1Problem Set 4: Oxidative Phosphorylation and PhotosynthesisThis Problem Set is due before 5pm on Thursday December 7. There are eight questions for atotal of 70 points.Question 1 (12 points). Complex I in the electron transport chain oxidizes NADH to NAD+ andreduces UQ to UQH2 in a 2-electron process.(a) Calculate the free energy of the process at 37 °C and pH8. Standard reduction potentials forthe half-reactions at pH7 can be found in Table 20.1(b) Based on your answer to (a), calculate how many moles of protons can be translocatedacross the inner mitochondrial membrane by complex I if translocation of 1 mole of protonsrequires 23 kJ.(c) Experimentally it is observed that oxidation of 1 NADH is coupled to the translocation of 4protons. If the concentrations of UQ and UQH2 are approximately equal, what is the ratio ofNADH to NAD+ necessary to accomplish this?(a) UQ + 2H+ + 2e- Æ UQH2 E°’ = 0.06 VNAD+ + 2H+ + 2e- Æ NADH + H+E°’ = -0.32 VThe NAD+ reaction must be reversed, so DE°’(reaction) = 0.38 VConverting the standard reduction potentials to free energies using DG°’ = -nFDE°’:DG°’ = -73.33 kJ/molCorrecting this for pH1 at 298 K (standard state) using DG°’ = DG° - RTln[H+]DG° = -113.26 kJ/molAnd recalculating for 310 K and pH8 DG = -65.79 kJ/mol(b) 65.79 kJ/mol will translocate 2.9 moles of protons(c) Translocation of 4 moles of protons requires 92 kJNADH + UQ + H+ = NAD+ + UQH2DG° = -113.26 kJ/molUsing DG = DG° + RTln[NAD+][UQH2]/[NADH][UQ][H+]-92,000 = -113,260 + RTln[NAD+]/[NADH][H+]At pH8, [NAD+]/[NADH] = 3.8 x 10-5, so to pump 4 protons NADH:NAD+ = 26,000:12Question 2 (12 points). Complex III in the electron transport chain oxidizes UQH2 to UQ andreduces cytochrome c from Fe3+ to Fe2+ via the Q-cycle. Calculate the redox potentials for eachof the 4 electron transfer steps at pH7. Standard reduction potentials for the half-reactions atpH7 can be found in Table 20.1Step 1: UQH2 is oxidized to UQ at the QP site in a 2 electron process. UQ is reduced to UQH. Atthe QN site.(i) UQH2 reduces cytochrome c in a 1-electron process:UQH2 Æ UQH. + H+ + e-E°’ = -0.19 Vcyt c (Fe3+) + e- Æ cyt c (Fe2+) E°’ = 0.254 VDE°’ = 0.064 V(ii) 1 electron is transferred from UQH. to UQ at the QN site:UQH. Æ UQ + H+ + e- E°’ = -0.03 VUQ + H+ + e- Æ UQH. E°’ = 0.03 VDE°’ = 0 VStep 2: UQH2 is oxidized to UQ at the QP site in a 2 electron process. UQH. is reduced to UQH2At the QN site.(i) UQH2 reduces cytochrome c in a 1-electron process:UQH2 Æ UQH. + H+ + e-E°’ = -0.19 Vcyt c (Fe3+) + e- Æ cyt c (Fe2+) E°’ = 0.254 VDE°’ = 0.064 V(ii) 1 electron is transferred from UQH. to UQ at the QN site:UQH. Æ UQ + H+ + e- E°’ = -0.03 VUQH. + H+ + e- Æ UQH2E°’ = 0.19 VDE°’ = 0.16 V3Question 3 (12 points). Normally ATP synthesis is tightly coupled to electron transfer throughthe electron transport chain. Under these conditions, the ratio of ATP produced per 2 electronstransferred to oxygen (P/O ratio) is about 2.5.(a) Predict the effect of a low concentration of an uncoupling agent on the P/O ratioUncouplers depolarize the inner mitochondrial membrane, and ATP synthesis is compromised.The electron transport chain remains unaffected, so protons are still pumped by complexes I, IIIand IV. Protons pumped from the matrix to the intermembrane space are rapidly returned to thematrix by the membrane-soluble uncoupler, so energy that would normally be used to activateATP synthase is ultimately expended as heat. Low concentrations of uncouplers dissipate someof the proton gradient necessary for ATP synthesis. Overall, more protons have to betransferred across the membrane for each ATP synthesized. This requires more NADH per ATPsynthesized, so the P/O ratio will decrease.(b) Predict the effect of DCCD on the P/O ratio (DCCD irreversibly blocks asp61 of the c subunitof F1Fo ATP synthase)With the proton channel through F1Fo ATP synthase blocked, ATP synthesis will be reduced andthe proton gradient will not dissipate through the synthase. Increasing [H+] in the intermembranespace will stop proton pumping by Complexes I, III, and IV and so will reduce delivery ofelectrons through the ETC to O2. The decrease in ATP synthesis will be exactly matched by adecrease in O2 reduction so the P/O ratio will be unaffected.Question 4 (6 points) Which will make more use of cyclic photophosphorylation, a C-3 plant ora C-4 plant? Briefly explain your reasoning.In order to minimize the deleterious effects of photorespiration, C-4 plants transfer CO2 frommesophyll cells at the surface to bundle sheath cells in the interior. This is accomplished byconverting PEP to oxaloacetate, then reducing (NADPH) oxaloacetate to malate which cancross the cell membranes. Oxaloacetate is regenerated (NADP+) and the CO2 used for Calvincycle glucose synthesis, leaving pyruvate. Pyruvate is transferred back to the mesophyll cellswhere it is converted to PEP using ATP.Cyclic photophosphorylation makes ATP, but not NADPH. C-4 plants have a higher demand forATP, and so make more use of cyclic photophosphorylation than C-3 plants.4Question 5 (10 points). 14CO2 is administered to a green plant in a closed system. After a shorttime (one pass through the Calvin-Benson cycle), the plant is harvested, and the followingsubstances are isolated: glucose, sedoheptulose-7-phosphate, ribose-5-phosphate, erythrose-4-phosphate and 3-phosphoglycerate. Draw the structures of these five molecules and indicatewhich atoms will be 14C labeled. glucose sedoheptulose ribose erythrose phosphoglycerateAfter equilibrium is reached, all the carbons of all the molecules will be labeled.Question 6 (8 points). List four similarities and four differences between photosynthesis andoxidative phosphorylation.Similarities: ATP synthase (Fo/F1 ATP synthase/CFoCF1 ATP synthase), chemiosmoticmechanism (electron transport coupled to proton translocation), lipophilic electron trasport pool(ubiquinone/plastoquinone), dedicated organelles (mitochondria/chloroplasts), small, water-soluble 1-electron transfer proteins (cytochrome c/plastocyanin).Differences: Source of chemical reduction (light/NADH), ATP/ADP transport (passive/active),products (photosynthesis produces NADPH), coupling to TCA (photosynthesis has no analog ofcomplex II), oxygen evolution (photosynthesis) vs. oxygen consumption (oxidativephosphorylation).CH2OHOHHOOHHOHHOHHCH2O PCH2O POHHHOOHHOHHCHOHCHOCH2O POHHOHHOHHCHOCH2O