Metabolism 6 Flashcards
Semester 1 year 1
What happens to the electron carriers that were reduced in glycolysis, link reaction, fatty acid oxidation and the Krebs cycle?
-they’re oxidised in the mitochondria
-makes ATP
What do NADH and FADH2 act as?
Electron donors to oxidative phosphorylation systems located in the mitochondrial inner matrix
What are electron transfers from donors coupled to and what does this form?
-coupled to proton transfer
-forms a proton motive force (pmf) for ATP synthesis
Describe the structure of the mitochondria
-outer + inner membrane, intermembrane space - space continuous with cristae
-matrix
-cristae + cristae junctions
-many ATP synthase along inner membrane - causes curves in cristae
What is redox potential?
A measure of the affinity of a redox couple for electrons
What does a more negative redox potential mean?
-more likely that the redox couple is going to donate electrons
-acts as a reductant
What does a more positive redox potential mean?
-more likely that the redox couple is going to accept electrons
-acts as an oxidant
What can the electron flow down their potential gradient be used for?
-free energy released when electrons move from -ive to +ive redox potential
-can be used to move protons from low conc. (matrix) to high (IMS)
How do you measure standard redox potentials?
-use 2 half cells linked together
-1 contains equimolar amounts of ‘A’ to be measured in oxidised + reduced states
-1 is a hydrogen half-cell with 10^-7 M sol. of H+ + 1 atm of H2 gas
What does it mean if electrons flow from the hydrogen half cell to ‘A’ or from ‘A’ to the hydrogen half cell?
-from hydrogen to ‘A’ = redox potential more +ive than H’s
-from ‘A’ to hydrogen = redox potential more -ive than H’s
How do you calculate ΔG°’ from redox potentials?
-ΔEm0 = Em0 (acceptor) - Em0 (donor)
-Em0 = redox potential (usually mV)
-ΔG°’ = zFΔEm0
-F = Faraday constant (J mol^-1 V^-1), z = no. of charges transferred
How do you calculate actual redox potential?
-Em = Em0 + (RT/nF) ln([Ox]/[Red])
-n = no. of electrons transferred
-R = gas constant
What is the order for the complexes in the respiratory electron transfer chain?
I. NADH dehydrogenase
II. succinate dehydrogenase
III. cytochrome bc1
IV. cytochrome c oxidase
V. ATP synthase
What is the overall equation for the reaction at NADH dehydrogenase?
NADH + 5H+(matrix) + UQ –> UQH2 + NAD+ + 4H+(IMS)
Describe the reaction that occurs at complex I
-oxidises NADH to NAD+ and electrons transferred to ubiquinone (UQ)
-UQ reduced to ubiquinol (UQH2)
-free energy released used to pump 4H+ from matrix to IMS
-engages in direct proton pumping
Describe direct proton pumping
-NADH + UQ bind to complex I, causing conformational change
-promotes H+ uptake in membrane arm of complex
-reduction of UQ causes another conformational change
-changes side of membrane that bound H+ are exposed to
-release of NAD+ + UQH2 causes drop in affinity for H+, so released into IMS
-loss of H+ resets conformation of complex
What is ubiquinone?
-lipid soluble electron carrier - takes electrons from complexes I + II to complex III
-takes up protons from matrix when reduced
-releases protons into IMS when oxidised
What is the overall equation for the reaction that occurs at succinate dehydrogenase?
succinate + UQ + 2H+(matrix) –> fumarate + UQH2
Describe the reaction that occurs at complex II
-oxidises succinate to fumarate as part of Krebs cycle
-electrons passed to FAD cofactor in the enzyme, forming FADH2
-2 electrons used to reduce UQ to UQH2
-no protons are directly pumped by this complex
What is the overall equation for the reaction that occurs at cytochrome bc1?
UQH2 + 2 cyt c (ox.) + 2H+(matrix) –> UQ + 4H+(IMS) + 2 cyt c (red.)
Describe the reaction that occurs at complex III
-oxidises UQH2 to UQ - electrons transferred to cytochrome c
-free energy used to translocate 4H+ from matrix to IMS
What is cytochrome c?
-small, soluble protein electron carrier in IMS
-reduces Fe3+ to Fe2+ in bound haem-cofactor by binding 1 e-
What does complex III form with complexes I and II?
-a H+ translocating loop
-UQH2 can be provided by complex I, II or other flavoproteins
-2H+ taken up by matrix when UQ reduced to UQH2 at complex I or II
-2 H+ released into IMS when UQH2 oxidised to UQ at complex III
-2 additional H+ moved from matrix to IMS for every UQH2 oxidised via Q-cycle
What is the equation for the overall reaction at cytochrome c oxidase?
2 cyt c (red.) + 4H+(matrix) + 1/2O2 –> 2 cyt c (ox.) + H2O + 2H+(IMS)
Describe the reaction that occurs at complex IV
-transfers electrons from cyt c to O2
-2 e- from 2 mol cyt c + 2 protons from matrix needed to reduce O2
-free energy released used to pump 2 protons from matrix to IMS
What is the equation for the overall reaction at ATP synthase?
12H+(IMS) + 3ADP + 3Pi –> 3ATP + 12H+(matrix)
Describe the reaction that occurs at complex V
-complex made of 2 domains
-potential energy stored in pmf used to drive energetically unfavourable ATP synthesis
-1 full turn of F0 ring carries 12H+ across membrane
-causes 1 full turn of F1 ATPase head, which forms 3 ATP
What 2 components is the proton motive force formed from?
-combination of membrane potential (Δψ) + proton potential gradient (ΔpH)
-Δψ - difference in charge between the 2 sides of the membrane
For each NADH and FADH2 oxidised, how many H+ are transferred across the MIM?
-NADH = 10H+ transferred
-FADH2 = 6H+ transferred
What is the total ATP yield per molecule of glucose in the entirety of respiration?
30
What was Mitchell’s theory of chemiosmosis?
Suggested that the electrochemical proton gradient generated by electron transport was used to generate ATP
What evidence was there to suggest electron transport was coupled to change in osmotic potential, providing evidence for chemiosmosis?
-when respiring, appearance of mitochondria changed in electron microscope imagery
-ratio of matrix to intermembrane space volume changed dramatically
What evidence is there to support chemiosmosis?
-if proton gradient formed by e- is needed to generate ATP, then its abolition should inhibit ATP formation
-abolished by an uncoupler
-ATP synthesis was abolished - e- transport + ATP synthesis were uncoupled
What is an uncoupler?
A molecule that facilitates diffusion of protons across a normally impermeable membrane
How did Walter Stoeckenius provide evidence for chemiosmosis?
-showed light driven proton pump ‘bacteriorhodopsin’ isolated from bacteria could drive ATP production
-proved no ‘high energy intermediates’ were required
