Electron Transport System

Transcription

1 Electron Transport System Lecture 29 Key Concepts Peter Mitchell's Chemiosmotic Theory The Electron Transport System is a series of Redox reactions Complex I: NADH-ubiquinone oxidoreductase Complex II: Succinate dehydrogenase Complex III: Ubiquinone-cytochrome c oxidoreductase Cytochrome C Complex IV: Cytochrome c oxidase The ATP currency exchange ratios for NADH and FADH 2 What is the Chemiosmotic Theory and how does it explain proton motive force? What is the role of coenzyme Q (ubiquinone) in the electron transport system?

2 Biochemical Application of the Electron Transport System Hydrogen cyanide is a deadly gas that kills cells by blocking electron transfer from cytochrome oxidase in complex IV to oxygen, the final electron acceptor in the electron transport system. Other electron transport inhibitors are rotenone, a poison, and amytal, a barbiturate, both of which block electron transfer from iron-sulfur centers. The Electron Transport System, also called the Electron Transport Chain, converts redox energy available from oxidation of NADH and FADH 2, into proton-motive force which is used to synthesize ATP through conformational changes in the ATP synthase complex through a process called oxidative phosphorylation.

3 Peter Mitchell's Chemiosmotic Theory Oxidation of NADH and FADH 2 in the mitochondrial matrix by the electron transport system links redox energy to ATP synthesis by oxidative phosphorylation (mitochondrial ATP synthesis) through the establishment of a proton (H + ) gradient across the mitochondrial inner membrane. "chemiosmotic" process was first proposed by Peter Mitchell, a British biochemist, in 1961 involves the outward pumping of H + from the mitochondrial matrix three protein complexes in the electron transport system (complexes I, III, IV) H + flow back down the gradient through the membrane-bound ATP synthase complex response to a chemical (H + concentration) and electrical (separation of charge) differential Overview of Chemiosmotic Theory

4 Basic Ideas of the Chemiosmotic Theory Energy from redox reactions or light is translated into vectorial energy coupling of electron transfer to membrane bound proton pumps that transverse a proton impermeable membrane thereby establishing an electrochemical proton gradient A "proton circuit" is established protons respond to the chemical and electrical gradient across the membrane flow back across the membrane through the ATP synthase protein complex to catalyze ATP synthesis Proton Circuit

5 Basic Ideas of the Chemiosmotic Theory Vectorial H + pumping results in both: a chemical gradient across the membrane represented by ph an electrical gradient due to the separation of charge which can be measured as a membrane potential Ψ ( psi) Separation of charge is due to: build-up of positively-charged protons (H + ) on one side of the membrane accumulation of negative charges (OH - ) on the other side of the membrane Basic Ideas of the Chemiosmotic Theory In mitochondria, the contribution of Ψ ( V) to G is actually greater than that of ph (the ph across the mitochondrial membrane is only 1 ph unit) In chloroplasts, the ph contribution to G is much more significant with ph close to 3 ph units Change in free energy ( G) for a membrane transport process is the sum of the ion concentration (RT ln(c2/c1)) and the membrane potential (ZF V) In mitochondria, the ZF V term makes a larger contribution than does RT ln(c2/c1).

6 The Mitochondrion, the Powerhouse of the Cell A critical feature of the mitochondrion is the extensive surface area of the inner mitochondrial membrane which forms the proton-impermeable barrier required for chemiosmosis. Electron microscopy studies have shown that the inner mitochondrial membrane forms structures called cristae which have been estimated to cover as much as 3,000 m 2 per cell (~5 m 2 per mitochondrion). Peter Mitchell He established the Glynn Research Institute in the early 1960s with a research staff of less than twenty, and remained a private research institution for almost 30 years. Mitchell's uncle was Sir Godfrey Mitchell who owned George Wimpy and Company Limited, the largest construction company in England at the time.

7 How was Mitchell s idea proven? Using biochemical approaches: 1. "inside-out" submitochondrial membrane vesicles that could be shown to pump protons into the interior of the vesicle when oxidizable substrate was made available 2. artificial vesicles containing bacterial rhodopsin protein were exposed to light proton pumping by the bacteriorhodopsin protein resulted in both inward proton pumping ATP synthesis on the vesicle surface The Nobel Prize in Chemistry 1978 "for his contribution to the understanding of biological energy transfer through the formulation of the chemiosmotic theory Peter Mitchell's speech at the Nobel Banquet, December 10, 1978: Your Majesties, Your Royal Highnesses, Ladies and Gentlemen, Emile Zola described a work of art as a corner of nature seen through a temperament. The philosopher Karl Popper, the economist F. A. Hayek, and the art historian K. H. Gombrich have shown that the creative process in science and art consists of two main activities: an imaginative jumping forward to a new abstraction or simplified representation, followed by a critical looking back to see how nature appears in the light of the new vision. The imaginative leap forward is a hazardous, unreasonable activity. Reason can be used only when looking critically back. Moreover, in the experimental sciences, the scientific fraternity must test a new theory to destruction, if possible. Meanwhile, the originator of a theory may have a very lonely time, especially if his colleagues find his views of nature unfamiliar, and difficult to appreciate. The final outcome cannot be known, either to the originator of a new theory, or to his colleagues and critics, who are bent on falsifying it. Thus, the scientific innovator may feel all the more lonely and uncertain. On the other hand, faced with a new theory, the members of the scientific establishment are often more vulnerable than the lonely innovator. For, if the innovator should happen to be right, the ensuing upheaval of the established order may be very painful and uncongenial to those who have long committed themselves to develop and serve it. Such, I believe, has been the case in the field of knowledge with which my work has been involved. Naturally, I have been deeply moved, and not a little astonished, by the accidents of fortune that have brought me to this point; and I have counted myself lucky that I have been greatly encouraged by the love and example of the late David Keilin, and that my research associate, Dr. Moyle, has skilfully helped to mitigate my intellectual loneliness at the most difficult times. Now, I am indeed a witness of the benevolent spirit of Alfred Nobel. Last, but not least, I would like to pay a most heartfelt tribute to my helpers and colleagues generally, and especially to those who were formerly my strongest critics, without whose altruistic and generous impulses, I feel sure that I would not be at this banquet today.

8 Pathway Questions 1. What does the electron transport system/oxidative phosphorylation accomplish for the cell? Generates ATP derived from oxidation of metabolic fuels accounting for 28 out of 32 ATP (88%) obtained from glucose catabolism. Tissue-specific expression of uncoupling protein-1 (UCP1) in brown adipose tissue of mammals short-circuits the electron transport system and thereby produces heat for thermoregulation. Alternative oxidase in certain plants produces heat for pollinator attractant and growth 2. What is the overall net reaction of NADH oxidation by the coupled electron transport and oxidative phosphorylation pathway? 2 NADH + 2 H ADP + 5 Pi + O 2 2 NAD ATP +2 H 2 O Pathway Questions 3. What are the key enzymes in the electron transport and oxidative phosphorylation pathway? ATP synthase complex the enzyme responsible for converting protonmotive force (energy available from the electrochemical proton gradient) into net ATP synthesis through a series of proton-driven conformational changes. NADH dehydrogenase also called complex I or NADH-ubiquinone oxidoreductase. This enzyme catalyzes the first redox reaction in the electron transport system in which NADH oxidation is coupled to FMN reduction and pumps 4 H + into the inter-membrane space. Ubiquinone-cytochrome c oxidoreductase - also called complex III, translocates 4 H + across the membrane via the Q cycle and has the important role of facilitating electron transfer from a two electron carrier (QH 2 ), to cytochrome c, a mobile protein carrier that transfers one electron at a time to complex IV. Cytochrome c oxidase - also called complex IV pumps 2 H + into the intermembrane space and catalyzes the last redox reaction in the electron transport system in which cytochrome a3 oxidation is coupled to the reduction of molecular oxygen to form water ( O e H + H 2 O).

9 Pathway Questions 4. What are examples of the electron transport system and oxidative phosphorylation? Cyanide binds to the heme group in cytochrome a3 of complex IV and blocks the electron transport system by preventing the reduction of oxygen to form H 2 O. Hydrogen cyanide gas is the lethal compound produced in prison gas chambers when sodium cyanide crystals are dropped into sulfuric acid. The Electron Transport System Is A Series Of Coupled Redox Reactions The electron transport system consists of five large protein complexes: 1. Complex I; NADH-ubiquinone oxidoreductase (NADH dehydrogenase 2. Complex II; succinate dehydrogenase (citrate cycle enzyme 3. Complex III; Ubiquinone-cytochrome c oxidoreductase 4. Complex IV; cytochrome c oxidase 5. F 1 F 0 ATP synthase complex consisting of a "stalk" (F 0 ) and a spherical "head" (F 1 )

10 It was possible to order the four electron transport system complexes because of: Specific redox reaction inhibitors (such as rotenone, antimycin A and cyanide) Known reduction potentials (Eº') of conjugate redox pairs Metabolic Fuel for Electron Transport NADH and FADH 2 feed into the electron transport system from the citrate cycle and fatty acid oxidation pathways. Pairs of electrons (2 e-) are donated by NADH and FADH 2 to complex I and II, respectively Pairs of electrons flow through the electron transport system until they are used to reduce oxygen to form water (O e H + H 2 O). The two mobile electron carriers in this series of reactions are coenzyme Q (Q), also called ubiquinone, and cytochrome c which transfer electrons between various complexes.

11 The stoichiometry of "proton pumping" is: 4 H + in complex I 4 H + in complex III and 2 H + in complex IV (10 H + /NADH and 6 H + /FADH 2 ) The four functional components of the electron transport system: Three large multisubunit protein complexes, I, III and IV, that transverse the inner mitochondrial membrane and function as proton "pumps". Coenzyme Q (Q), also called ubiquinone, a small hydrophobic electron carrier that diffuses laterally within the membrane to donate electrons to complex III. Three membrane-associated FAD-containing enzymes (succinate dehydrogenase; complex II, electron-transferring flavoprotein; ETF, and glycerol-3-phosphate dehydrogenase) that pick up electrons from linked metabolic pathways and donate them to coenzyme Q. Cytochrome c, a small water-soluble protein that associates with the cytosolic side of the membrane and carries electrons one at a time from complex III to complex IV.

12 How is the energy released by redox reactions used to "pump" protons into the inter-membrane space? Answer: we don t completely know yet, but, it is thought to involve: a redox loop mechanism Q cycle in complex III redox-driven conformational changes : proton pump complexes I and IV

13 separation of the H + and e - on opposite sides of the membrane The Q cycle in complex III uses this mechanism to translocate protons across the membrane Redox-driven conformational changes in the protein complex "pump" protons across the membrane by altering pka values of functional groups located on the inner and outer faces of the membrane. Both complexes I and IV have properties that are consistent with such a proton pumping mechanism

14 Complex I: NADH-ubiquinone oxidoreductase Complex 1 passes 2 e - obtained from the oxidation of NADH to Q using a coupled reaction mechanism that results in the net movement of 4 H + across the membrane Contains a covalently bound flavin mononucleotide (FMN) that accepts the two electrons from NADH, as well as at least six different iron-sulfur centers (Fe-S) that carry one electron at a time from one end of the complex to the other. The poison rotenone blocks electron transfer within complex I by preventing a redox reaction between two Fe-S centers. Complex II: Succinate dehydrogenase The citrate cycle enzyme we first encountered in lecture 28. It catalyzes an oxidation reaction that converts succinate to fumarate in a coupled redox reaction involving FAD. The 2 e - extracted from succinate in the citrate cycle is passed through the other protein subunits in the complex to Q as shown below. No protons are translocated across the inner mitochondrial membrane by complex II.

15 Alternative Oxidase in Plants Fig Alternative Oxidase is involved in thermogenesis Titan Arum (Amorphophallus titanum) Voodoo Lily

16 Alternative Oxidase acts here The stoichiometry of "proton pumping" is normally: 4 H + in complex I 4 H + in complex III and 2 H + in complex IV (10 H + /NADH and 6 H + /FADH 2 ) But w/ Alternative Oxidase, The total H + pumped is only 4 Electrons from Succinate lead to NO ATP FORMATION. Complex III: Ubiquinone-cytochrome c oxidoreductase The docking site for QH 2 (ubiquinol) and consists of 11 protein subunits in each of two monomer subunits. Note the relative position of the electron carriers and the presence of two distinct binding sites for ubiquinone called QP and QN, which play a crucial role in diverting one electron at a time to cytochrome c via the Q cycle. The terms QP and QN refer to the proximity of the sites to the positive (inter-membrane space) and negative (matrix) sides of the membrane.

17 The Q Cycle (No, it isn t an invention for James Bond) Functions a both a mobile electron carrier and a "transformer" that converts the 2 e - transport system used by complexes I and II, into a 1 e - transport system required by cytochrome C. The Q cycle requires that 2 QH 2 molecules get oxidized by complex III, with one of QH 2 molecule being reformed by reduction to give a net oxidation of one QH 2 molecule. 4 Steps of the Q Cycle 1. Oxidation of QH 2 at the QP site results in transfer of one electron to the Rieske Fe-S center which is transferred to cytochrome c1 and then passed off to Cyt c. The second electron is transferred to cytochrome bl which "stores" it temporarily. The oxidation of QH 2 in this first step contributes 2 H + P to the inter-membrane space. 2. The oxidized Q molecule moves from the QP site to the QN site through a proposed substrate channel within the protein complex. This stimulates electron transfer from bl to bh which then reduces Q in the QN site to form the semiquinone Q - intermediate.

18 4 Steps of the Q Cycle 3. A new QH 2 molecule binds in the vacated QP site and is oxidized in the same way as step 1 such that one electron is transferred to cytochrome c1 and then to a new molecule of Cyt c. Oxidation of this second QH2 molecule translocates another 2 H + P into the intermembrane space (4 H + P total) and the resulting Q molecule is released into the membrane (the QN site is occupied with Q - ). 4. The second electron from the QH 2 oxidation in step 3 is passed directly from bl to bh and then used to reduce the semiquinone Q - intermediate already sitting in the QN site which uses 2 H + N to regenerate a QH 2 molecule. 4 Steps of the Q Cycle

19 To see how the Q cycle accomplishes the 2 e - 1 e e - conversion process, write out two separate QH2 oxidation reactions and then sum them to get the net reaction for complex III: QH 2 + Cyt c (oxidized) Q H + P + Cyt c (reduced) QH 2 + Q H + N + Cyt c (oxidized) Q + QH H + P + Cyt c (reduced) QH H + N + 2 Cyt c (oxidized) Q + 4 H + P + 2 Cyt c (reduced) Note that the Q cycle reactions require that 2H + N from the matrix be used to regenerate QH 2, even though 4H + P are translocated. However, this apparent imbalance of 2H + N is corrected by the redox reactions of complex IV where 2H + N are required to reduce oxygen to water and 2H + N are pumped across the membrane. Therefore, the net translocation of protons across the membrane in the combined redox reactions of complexes III and IV becomes 6 H + N 6 H + P. Cytochrome C Cytochrome c (Cyt c) is a small protein of ~13 kda that associates with the cytosolic side of the inner mitochondrial membrane and is responsible for transporting one electron at a time from complex III to complex IV using an ironcontaining heme prosthetic group. Oxidized Cyt c contains ferric iron (Fe 3+ ) in the heme group and reduced Cyt c contains ferrous iron (Fe 2+ ). A version of the Cyt c molecular structure is used in the Bioc460 website header.

20 Complex IV: Cytochrome c oxidase Complex IV accepts electrons one at a time from Cyt c and donates them to oxygen to form water. In the process, two net H + are pumped across the membrane using a conformational-type mechanism similar to complex I. Cyt c docks on the P side of the membrane to complex IV near CuA which accepts the electron leading to oxidation of the heme group in Cyt c (Fe 2+ --> Fe 3+ ). Cyanide blocks electron transfer in complex IV. ATP Currency Exchange Ratios of NADH and FADH 2 Experimental measurements demonstrate 3 H + are required to synthesize 1 ATP when they flow back down the electrochemical proton gradient through the ATP synthase complex, and 1 H + is needed to transport each negatively-charged Pi molecule into the matrix.

21 ATP Currency Exchange Ratios of NADH and FADH 2 Taking into account the requirement of 3 H + /ATP synthesized, and the use of 1 H + to translocate ADP, we can now see where the ATP currency exchange ratios of ~2.5 ATP/NADH and ~1.5 ATP/FADH 2 come from: oxidation of NADH by complex I leads to 10 H + /4 H + = 2.5 ATP oxidation of FADH 2 by complex II yields 6 H + /4 H + = 1.5 ATP for FADH 2