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Electron transport chain and oxidative phosphorylation Where DOES most of the ATP come from? Remember that we have 10 molecules of NADH and 2 molecules of FADH2 These contain high-energy electrons Electron transport chain: the controlled release of this energy Oxidative phosphorylation: ATP synthesis Linked by electrochemical proton gradient
What are the electron carriers? • All (except coenzyme Q) have prosthetic groups • that can be oxidized or reduced • All (except cytochrome c) are hydrophobic (how • do you know?) • Can be distinguished by absorption spectra • Flavoproteins- use FAD or FMN as a prosthetic • group. Can transfer both electrons and protons • Iron-sulfur proteins- Fe and S complexed to • cysteine groups. Fe is electron donor and • Acceptor; does not pick up protons
Cytochromes- also contain iron, but as heme. At least 5 different kinds: b,c,c1,a, a3 All are integral membrane proteins except cytochrome c. Cytochrome c is not part of a complex and can diffuse rapidly. Cytochromes a and a3 also contain copper; they form iron-copper centers. Donates and accepts electrons Also binds oxygen until 4 protons and electrons are also bound, to form water That is one reason that you need iron and copper in your diet!
Coenzyme Q- a quinone, not a protein also known as ubiquinone Found in interior of inner membrane Can transfer electrons and protons Can itself pick up and transfer protons to intermembrane space Carriers are arranged according to their reduction potentials
I II III IV
Comparison of the complexes Complex electrons electrons protons from to transferred? I NADH coenzyme Q yes II succinate coenzyme Q no (via FAD) III coenzyme Q cytochrome c yes IV cytochrome c oxygen yes (see previous diagram for complex components)
Complex IV (cytochrome oxidase) is a terminal oxidase: passes electrons directly to oxygen Cyanide and azide bind directly to Fe-Cu center of this and therefore block all electron transport These complexes are mobile within the membrane lots of unsaturated phospholipids and very little cholesterol (very fluid membranes)
Electron transport does not yield ATP but the energy released is couple to ATP synthesis Chemiosmosis model (Mitchell, 1961) energy released from electron transfer is accompanied by pumping of protons across membrane. Gradient provides driving force for ATP synthesis Took almost 20 years to confirm this; the connection is provided by the F0F1-ATP complex (ATP synthase)
The evidence: • Protons are pumped out of the mitochondrial • Matrix. Mitochondria were suspended in • medium • In the presence of oxygen, pH of medium fell • (same with chloroplasts). Conclusion: • Protons are being pumped out. (How is still • not entirely clear) • 2. ETC carriers must be oriented so that • pumping occurs in one direction. Labeling • experiments confirmed this.
3. Synthetic vesicles with complexes I, III or IV produced proton gradients 4. Oxidative phosphorylation will not occur unless an intact membrane is formed 5. Anything that abolished the proton gradient (an “uncoupling” agent) abolishes ATP formation 6. Proton gradient drives ATP synthesis “proton motive force” tends to drive protons back across membrane (about 3-4 protons per ATP molecule
7. Artificial proton gradients can also drive ATP synthesis ATP synthase has distinct components F1- binding site/catalytic site for ATP formation F0- forms and stabilizes proton channel “stalk” (within F0) transmits energy from F0 to F1
Chemiosmosis uses energy stored in a proton gradient to drive cellular work Mitochondria- ATP synthesis Chloroplasts- likewise (except that high-energy electrons are excited by light) Prokaryotes- ATP synthesis (on plasma membrane) pumping of various molecules flagellar movement
Approximately 3 ATP per NADH 2 per FADH2 Prokaryotes generally gain more ATP from a molecule of glucose than do eukaryotes Why? NADH produced during glycolysis (in cyto- plasm) must be shuttled into mitochondrion Process ultimately requires transfer of electrons to FAD, and thus 2 fewer ATP molecules
Actual yield is probably less than that, because proton pump is used to drive transport of other Materials across inner membrane. Nevertheless, the process is highly efficient (40-50% of energy released through oxidation of glucose is actually conserved, through ATP formation
Control of cellular respiration Feedback control ATP synthesis increases if demand is high Demand is low: intermediates can be diverted into other pathways Regulation of certain enzymes is critical