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Lecture 25. Tuesday, December 5. Redox Potentials

Lecture 25. Tuesday, December 5. Redox Potentials -...

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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 1 John Kuriyan: University of California, Berkeley Chem C130/MCB 100A, Fall 2006, Lecture 25 Electrochemistry Redox reactions involving metals or organic compounds are crucial in biology for energy transduction. Example of a metal reaction: Fe 3 + + e Oxidation Re duction Fe 2 + (1) Example of a biological organic co-factor:
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 2 Metals and co-factors such as NADH carry out redox reactions when bound to proteins. The proteins are assembled into macro-molecular complexes that carry out redox reactions to generate energy (ATP stores) via proton gradients.
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 3 In chloroplasts: To understand the energetics of these processes, we need to understand the free energy of redox reactions. The complete oxidation of one molecule of glucose releases considerable energy, which is ultimately coupled to ATP production: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O (2) = -2823 kJ mol -1 This is actually a redox reaction, although a complicated one: (1) C 6 H 12 O 6 + 6H 2 O 6CO 2 + 24H + + 24e - (3) (2) 6O 2 + 24H + + 24e - 12H 2 O (4)
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 4 The protons (H + ) and electrons (e - ) move separately in biochemical reactions until they come together to form water, with oxygen. If the reaction is allowed to proceed all at once, then the huge drop in free energy would be dissipated as heat. Instead, the reaction is coupled to a series of intermediate chemical steps, of intermediate energy, and this is coupled to the production of NADH: The coupling of free energy of electrons to ATP production is done by generating a proton gradient, which is accomplished by the Electron Transport Chain, a complex of membrane bound proteins:
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 5 The reduction potentials (i.e., ability to be reduced) of components of the electron transport chain are tuned so that electrons hop down the chain. The reduction potential is denoted in volts, and in this lecture we study how to relate the redox potential to the change in free energy. + 2e -
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 6 The protein complexes of the electron transport chain contain metal centers that are tuned to different reduction potentials by tinkering with chemical bonds and electrical environment. Eg. iron-sulfur clusters heme-bound metals The reduction potentials of these metals are perturbed by the protein linkages so that the energy steps in electron transfer are appropriate.
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY Lecture 25 7 Consider a piece of Zinc (Zn, solid) placed in a solution of CuSO 4 (Cu 2+ , aq.; SO 4 2- , aq).
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