Biochemistry: A Short Course

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1 Tymoczko Berg Stryer Biochemistry: A Short Course Second Edition CHAPTER 20 The Electron-Transport Chain 2013 W. H. Freeman and Company

3 Chapter 20 Outline

Oxidative phosphorylation captures the energy of high energy electrons to synthesize ATP. The flow of electrons from NADH and FADH 2 to O 2 occurs in the electrontransport chain or respiratory chain.

4 Oxidative phosphorylation captures the energy of high energy electrons to synthesize ATP. The flow of electrons from NADH and FADH 2 to O 2 occurs in the electrontransport chain or respiratory chain. This exergonic set of oxidation reduction reactions generates a proton gradient. The proton gradient is used to power the synthesis of ATP. Collectively, the citric acid cycle and oxidative phosphorylation are called cellular respiration or simply respiration.

6 The electron transport chain and ATP synthesis occur in the mitochondria. Recall that the citric acid cycle occurs in the mitochondrial matrix.

7 The outer mitochondrial membrane is permeable to most small ions and molecules because of the channel protein mitochondrial porin. The inner membrane, which is folded into ridges called cristae, is impermeable to most molecules. The inner membrane is the site of electron transport and ATP synthesis. The citric acid cycle and fatty acid oxidation occur in the matrix.

10 Sequence data suggest that all mitochondria are descendants of an ancestor of Rickettsia prowazekii, which was engulfed by another cell.

12 The electron transport chain is a series of coupled redox reactions that transfer electrons from NADH and FADH 2 to oxygen.

A strong oxidizing agent readily accepts electrons and has a positive E 0.

13 The reduction potential E 0, or redox potential, is a measure of a molecule s tendency to donate or accept electrons. A strong reducing agent readily donates electrons and has a negative E 0. A strong oxidizing agent readily accepts electrons and has a positive E 0. The standard free energy change is related to the change in reduction potential. where n is the number of electrons transferred and F is the Faraday constant.

16 Energy is released when high energy electrons are transferred to oxygen. The energy is used to establish a proton gradient.

17 The electron transport chain is composed of four large protein complexes. The electrons donated by NADH and FADH 2 are passed to electron carriers in the protein complexes. The carriers include flavin mononucleotide (FMN), iron associated with sulfur in proteins (iron sulfur proteins), iron incorporated into hemes that are embedded in proteins called cytochromes, and a mobile electron carrier called coenzyme Q (Q).

22 Coenzyme Q is derived from isoprene. Coenzyme Q binds protons (QH 2 ) as well as electrons, and can exist in several oxidation states. Oxidized and reduced Q are present in the inner mitochondrial membrane in what is called the Q pool.

24 Electrons flow from NADH to O 2 through three large protein complexes embedded in the inner mitochondria membrane. These complexes pump protons out of the mitochondria, generating a proton gradient. The complexes are: NADH Q oxidoreductase (Complex I) Q cytochrome c oxidoreductase (Complex III) Cytochrome c oxidase (Complex IV) An additional complex, succinate Q reductase (Complex II), delivers electrons from FADH 2 to Complex III. Succinate Q reductase is not a proton pump.

27 Electron flow within the complexes in the inner mitochondrial membrane generate a proton gradient. These complexes appear to be associated with one another in what is called the respirasome.

28 The electrons from NADH are passed along to Q to form QH 2 by Complex I. QH 2 leaves the enzyme for the Q pool in the hydrophobic interior of the inner mitochondrial membrane. Four protons are simultaneously pumped out of the mitochondria by Complex I.

30 Succinate dehydrogenase of the citric acid cycle is a part of the succinate Q reductase complex (Complex II). The FADH 2 generated in the citric acid cycle reduces Q to QH 2, which then enters the Q pool.

31 Electrons from QH 2 are used to reduce two molecules of cytochrome c in a reaction catalyzed by the Q cytochrome c oxidoreductase or Complex III. Complex III is also a proton pump.

32 QH 2 carries two electrons while cytochrome c carries only one electron. The mechanism for coupling electron transfer from QH 2 to cytochrome c is called the Q cycle. In one cycle, four protons are pumped out of the mitochondria and two more are removed from the matrix.

34 Cytochrome c oxidase accepts four electrons from four molecules of cytochrome c in order to catalyze the reduction of O 2 to two molecules of H 2 O. In the cytochrome c oxidase reaction, eight protons are removed from the matrix. Four protons, called chemical protons, are used to reduce oxygen. In addition, four protons are pumped into the intermembrane space.

40 The area at the mouth of the Mississippi is nutrient rich because of agricultural runoff. Phytoplankton and aerobic bacteria proliferate to such an extent that the oxygen concentration in the water falls. Fish cannot survive under these conditions. Consequently, the area is called a dead zone.

Partial reduction of O 2 generates highly reactive oxygen derivatives, called reactive oxygen species (ROS). ROS are implicated in many pathological conditions.

42 Partial reduction of O 2 generates highly reactive oxygen derivatives, called reactive oxygen species (ROS). ROS are implicated in many pathological conditions. ROS include superoxide ion, peroxide ion, and hydroxyl radical. Two to four percent of oxygen molecules consumed by mitochondria are converted into superoxide ions.

43 Superoxide dismutase and catalase help protect against ROS damage.

Oxidative Phosphorylation

Oxidative Phosphorylation Energy from Reduced Fuels Is Used to Synthesize ATP in Animals Carbohydrates, lipids, and amino acids are the main reduced fuels for the cell. Electrons from reduced fuels are