Fundamentals of Biochemistry Third Edition Donald Voet • Judith G. Voet • Charlotte W. Pratt Chapter 18 Electron Transport and Oxidative Phosphorylation Copyright © 2008 by John Wiley & Sons, Inc. We have made small amounts of ATP so far The main product is the 24 H+ (24 e-) that are produced Mitochondrion Cristae are not necessarily uniform structures. They can vary greatly, restricting local ion movement, creating a large pH gradient. How does NADH get into Matrix • Aspartate-Malate shuttle • Glycerophosphate shuttle – Insect muscle – Same power to weight ratio as a small car engine Transport of Materials • ATP/ADP – Translocator – One for one – Charge problems • Phosphate – Symporter – Phosphate and H+ • Both driven by pH gradient Efficiency of ETC NAD H 2e NADH 0.5O2 2 H 2e H 2O Δε°’= -0.315 V Δε°’= 0.815 V 0.5O2 NADH H H 2O NAD ' 0.815V 0.315V 1.130V G ' n ' G ' 218kJ m ol1 If 1 NADH makes 2.5 ATP and it takes 30.5 kJ/mol to make 1 ATP Efficiency of ATP synthesis is ~100kJ/mole / 218kJ/mol = 35% Inhibitors of ETC Complex I Complex IV Complex III 10 in humans Complex I Rhodopsin – Model Proton Pump Complex II Complex I and Complex II are not sequential, but accept electrons from different sources. Complex I – NADH Complex II - succinate Complex III Pass electrons from CoQ to cytochrome c Electrons in Complex III must go from a 2 e- carrier (CoQH2) to a 1 e- carrier, cytochrome c. Q cycle Q-cycle involves two cycles Proton pumping of complex III is different from complex I or II Complex IV Requires 4 electrons Contains Cu and Fe Reaction center requires 4 electrons at once Fe(II)-Cu(I) contributes only 3 electrons Fourth electron comes from Y244 forming a tyrosyl radical • Chemiosmotic Theory – 1961 Peter Mitchell – Observations • • • • OP needs intact inner mitochondrial membrane Inner MM impermeable to H+, OH-, K+, ClET sends H+ out Compounds that increase permeability of the MM inhibit ATP synthesis Harnessing the Energy • The free energy of sending a proton against the gradient is approximately 22 kJ/mol • The free energy required to synthesize one ATP is approximately 40-50 kJ/mol • It takes 2-3 H+ ions moving with the gradient to provide the energy to synthesize 1 ATP ATPase portion Membrane portion F1 region of ATP synthase Only beta subunits can catalyze ATP synthesis ATP synthesis • ADP + Pi bind to L site • Free energy drives conformational change – Only T site catalyzes ATP synthesis • ATP is release following next conformational change pH=8 pH=5 Only the c ring moves, not alpha and beta parts Arg210 PO Ratios • ATP synthesized(P)/Oxygen reduce(O) – 2 electrons through Complex I, III, IV result in 10 protons which rotates the c subunits one revolution thereby synthesizing 3 ATP – 2 electrons from FADH2 enter complex II and result in 6 protons or ~2ATP – Estimates for glucose • • • • • 2.5 ATP/NADH; 10 NADH/glucose 1.5 ATP/FADH2; 2 FADH2/glucose 2 ATP/glucose from glycolysis 2 ATP/glucose from citric acid cycle Total = 32 ATP/glucose Uncoupling Leads to Generation of Heat Diet pill?