Electron Transport System. May 16, 2014 Hagop Atamian

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1 Electron Transport System May 16, 2014 Hagop Atamian

2 What did We learn so far? Glucose is converted to pyruvate in glycolysis. The process generates two ATPs. Pyruvate is taken into the mitochondia and is further oxidized in the TCA cycle. First PDHc converts pyruvate to Acetyl-coA Acetyl-CoA which enters TCA-cycle. Here energy is captured as NADH. NADH Series of eight oxidation reactions resulted in production of two CO 2 while the energy was collected in the form of NADH and FADH 2. One ATP was produced in the process.

3 Review of the TCA-cycle NADH Malate dehydrogenase Fumarase Malate Acetyl CoA OAA Citrate Synthase Citrate Isocitrate Aconitase Isocitrate dehydrogenase Fumarate FADH 2 Succinate dehydrogenase GTP Succinate Succinyl CoA Succinyl CoA syhthetase NADH & CO 2 α-ketaglutarate α-ketaglutarate dehydrogenase NADH & CO 2 h"p://tru- wealth.blogspot.com/2013/05/your- bodys- chemistry- pyruvate- for.html

4 Electron Transport Chain The electron transport chain is the final stage of aerobic respiration leading to the forming of ATP. Now finally we are about to obtain the ATP necessary for survival by using the reduced cofactors, NADH and FADH 2, formed in the TCA-cycle. Note that there is a difference between the way ATPs were generated so far and the way ETC makes ATP.

5 Substrate Level Phosphorylation VS Oxidative Phosphorylation Substrate Level Phosphorylation: results in the formation of ATP by the direct enzymatic transfer of phosphate group to ADP. Oxidative Phosphorylation: uses an electrochemical or chemiosmotic gradient of protons (H + ) across the inner mitochondrial membrane to generate ATP from ADP. h"p://en.wikipedia.org

6 Reaction 5: Oxidation Reaction O O CoA-SH O - C CH 2 CH 2 C 2 3 S CoA Succinyl CoA Succinyl CoA synthetase GDP GTP O O O - C CH 2 CH 2 C 2 3 O - Succinate

7 Mitochondrial Compartments

8 Molecular Species Involved Flavoproteins Ubiquininone Complex I Succinate dehydrogenase Complex II Cytochromes Complex III Iron-sulfur proteins Complex IV Protein bound copper

9 The Overall Picture of ETC NADH FADH 2 Oxidation steps Protein complex I Protein complex II Protein complex III Purpose Use the released energy to pump protons. Protein complex IV O 2 H 2 O Electrons move along the electron transport chain going from donor to acceptor until they reach Oxygen.

10 Standard Reduction Potential Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Molecules with a more positive Standard Reduction Potential will pull electrons away from molecules with a more negative Standard Reduction Potential wikispaces.psu.edu

11 Electron Flow Direction in ETS The flow of electrons through the cell's electron transport chain is strictly in the direction of molecules with increasing Standard Reduction Potentials.

12 Summary h"p://hyperphysics.phy- astr.gsu.edu/hbase/biology/etrans.html Key point: The electrons lose energy as they pass down the electron transport system. Some of this energy is used to pump protons (hydrogen ions) into the outer compartment of the mitochondrion.

13 Reduction Potential and Free Energy The relationship between Standard Free Energy change ( Go ) and Eo is: ΔG o = -nf Eo Standard reduction potential difference: ΔE o = Eo (e acceptor) - Eo (e donor) REMEMBER NADH and FADH 2 are the first electron donors. O 2 is the last electron acceptor.

14 O 2 reduction by NADH: Overall Reaction 1/2 O 2 + NADH + H + H 2 O + NAD + 1/2 O 2 + 2H + + 2e - H 2 O NAD + + H + + 2e - NADH Eo = V Eo = V ΔE o = Eo (e acceptor) - Eo (e donor) ΔE o = (-0.315) = V ΔG o = -nf Eo = -218 KJ/mole

15 O 2 reduction by NADH: Overall Reaction Oxidation of O 2 (2e - transfer) under standard biochemical conditions releases 218KJ of free energy. An ATP synthesis requires 30.5 KJ/mole free energy. Therefore several moles of ATP can be synthesized.

16 Specifics of ETC ETC is carried out by 4 complexes. 1. Complex I: also known as the NADH-coenzyme Q reductase or NADH dehydrogenase. 2. Complex II: also known as succinate-coenzyme Q reductase or succinate dehydrogenase. 3. Complex III: also known as coenzyme Q reductase. 4. Complex IV: also known as cytochrome c reductase. Each complex contains several different electron carriers.

17 Complex I: NADH-coenzyme Q reductase Coenzyme Q = Ubiquinone The process of transferring electrons from NADH to CoQ by complex I resul

18 Summary Complex I is called NADH-Coenzyme Q reductase. Transfers two electrons from NADH to coenzyme Q. Electrons are transferred through Complex I using FMN (flavin mononucleotide) and a series of Fe-S clusters. The process accomplishes the pumping of four protons across the inner mitochondrial membrane to the intermembrane space. The NAD + can now go back to the Krebs Cycle to pick up another pair of electrons.

19 Complex III Coenzyme Q-cytochrome c reductase

20 Complex I: NADH-coenzyme Q reductase Coenzyme Q = Ubiquinone The process of transferring electrons from NADH to CoQ by complex I resul

21 Summary This complex accomplishes the oxidation of ubiquinol and the reduction of two molecules of cytochrome-c. Four hydrogens are pumped across the membrane to the intermembrane space.

22 Complex IV Cytochrome c Oxidase

23 Summary This final complex in the electron transport chain accomplishes the final transfer of the electrons to oxygen. It pumps two protons across the membrane. This makes a total of ten protons across the membrane for one NADH into the electron transfer chain.

24 The function of Cyt a3 Oxygen loves electrons, but adding fewer than 4 electrons ta a time makes oxygen unstable. So Cyt a3 holds them apart until 4 electrons accumulate. When these four once energetic electrons have accumulated at Cyt a3, they are fed all at once to oxygen.

25 Complex II Succinate Deyhydrogenase Complex II includes succinate dehydrogenase and serves as a direct link between the citric acid cycle and the electron transport chain.

26 Reaction 6: Oxidation Reaction O O O - C CH 2 CH 2 C 2 3 O - Succinate Succinate Dehydrogenase (membrane bound) FAD FADH 2 O O O - C CH CH C 2 3 O - Fumarate

27 No Protons pumped FADH 2 + UQ (oxidized) FAD + UQ (reduced) FAD + 2H + + 2e - FADH 2 Eo = V UQH 2 UQ + 2H + + 2e - Eo = 0.06 V ΔE o = Eo (e acceptor) - Eo (e donor) ΔE o = 0.06 (-0.22) = V ΔG o = -nf Eo = -5.4 KJ/mole

28 Overall View of ETC This complex forms a second entry point into the electron transport chain using the succinate product of the TCA cycle. This is independent of NADH pathway. FADH2 is generated in succinate dehydrogenase in the citric acid cycle,

29 ATP Synthase

30 The role of Oxygen The role of oxygen in cellular respiration is substantial. As a final electron receptor, it is responsible for removing electrons from the system. If oxygen were not available: Electrons could not be passed among the coenzymes. The energy in electrons could not be released. The proton pump could not be established. ATP could not be produced. Without sufficient ATP, a cell depending upon mitochondrial ATP for energy will die. When enough cells die, the entire organism will die.

31 ETC Inhibitors Cyanide binds cytochrome oxidase so as to prevent the binding of oxygen.

32 Carbon Monoxide Carbon Monoxide- binds Nghtly to Fe+2 form of cytochrome a3 (Complex VI).

33 ATP Synthase

34 THE END More questions?? Please hatamian@ucdavis.edu