Electron Transport and Oxidative Phosphorylation

Electron Transport and Oxidative Phosphorylation - C H A P...

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173 C H A P T E R 14 ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION Oxidation and Reduction The Electron Transport Chain Connections Regulation P/O Ratios Uncouplers Inhibitors You only have to look at the ATP yield from the TCA cycle, 12 of them per molecule of acetyl-CoA, to know that oxidative phosphorylation must be important. That’s where all the electrons from NADH and FADH 2 go after they’re made by the TCA cycle. Electrons usually aren’t floating around in space; they’re stuck on some atom or other. The simple consequence of this is that when one OXIDATION AND REDUCTION Oxidation is the loss of electrons. NADH is oxidized to NAD 1 . Reduction is the gain of electrons. O 2 is reduced to H 2 O.
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174 Basic Concepts in Biochemistry thing loses electrons, something else must gain them. Every oxidation of something must be coupled to the reduction of something else. The mol- ecule or atom that loses the electrons has been oxidized; the one that gains them has been reduced. Oxidants, or oxidizing agents, are com- pounds that oxidize other compounds—they are reduced in the process. Reductants, or reducing agents, are compounds that reduce other com- pounds—they are oxidized in the process. Pyruvate is reduced to lactate. Lactate is oxidized to pyruvate. NADH is oxidized to NAD 1 . NAD 1 is reduced to NADH. Pyruvate and NAD 1 are oxidizing agents. Lactate and NADH are reducing agents. As fuel molecules are oxidized, the electrons they have lost are used to make NADH and FADH 2 . The function of the electron transport chain and oxidative phosphorylation is to take electrons from these molecules and transfer them to oxygen, making ATP in the process. As electrons move down the electron transport chain, the carriers become reduced (Fig. 14-1). The next carrier oxidizes the previous car- rier, taking its electrons and transferring them on to the next carrier. Finally the electrons end up reducing oxygen to water. The cytochromes are named with letters in no particular order, making them tough to mem- orize, but you probably should learn them, at least right before the exam—after that you can look them up if you ever need to. The energetics of the electron transport steps makes the process work. Overall there’s a lot of free energy lost in the tranfer of electrons from NADH to oxygen—the overall reaction is very favorable, with an equilibrium constant that’s overwhelmingly large. At the three sites where ATPs are made (labeled I, II, and III), the reaction is the most downhill. THE ELECTRON TRANSPORT CHAIN Two electrons flowing down the chain make 3 ATP/NADH 2 ATP/FADH 2 CH 3 CHCO 2 2 1 NAD 1 OH Lactate CH 3 CCO 2 2 —— 1 NADH 1 H 1 O Pyruvate
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1 4 E e c r o n T a p d O x v P h y 7 5 H + H + H + Pyruvate TCA cycle Malate NADH NADH-CoQ reductase (flavoprotein Fe-S center) Site I Site Il Rotenone Amytal CoQ Cyt b
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This note was uploaded on 04/09/2008 for the course BIO SCI 98 taught by Professor Goulding during the Spring '08 term at UC Irvine.

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Electron Transport and Oxidative Phosphorylation - C H A P...

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