(1-13) Fuller-Electron+Transport+Chain+and+sysnthsis+of+ATP

(1-13) Fuller-Electron+Transport+Chain+and+sysnthsis+of+ATP...

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Unformatted text preview: 1/12/14 The electron transport chain and ! ATP synthesis via the proton motive force! ! M. T. Fuller! ! ECB3: Ch 14 pp.453 - 476 ! Respiration and ATP synthesis • Last Thursday Glycolysis and the Citric Acid Cycle • Today Electrons from NADH to the cytochrome proteins Onward to oxygen where 217 kj of energy is released per mole of NADH The proton gradient and proton pumps ATP synthase, a rotary molecular motor In one turn of the Citric Acid Cycle CH3-COO~ ⇒ 2CO2 and 3 NADH 1 GTP 1 FADH2 GTP: synthesized by adding terminal phosphate to ATP NAD: Oxidized, has lost extra electrons, has given up electrons to oxygen NADH: reduced form NADH + H+ + 1/2 O2 ⇒ NAD+ + H2O (Reduced) (Oxidized) ∆G = - 217 kJ Figure 13-11 ECB3 (© Garland Science 2010) 1 1/12/14 Addition of an electron can draw in a proton (H+ ion) from water Reduced Form (NAD+ ) (NADH) (NAD+ ) (NADH) Figure 14-15 Essential Cell Biology (© Garland Science 2010) -Ring molecule with nitrogen allows switch -Reduced from has more energy than the oxidized state (Reduced) (Oxidized) Figure 14-5 Essential Cell Biology (© Garland Science 2010) In mitochondria, 3 cyclic processes are coupled to transport electrons H 2O NAD Citric Acid Cycle -To make NAD: add two electrons to it and you get NADH Cytochromes in respiratory chain NADH Problem: O2 Sprinting requires 60 µmoles of NAD per gram of muscle per minute, but a muscle only has 1 µmole of NAD per gram. 2 1/12/14 Mitochondria are the powerhouse of the cell Matrix: Site of CA cycle Cristae: expanded inner membrane, locus of cytochrome based electron transport chain Lots of surface area, right next to the matrix ECB3 Figure 1-18 (@2010 Garland Science) Note double membrane Dissecting proteins from the inner mitochondrial membrane Cytochrome C Coenzyme Q 43 different proteins 3 proteins Figure 14-9 Essential Cell Biology, 3/e. lowest Electrons flow down an energy hill from NADH to oxygen, releasing their energy in small steps. -As electrons move from NADH to various proteins they lose free energy and they go from a molecule that has the lowest affinity highest Electron affinity -At each step there is lower free energy -Change in energy is small enough at step to be harvested by biological mechanisms Distance along the electron transport chain As in glycolysis, small -ΔG steps between carriers allow proteins to collect the energy released when the proteins change their shape. 3 1/12/14 -Cytochromes: hold an iron ion -Cytochrome 1 can pass electrons to cytochrome 2 very easily Outer Mitochondrial membrane Inner Mitochondrial membrane (Cristae) Cytochromes NADH Matrix Space NAD+ O2 H 2O Citric Acid Cycle Enzymes Electron transfer between cytochromes Cyt-1 Fe2+ + Cyt-2 Fe3+ Cyt-1 Fe3+ + Cyt-2 Fe2+ Rust color flags an iron atom transporting an electron Structure of cytochrome C Close up of heme and its iron atom, showing the bonds to amino acid side chains in cytochrome heme - blue iron - orange -Each different cytochrome has a higher affinity as you move down -Structure of cytochrome influences how it obtains and holds electrons Heme group holding an Iron atom Tracking electrons -Coenzyme Q: small hydrophobic molecule, it can move through hydrophobic center of lipid bilayer and it can move carrying its electrons Cytochrome C Coenzyme QH2, also from succinate dehydrogenase in the inner membrane (FADH2) Figure 14-9 Essential Cell Biology, 3/e. 4 1/12/14 Mobile electron carriers CoQ and Cytochrome C A small hydophillic protein + charged surface Skates along - charged phospholipid head groups Cytochrome C -To make a molecule of water (2 from 1 oxygen) you need 4 electron to go with the 4 hydrogen nuclei -Reactive Oxygen Species: extremely dangerous, trying to get electrons from everywhere Coenzyme Q a small hydrophobic hydrocarbon that can diffuse within the lipid bilayer Electrons meet oxygen only at one place Cytochrome Oxidase Complex binds oxygen -Holds intermediates until 4 electrons have arrived Heme a3 (red) with an Fe-Cu Center, (Cu is green) to which oxygen or cyanide bind. Cyanide poisoning shows: 1. Cytochrome oxidase is responsible for 90% of total oxygen uptake by tissue. 2. When the final electron transfer step is blocked, electrons cannot flow in any part of the chain. 3. Because NADH cannot unload its electrons, the citric acid cycle stops, ATP production stops, and the victim suffocates. 4. Intermediates in the stepwise transfer of electrons to O2 are reactive oxygen species (ROS) that are powerful carcinogens and are guarded from escape. 5 1/12/14 Looking for large energy changes (CoQ) The big energy changes coincide with H+ pumps A Hydrogen ion is drawn in on the matrix side and released on the opposite side of the membrane due to conformational changes in the proteins Matrix Intermembrane space Figure 14-16 Essential Cell Biology (© Garland 2010) 6 1/12/14 Proton pumping produces a gradient across the inner mitochondrial membrane Intermembrane space Matrix Results in lower concentration of H+ in the matrix Extracted from ECB, 3/e Figure 14-11 Measure protons pumped by changes in pH Lodish MCB Fig 8-14 Mitochondria in brown fat have a special pore that allows protons to leak through the inner membrane -Ions can cross lipid bilayer, holds energy -Difference in concentration inside and outside of membrane -Brown fat is used as a heat generating source H+ thermogenin channel Hairless babies of mammals have brown fat at their neck and upper back 7 1/12/14 The proton gradient across the inner mitochondrial membrane is used to drive synthesis of ATP Figure 14-7 ECB3 (© Garland Science 2010) F1 particles are ATP synthase Electron micrographs of the inner membrane of a mitochondrion. F1 particles are lollipops: they have a spherical head 9 nm in diameter that is attached to the membrane by a stalk. Surface of Membrane (cristae) Matrix space Protons drive the synthesis of ATP when they flow through ATP synthase, and cause it to rotate -Enzyme that converts ADP to ATP: uses energy of hydrogen ions Fig 14-11 ECB, 3/e 8 1/12/14 -Matrix: top structure Rotating ATP synthase Matrix —> ß-subunit conformation Open ADP + Pi ATP Release Grab Squeeze <—Intermembrane Space Matrix Structure based on John Walker s crystallography. Image courtesy Karplus (2006) ARBBS 35: 1-47. See also Fig 14-12, ECB. Itoh et al 2004, Mechanically driven ATP synthesis by F1, Nature 427, 465. The force exerted by ATP synthase drives ADP and Pi close enough together for the covalent phosphoanhydride bond to form, regenerating ATP Energy from food via mitochondria 9 1/12/14 Enough energy is released in the oxidation of NADH to make several ATP molecules ∆G o = - 52.4 kcal/mole o ∆G = 7.3 kcal/mole Figure 14-8 ECB3 (© Garland Science 2010) Almost 50% of the energy released in the conversion of glucose to CO2 and water is captured and used to make ATP ~30 ATPs, which can be used to do work in the cell If only we could design engines to be this efficient! Figure 13-1 Essential Cell Biology (© Garland Science 2010) 10 ...
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