Ch 9 (Part 3): 9.4 - E.T.C./ Oxidative Phosphorylation ● So far, in glycolysis & the Krebs cycle, 1 glucose molecule has resulted in: 4 ATPs (2 from glycolysis, 2 from Krebs) 10 NADH (2 from gly., 2 from acetyl- CoA step, 6 from Krebs Cycle) 2 FADH2 (from Krebs Cycle) ● Following glycolysis and the Krebs cycle, NADH and FADH2 account for most of the energy extracted from food ● These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation ELECTRON TRANSPORT CHAIN (E.T.C.) ● E.T.C. = a collection of molecules (mostly protein complexes) embedded in the inner membrane of mitochondrion (foldings of inner membrane form CRISTAE) The Pathway of Electron Transport ● the groups along the chain alternate between reduced & oxidized states as they accept and donate electrons ● each successive group is more electronegative than the group before it, so the electrons are “pulled downhill” towards OXYGEN (the final electron carrier!) NADH 50 FADH2 Multiprotein I complexes 40 FMN FAD Fe•S Fe•S II Q III Cyt b Oxidative Glycolysis Citric phosphorylation: Fe•S acid electron transport cycle 30 and chemiosmosis Cyt c1 IV Cyt c ) relative to O2 (kcal/mol) to O2 relative ) Cyt a ATP ATP ATP G Cyt a3 20 Free energy ( energy Free 10 + 1 0 2 H + /2 O2 H2O ● as molecular oxygen (O2) is reduced, it also picks up H+ from the environment to form water (H2O) ATP Production of the E.T.C. Typically, the ATP produced is as follows: 1 NADH 3 ATP “exchange rate” 1 FADH2 2 ATP (FADH2 is “dropped off” at a lower point in the E.T.C., so it generates fewer ATPs) Chemiosmosis: The Energy- Coupling Mechanism ● Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space (active transport) ● H+ (protons) then move back across the membrane, passing through channels in ATP synthase Chemiosmosis: The Energy- Coupling Mechanism ● ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP ● This is an example of CHEMIOSMOSIS, the use of energy in a H+ gradient to drive cellular work ● The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis ● The H+ gradient is referred to as a PROTON-MOTIVE FORCE, emphasizing its capacity to do work (inner matrix) ● protons then diffuse back across the membrane through the ATP synthase complex which causes the phosphorylation of ADP to form ATP! (intermembrane space) INTERMEMBRANE SPACE H+ A rotor within the membrane spins H+ H+ as shown when H+ flows past H+ H+ it down the H+ gradient. H+ H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ Three catalytic sites in the ADP stationary knob + join inorganic phosphate to P ATP i ADP to make ATP. MITOCHONDRAL MATRIX Inner mitochondrial membrane Oxidative Glycolysis Citric acid phosphorylation: cycle electron transport and chemiosmosis ATP ATP ATP H+ H+ H+ H+ Protein complex Cyt c Intermembrane of electron space carriers Q IV I III ATP synthase Inner II 2H+ + 1/2 O H O FADH 2 2 mitochondrial 2 FAD membrane + NADH + H+ NAD ADP + P i ATP (carrying electrons from food) H+ Mitochondrial Electron transport chain Chemiosmosis matrix Electron transport and pumping of protons (H+), ATP synthesis powered by the flow Which create an H+ gradient across the membrane of H+ back across the membrane Oxidative phosphorylation SUMMARY: ● most energy flows in this sequence: Glucose NADH E.T.C. proton ATP motive force ATP ATP PROCESS produced Reduced produced by oxid. phos. TOTAL by subs. coenz. (in the ATPs phos. E.T.C.) Glycolysis 2 ATP 2 NADH 4-6 ATP 6-8 (go to ETC) oxid. of 2 NADH 6 ATP 6 pyruvate to (go to ETC) acetyl CoA Krebs 2 ATP 6 NADH 18 ATP 24 cycle 2 FADH2 4 ATP (go to ETC) TOTAL 36-38! ATPs ● approximately 40% of energy in glucose is converted to ATP ● the remaining energy is lost as heat CYTOSOL Electron shuttles MITOCHONDRION span membrane 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Oxidative 2 2 Citric phosphorylation: Glucose Pyruvate Acetyl acid electron transport CoA cycle and chemiosmosis + 2 ATP + 2 ATP + about 32 or 34 ATP by substrate-level by substrate-level by oxidation phosphorylation, depending phosphorylation phosphorylation on which shuttle transports electrons form NADH in cytosol About Maximum per glucose: 36 or 38 ATP **actual ATP total’s are slightly less – when we factor in “real” exchange rates and the energetic cost of moving the ATP formed in the mitochondrion out into the cytosol, where it will be used**.