The Electron Transport Chain

The Electron Transport ChainIn the electron point chain, electrons flow downward in energy from coenzyme nicotinamide adenine dinucleotideH and flavoprotein FADH2 to the terminal electron acceptor, molecular(a) oxygen, O2. Electrons move spontaneously from carriers of disgrace step-down potential (Eo) to carriers of higher decline potential. Molecules involved in the ETC have step-down potentials between the values for nicotinamide adenine dinucleotide+/nicotinamide adenine dinucleotideH couples and oxygen/H2O couples. Energy extracted from the transfer of training of electrons is most efficiently maintain when it is released in a step wise fashion, and is accomplished with quadruple unmistakable protein complexes in the mitochondrial membrane coordination compound I = nicotinamide adenine dinucleotideH-coenzyme Q reductase (NADH dedhydrogenase) composite plant II = succinate-conenzyme Q reductase (succinate dehydrogenase) labyrinthian III = coenzyme Q-cytochrome c reducta se mazy IV = cytochrome c oxidaseComplex I oxidizes NADH and reduces coenzyme Q (UQ), transferring a pair of electrons from NADH to UQ. The oxidation of one NADH and diminution of UQ results in a net enthral of protons from the matrix side to the intermembrane space. Complex II oxidizes succinate and reduces UQ, concession a net drop-off potential of +0.029 V, which does not contribute to the transport of protons crosswise the inner mitochondrial membrane. Complex III facilitates the transfer of electrons from UQ to cytochrome c (cyto c) via the Q cycle, which oxidizes UQH2 and reduces cyto c, evacuant four protons into the intermembrane space for every two electrons that pass with the Q cycle. Complex IV accepts electrons from cyto c and reduces oxygen to form H2O, driving proton transport across the inner mitochondrial membrane into the intermembrance space. For every four electrons used to reduce oxygen, four protons ar released into the intermembrance space.Components of t he ETC atomic number 18 arranged in line with the flow of electrons from donors with lower affinity for electrons toward acceptors with higher affinity for electrons. Affinity for electrons is measured by the reduction potential. The transfer of electrons does not occur in a simple running(a) sequence. Electrons gutter enter the ETC at different entry points, either through Complex I or Complex II, and then the pathways converge at Complex III. As Fig. 1 shows, electrons move from more than negative to more commanding reduction potentials on the energy scale.Table 13-7 presents the following reduction potentials for reactions that occur in the ETCNAD+ + 2H+ 2e- NADH + H+ Eo = -0.320 VFAD + 2H+ +2e- FADH2 Eo = -0.219 VFumarate + 2H+ + 2e- Succinate Eo = +0.031 VQ + 2H+ + 2e- QH2 Eo = +0.045 Vcyt c1(Fe3+) + e- cyt c1(Fe2+) Eo = +0.220 Vcyt c(Fe3+) + e- cyt c(Fe2+) Eo = +0.254 V O2 + 2H+ + 2e- H2O Eo = +0.816 VAs mentioned, molecules involved in the ETC have reduction pote ntials between the values for NAD+/NADH couples and oxygen/H2O couples. Electrons move from more negative to more positive reductions potentials in the following orderNADH Q cytochrome c1 cytochrome c O2Reactions that have positive reduction potentials have negative free energy and are energetically favorable. Complex III has a more positive reduction potential than Complex I and II, and Complex IV has a more positive reduction potential than Complex III. The reduction potential for each complex can be estimated with the half reactions and reduction potentials provided in Table 13-7. Below are the net equations for each complexComplex I NADH + 5H+N + Q NAD+ + QH2 + 4H+PComplex II Succinate + Q fumarate + QH2Complex III QH2 + 2 cyt c1 + 2H+N Q + 2 cyt c1 + 4 H+PComplex IV 4 cyt c + 8 H+N + O2 4 cyt c + 4 H+P + 2 H2OFor exampleComplex I NADH + 5H+N + Q NAD+ + QH2 + 4H+PNAD+ + 2H+ 2e- NADH + H+ Eo = -0.320 VQ + 2H+ + 2e- QH2 Eo = +0.045 VEo = Eoacceptor EodonorEo = 0.045 (-0.320) = +0.365 VComplex III QH2 + 2 cyt c1 + 2H+N Q + 2 cyt c1 + 4 H+PQ + 2H+ + 2e- QH2 Eo = +0.045 Vcyt c1(Fe3+) + e- cyt c1(Fe2+) Eo = +0.220 VEo = 2 x 0.220 0.045 = +0.395 VThe reduction potential for Complex III is greater than that of Complex I, correlating to flow of electrons in the ETC. Electrons move from more negative to more positive reductions potentials.In addition, both overall reactions for NADH/FADH2 to O2 are positive values, another indication that electrons wretched from Complex I/II to Complex IV is energetically favorable. The calculations are provided below.This is the overall reaction for electrons that travel from NADH to O2NADH + H+ + O2 NAD+ + H2ONAD+ + 2H+ 2e- NADH + H+ Eo = -0.320 V O2 + 2H+ + 2e- H2O Eo = + 0.816 VEo = 0.816 (-0.320) = +1.136 VThis is the overall reaction for electrons that travel from FADH2 to O2FADH2 + O2 FAD + H2OFAD + 2H+ +2e- FADH2 Eo = 0.219 V O2 + 2H+ + 2e- H2O Eo = + 0.816 VEo = 0.816 (-0.219) = +1.035 VAs a re sult of the ETC, the net reaction for the transfer of two electrons from NADH through the respiratory chain to molecular oxygen is super exergonic (positive reduction potentials and negative free energy). For each pair of electrons transferred to O2, four protons are pumped out of the matrix into the intermembrane space by Complex I, four by Complex III and two by Complex IV, producing a proton gradient that drives ATP synthesis (Fig.2).