IV
7) Complex IV reduces oxygen= Forms O2- which is superoxide which is a radical which will react with anything
What is redox potential?
The redox potential of a substance is a measure of its affinity for electrons, relatively to hydrogen
Positive redox potential: Substance has a higher affinity for electrons than hydrogen does= Accept electrons from hydrogen
Electron transfer is driven by redox potential
How is electron transfer driven by redox potential?
NADH: Redox potential= -52.6 kcal mol-1 which is relatively large= low affinity for electrons= able to release electrons
Oxygen has very high affinity for electrons
Electrons are passed from negative redox potential to very positive (oxygen)
Cofactors have low affinity for electrons and want to donate electrons down towards compounds which have much higher affinity for electrons
Each step involves release of free energy: Oxidation of NADH releases sufficient energy to drive the synthesis of several molecules of ATP
What actually acts as the electron carriers?
Can’t just add electrons to proteins= Reduction and oxidation do not occur on the amino acid side chains=
PROSTHETIC GROUPS associated with the proteins are the electron carriers
Prosthetic groups= Can be organic or inorganic material that is bound tightly to proteins
1) Complex I= Binds to and oxidises NADH to NAD+ and passes 2 electrons from the NADH to FMN prosthetic group= Produces FMNH2
Each electron is accepted together with a hydrogen ion= accepts 2 electrons and 2 hydrogens
2) Electrons are transferred within Complex I to iron-sulfur clusters= Electron is carried by iron atom= Changes from Fe2+ to Fe3+ and releases the electron to CoQ
Complex II= FAD, Fe-S
Complex III= Fe-S, Cyt B
Complex IV= Cu, Cyt a
Basically: Complexes 1 and 2 accept 2 electrons and 2 hydrogens by their prosthetic groups, and bind to Fe3+
Complex III= Only accepts 1 electron
Complex IV= Copper atoms cycle between Cu2+ and Cu+ and transfers 4 electrons form 4 cytochrome c molecules and 4 H+ ions to molecular oxygen= To form 2 molecules of water
How could you experimentally demonstrate that electron transfer to oxygen is coupled to proton transfer across the mitochondrial membrane?
Test tube: Mitochondria in saline solution= Air tight + Degassed to make sure NO OXYGEN
- Measure pH= pH remains stable UNTIL solution with oxygen is injected
- Oxygenated buffer= pH starts to drop due to increase in protons being pumped out
1) No oxygen and NADH is added= No oxidation
2) Addition of small amounts of oxygen= Protons pumped out= Lowering pH
3) As oxygen is being used up, protons move back into mitochondria= ATP synthesis as pH returns to initial value
Where does the pumping of protons occur?
Change in redox potential in chain= Gets more positive
ONLY 3 steps generate enough free energy to pump protons:
1) Complex I
2) Complex III
3) Complex IV
NOT COMPLEX II
As the free energy is released, it is used by the complexes to drive proton ions from the mitochondrial matrix across the inner mitochondria membrane and into intermembrane space
Overall: electron transport along the chain from NADH releases energy that is used to create an H+ gradient
Where does FADH enter?
Succinate- coenzyme Q reductase (Complex II)
Succinate dehydrogenase: Oxidises succinate to fumigate in citric acid cycle= Contains bound FAD that is reduced to FADH2 in the reaction
Enters via complex II: Contains Fe-S clusters= FADH2 gets oxidised as it reduces Complex II= 2 electrons passed to Fe-S clusters in complex II which are then passed to CoQ
Why are protons not pumped across in complex II?
FADH2 has higher affinity for electrons than NADH= less negative free energy
This means that there is less free energy released when the electrons are transferred= Not enough to drive the proton pump
What are some properties to Cyt C?
It is a small protein which is associated with the outer side of the inner mitochondrial membrane= Useful properties
One way of trigger cell death= Trigger release of Cyt C
IF mitochondria and membrane is damaged= release of Cyt C which can trigger a variety of processes which can lead to cell death
What type of proteins are cytochromes?
Covalently linked Haem containing proteins= Same basic structure, similar to haemoglobin or myoglobin
Iron atom in centre= But differs from haemoglobin as they have different side chains
During synthesis of the haem structures, you get differences in synthetic groups = Alter the affinity for electrons in the iron
Where does oxidation and reduction of the complexes actually take place?
Electron transport occurs by oxidation and reduction of the Fe atom in the centre of the haem group
NOT change in the oxidation of protein as it is a change in the oxidation of the IRON in the protein
Where is Cytochrome C located? Where is CoQ?
Cyt C= Water soluble protein and is in the intermembrane space
CoQ= Lipid soluble protein and is in the inner membrane
What does the Q cycle allow?
Allows transport of additional protons across the membrane
What is the problem during the transfer of electrons between CoQ to Complex III?
Oxidation of CoQ= leads to the release of 2 electrons to be passed down the chain
HOWEVER: Cyt C can ONLY accept 1 electron
Q cycle: Single molecules of Cyt C can be reduced= Leads to additional protons being pumped across
What are the different states which CoQ can exist in?
1) Fully reduced state= Ubiquinol= COQH2
1) Oxidised state=Ubiquinone= CoQ
3) Semiquinone radical= Ubisemiquinone= COQH
What happens during the Q cycle?
1) CoQ receives 2 electrons and 2 H+= Reduced COQH2 which then binds to Complex III on the Q0 site in the inter membrane space (outer side)
2) CoQH2 releases 2 protons (one at a time, so that you get CoQH at some point) into the inter membrane space and 2 electrons= one binds to the Cyt C= reduces Cyt C
3) The other electron moves through cyt B and partially reduces an oxidised CoQ that is bound at another site of the Complex III protein (its on the inner side)= Forms the semiquinone radical, CoQH’
4) Process is repeated with the binding of a second CoQH2 at the Q0 site, 2 protons released, and reduces another Cyt C
5) Also get another electron transferred to CoQ= COQ’, 2 protons from the matrix attach= CoQH2 at the semiquinone site which then dissociates and a new molecule of CoQ can be attached so that the Q cycle starts again
What is the problem with cytochrome c oxidase?
Complex IV= Need safe reduction of O2
Partially reduced intermediates are very dangerous
Can produce OH-= One of the most dangerous radicals that can be generated which will react with DNA, proteins etc…
Can get proteins suddenly stuck onto lipids + induces damage on DNA= Mutations
Process of adding electrons to oxygen must be controlled in order to minimise free radicals
What does the cytochrome oxidase mechanism do?
Cyt C transfer 1 electron to the copper centre, Cu, then to haem a then fairly a binuclear centre consisting of Cub and haem a3= Where final transfer to O2 occurs
4 reduced molecules of Cyt C needed to reduce 1 oxygen molecule
-Cyt C has single binding side= can donate electron and get reduced as it passes the electron to Cu centre= 2 molecules of Cu= Passes them to an iron molecule in haem (Cyt A)= Another Cu Centre and another Iron haem group (Cyt B)
= Transfers 4 electrons from 4 cyt c and 4H+ ions to molecular oxygen
Why is there many haems and Cu?
Allows repeated addition of electrons to this = Electrons are added together
Complexes III and IV may be associated with each other in the mitochondrial membrane= Increase kinetic efficiency as Cyt C does not need to diffuse the distance between them
