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Transcript
Electron Transport System 1 There are 2 Ways to Make ATP 1. Substrate phosphorylation 2. Electron transfer-dependent oxidative phosphorylation 2 2 Glycolytic Reactions Make ATP by Substrate-level Phosphorylation --1,3-BPG is an energy –rich molecule with a greater phosphoryl-transfer potential than that of ATP. Thus, it can be used to power the ATP synthesis from ADP. --This is called substrate-level phosphorylation because the phosphate donor is a Substrate with high phosphoryl-transfer potential. 3 2 Glycolytic Reactions Make ATP by Substrate-level Phosphorylation PEP has high phosphoryl-transfer potential, pyruvate (ketone) is much more stable than enol form. 4 There are 2 Ways to Make ATP 1. Substrate phosphorylation 2. Electron transfer-dependent oxidative phosphorylation 5 How do we obtain lots of ATP? Food (carbohydrates) Glucose Glycolysis TCA Glycolysis ATP Little (~4 ATP) After TCA cycle, energy is extracted In the form of reduced Coenzymes, FADH2 and NADH Reduced coenzymes (NADH + H+, FADH2) O2 ETC H2O Lots (~28-30 ATP) ATP Electron transport and Oxidative phosphorylation: Involved many steps, Sequestered in special environment. 6 Glucose Minimal TCA Cycle NADH + H+ Pyruvate O NADH + H+ CH3C-SCoA (2C) CoASH 4C 6C NADH + H+ NADH + H+ CO2 FADH2 4C GTP GDP NADH + CO2 H+ 1 GTP 3 NADH +1 FADH2 10 ATP/cycle And releases two CO2 NOTE: 1 NADH 2.5 ATP; 1 FADH2 1.5 ATP; 1 GTP 1 ATP so get 1 + 7.5 + 1.5 = 10 ATP/cycle 7 Where in the cell does electron transport and oxidative phosphorylation occur? 8 9 Mitochondria TCA enzymes b-oxidation ATP synthase Permeable Outer Mtch Membrane Intermembrane Space Inner Mtch Membrane e- transport chain M DNA Matrix 10 Mitochondria --A mitochondrion is bounded by a double membrane, with an intermembrane space. --Outer M: permeable to most ions and small molecules --The inner membrane: highly impermeable, Highly folded “cristae”. most molecules require transporters (exceptions: O2, CO2). provide large surface area for the transport proteins, several FAD-dependent dehydrogenases and all enzymes and proteins of oxidative phosphorylation --The matrix is the fluid-filled interior of the mitochondrion. oxidative enzymes like pyruvate dehydrogenase (acetyl Co A formation) glutamate dehydrogenase, TCA cycle enzymes, fatty acid oxidation enzymes --Note that glycolysis occurs outside the mitochondrion in the cytosol, whereas the citric acid cycle occurs in the matrix. --The electron transport system is located on the cristae, both TCA cycle and11 oxidative phosphorylation occur within the mitochondrion. Electron Transport System (ETS) The electron transport system is located in the cristae of mitochondria It is a series of protein/prosthetic group carriers that pass electrons from one to the other. Electrons are donated to the ETS by NADH and FADH2 As a pair of electrons is passed from carrier to carrier, energy is released and is used to form ATP At the end of the electron transport chain, oxygen receives the energy-spent electrons, resulting in the production of water. ½ O2 + 2 e- + 2 H+ → H2O (Oxygen is the final electron acceptor) 12 Redox Reactions reduction e- A e+ B A + B oxidation O oxidation R I is I L loss of electrons G reduction is gain of electrons Reductant (A): is oxidized, electron donor Oxidant (B): is reduced, electron acceptor How are redox potentials determined? 14 Half cell reactions measure electromovtive force Ethanol gives up e to H+ to form H2 H2 gives up e to Fe3+ to form H+ Oxidant Reductant Reductant Oxidant Standard: 1M H+ 1atm H2 gas E0’ of H+/H2 is 0 volts Sample Reference Neg value = oxidized form has a lower affinity for electrons than does H2 (e.g., NADH a strong reducing agent has a negative reduction potential) Pos value = oxidized form has a higher affinity for electrons than does H2 (e.g., Oxygen a strong oxidizing agent has a positive reduction potential) 15 --Biochemists use E0’, the value at pH 7. --Chemists use E0, the value in 1M H+. --The prime denotes that pH 7 is the standard state. --Thus, these values are different in chem textbooks. A strong reducing agent, NADH is poised to donate electrons, has a negative reduction potential, whereas a strong oxidizing agent O2 is ready 16 to accept electrons and has a positive reduction potential. Partial reactions By convention, reduction potentials (as in Table 18.1) refer to partial reactions are written as: oxidant + e- reductant OVERALL REACTION 17 Redox reactions Redox pairs act as e- carriers Reductant + oxidant oxidized reductant + reduced oxidant Free energy is released in the transfer of ereduction (RIG) e- A e+ B A + B oxidation (OIL) 18 Standard free-energy changes of an oxidationreduction reaction can be determined DG0’: standard free energy change – for a redox reaction • is related to the difference in E0 between the e- acceptor and donor DG0’ = -nFDE’0 DG0’ = standard free-energy change F= faraday constant = 23.06 kcal/mol/V (required to remember!) n = number of electrons DE’0 = Change in reduction potential 19 Determining: DG0’: standard free energy change DE’0 = E’0 (acceptor) - E’0 (doner) Pyruvate NADH DG0’ = -nFDE’0 F= faraday constant = 23.06 kcal/mol/V n = number of electrons DG0’ = = = -2 x 23.06 kcal/mol/V x [-0.19 – (-0.32) V ] -2 x 23.06 kcal/mol/V x 0.13V -6.0 kcal/mol 20 1.14 Volt potential favors formation of proton gradient Acceptor donor DG0’ = -nFDE’0 = -nF (E’0 acceptor – E’0 donor) = -2 x 23.06 kcal/mol/V x [0.82V- (-0.32V)] = -2 X 23.06 kcal/mol/V x 1.14V = -52.6 kcal/mol Note: DG0’ = -7.3 kcal/mol for the hydrolysis of ATP The driving force of oxi phos is the elec-trans potential of NADH or FADH2 rel. 21 to that of O2. The released energy is used to generate a proton gradient, then for ATP synthesis Driving e- Transport Electron carriers at the beginning of the chain are more - E0’ than those at the end – so e- flow spontaneously from NADH (E’0 = –0.32 v) or FADH2 (E’0 = –0.22V) to O2 (E’0 = +0.82 volts) Neg reduction potential = oxidized form has a lower affinity for electrons and so transfers them most easily to an acceptor Pos reduction potential = will be the strongest oxidizing substance and 22 have a higher affinity for electrons The electron transport system consists of four protein complexes and two mobile carriers. NADH-Q Oxidoreductase Succinate-Q reductase complexes Q-cytochrome c Oxidoreductase Cytochrome c Oxidase Coenzyme Q carrier Cytochome c The mobile carriers transport electrons between the complexes, which also contain electron carriers. The carriers use the energy released by electrons as they move down the carriers to pump H+ from the matrix into the intermembrane space of the mitochondrion. 23 NAD+/NADH Fumarate/ Succinate Cytochrome C (+3) / (+2) 24 A very strong electrochemical gradient is established with few H+ in the matrix and many in the intermembrane space. The cristae also contain an ATP synthase complex through which hydrogen ions flow down their gradient from the intermembrane space into the matrix. The flow of three H+ through an ATP synthase complex causes a conformational change, which causes the ATP synthase to synthesize ATP from ADP + P. 25 Mitochondria produce ATP by chemiosmosis, so called because ATP production is tied to an electrochemical gradient, namely an H+ gradient. Once formed, ATP molecules are transported out of the mitochondrial matrix. 26 Mitchell’s Postulates for Oxidative Phosphorylation 1. The respiratory and photosynthetic electron transfer chains should be able to establish a proton gradient 2. The ATP synthases should use the proton-motive force to drive the phosphorylation of ADP 3. Energy-transducing membranes should be “impermeable” to protons. If proton conductance is established (uncouplers), a proton-motive force should not form and ATP synthesis should not occur. 4. Energy-transducing membranes should possess specific exchange carriers to permit metabolites to permeate in the presence of high membrane potential Intermembrane ADP ATP-ADP Antiporter Mitochondrial matrix ATP H+ H+ H+ H+ e27 ADP + Pi ATP