Oxidative Phosphorylation Flashcards
Where do the reduced cofactors NADH and FADH2 come from and what is their role in oxidative phosphorylation. Give the equation.
- Glycolysis
- TCA cycle
- Fatty acid oxidation
- Role in oxidative phosphorylation is their high electron pair transfer potential which is converted to phosphate transfer of ADP to ATP
- NADH —> NAD+ + H+ + 2electrons
Where does oxidative phosphorylation occur
- In the mithochondrial inner membrane
- The invaginations of the inner membrane are called Cristae
What is the proton motive force and how is it generated
- The proton motive force is composed of a proton gradient and voltage gradient across the inner mitochondrial membrane
- Oxidation of NADH and FADH2 simultaneously generates a proton gradient and thus an electrical gradient. There is a proton gradient since there’s a greater concentration of H+ in the intermembrane space than in the matrix. The electrical gradient across the inner membrane is a byproduct of the relatively more positive intermembrane space due to high H+ concentration and a relative negative matrix.
Explain how energy is released along the ETC
- A large amount of free energy is released upon glucose oxidation to carbon dioxide
- This free energy is retained within reduced cofactors NADH and FADH2
- Pairs of electrons from NADH are deposited at one of two electron carriers transferring the energy to proton pumping into the intermembrane space
- Electron flow through the ETC releases energy and protons are continually pumped
- Via proton chemiosmosis through ATP synthase, energy is used to phosphorylate ADP + P to ATP
- Electron transfer to oxygen is coupled to proton transfer across the inner membrane, thus it is essential electrons have a terminal acceptor to set up a chemiosmotic gradient
Name and describe the multi protein complexes and electron carriers along the ETC
Complex 1: NADH-CoQ reductase complex (NADH) PUMPS H+ OUT
Complex 2: Succinate-CoQ Reductase complex (FADH2 donates to flavin prosthetic group within Cp2)
CoQ/Ubiquinone: electron carrier.
Complex 3: CoQH2-Cytochrome c Reductase complex / Cytochrome bc1 complex. PUMPS H+ OUT
Cytochrome C: electron carrier
Complex 4: Cytochrome C oxidase complex. PUMPS H+ OUT
Explain the concept of redox potentials and its importance along the ETC
X+ + e- —> X
1. A negative redox potential for a substance means that it has lower affinity for electrons than hydrogen, essentially harder to reduce
2. Positive redox potential = higher affinity than hydrogen, eg. Oxygen
3. Strong oxidising agents have a positive redox potential, reducing agents have low redox potential
4. Electron carriers have higher redox potentials along the ETC generating a redox potential
5. These electron carriers increase in oxidising agent strength and thus electron transfer along the chain is driven by redox potential
Name the prosthetic groups within the electron carriers that receive electrons
- For complex 1, flavin mononucleotide (FMN) and Fe-S
- For complex 2, flavin adenine dinucleotide (FAD) and Fe-S
- For complex 3, Fe-S and Cytochrome b
- For complex 4, Cu and Cytochrome a
- Typically FMN would have a lower redox potential than FAD
Give the physical characteristics of complexes and electron carriers in the ETC
One physical for CoQ
Two physical for Cytochrome C
- Complexes I, III and IV are transmembrane proteins
- Coenzyme Q shuttles electrons between Cplx I/II to Cplx III and Cytochrome C shuttles electrons between Cplx III and Cplx IV.
- CoQ is a lipid soluble small molecule in the inner membrane and maximises rate of electron flow though ETC via the Q-cycle
- Cytochrome C is a water soluble protein situated in the intermembrane space.
- Cytochromes are heme containing proteins. Heme is an iron containing prosthetic group. Electron transfer is facilitated by oxidation and reduction of the Fe atom at the centre of heme. The various cytochromes have different heme groups affecting tendency of the Fe ion to accept an electron
Describe the role of cytochrome c oxidase and its mechanism to fulfil the role
- Electrons from cytochrome c reduce complex IV, as electrons are passed to oxygen to form water, protons are pumped into the intermembrane space
- Reduces oxygen to ROS such as superoxide anion (O2-)
- ROS intermediates such as superoxide and peroxide are toxic and lead to oxidative damage
- Superoxide dismutase enzyme (SOD) catalyses superoxide to peroxide
- Catalase catalyses peroxide to produce oxygen and water
Describe the structure of ATP synthase and give the evidence for rotation
- ATP synthase is composed of an F0 and F1 subunit
- The F1 particle is the catalytic domain of ATP synthase, protruding into the matrix
- The Fo particle is embedded in the inner membrane, forming a proton pore which protons flow through from the intermembrane space to the matrix
- Proton flow through the Fo particle causes rotation of subunits in the F1 particle, resulting in the synthesis of ATP from ADP + P
- Each complete rotation of the y subunit results in the synthesis of 3 ATP molecules
- Movement of fluorescently labelled actin was detected by microscopy
Explain some of the toxins that block oxidative phosphorylation
- Interference of electron flow : both CO and Azide compete with O2 for binding to cytochrome c oxidase
- Interference of proton flow
- Miscellaneous compounds
Describe the transporters in the inner membrane
- ATP/ADP translocase is an antiporter sending ATP into the cytoplasm and drawing ADP into the matrix
- Phosphate transporter draws HPO4 into the matrix and OH- into the cytoplasm
- H+ pumps coupled to electron transfer ensure the OH- combine with H+ to form water
What is required for oxidation of cytosolic NADH, and why is this oxidation needed
- When more electrons are required to couple with proton pumping, the glycerol phosphate shuttle provides reducing equivalents from cytosol to mitochondria
- The inner membrane is impermeable to NADH
- So electrons from cytosolic NADH are transported via intermediates to FADH2 in the mitochondria
- This enables a high rate of oxidative phosphorylation since extra electrons can be provided to the ETC from outside the mitochondria
- Occurs especially in muscle
Explain the malate-aspartate shuttle
- Occurs in the heart and liver
- A reversible shuttle that can interchange between malate and aspartate
- Aspartate is converted to oxaloacetate via a ketoglutarate and aspartate, which is reduced to form malate
- Malate can be oxidised to oxaloacetate and converted back to aspartate via glutamate and a ketoglutarate
- Can also shuttle metabolic intermediates such as aKG between mitochondria and cytosol
What is unique about brown-fat mitochondria
- Brown fat mitochondria contain an uncoupler of oxidative phosphorylation
- The uncoupler protein is a thermogenin proton transporter protein uncoupled to ATP synthesis
- The energy released from NADH oxidation/Proton diffusion is converted to heat
- Brown fat is thus specialised to produce heat, especially present in newborns
