Oxidative phosphorylation & Photophosphorylation

Transcription

1 Oxidative phosphorylation & Photophosphorylation

2 Oxidative phosphorylation is the last step in the formation of energy-yielding metabolism in aerobic organisms. All oxidative steps in the degradation of carbohydrates, fats, and amino acids converge at this final stage of cellular respiration to synthesize ATP. Photophosphorylation is the means by which photosynthetic organisms capture the energy of sunlight the ultimate source of energy in the biosphere and harness it to make ATP. The energy of oxidation drives the synthesis of ATP. Together, oxidative phosphorylation and photophosphorylation account for most of the ATP synthesized by most organisms.

3 In eukaryotes, oxidative phosphorylation occurs in mitochondria, In plants photophosphorylation in chloroplasts.

4 chemiosmotic theory Our current understanding of ATP synthesis in mitochondria and chloroplasts is based on the hypothesis,introduced by Peter Mitchell in 1961, transmembrane differences in proton concentration are the the energy extracted from biological oxidation reactions. reservoir for This chemiosmotic theory provides insight into the processes of oxidative phosphorylation and photophosphorylation,

5 OXIDATIVE PHOSPHORYLATION In eukaryotes, oxidative phosphorylation occurs in mitochondria. The electrons from NADH and FADH 2 are used to reduce O 2 to form H 2 O. This process is independent to light energy. PHOTO PHOSPHORYLATION In plants photophosphorylation takes place in chloroplasts. In photophosphorylation NADP + as electron acceptor, H 2 O is oxidized to O 2. This process is dependent on light energy

6 Mitochondria, like gram negative bacteria, have two membranes The outer mitochondrial membrane is readily permeable to small molecules (Mr,5,000) and ions, which move freely through transmembrane channels, The inner membrane is impermeable to most small molecules and ions, including protons (H); the only species that cross this membrane do so through specific transporters. The inner membrane contain the components of the respiratory chain and the ATP synthase.

7 The mitochondrial matrix, The mitochondrial matrix enclosed by the inner membrane, contains the pyruvate dehydrogenase complex the enzymes of the citric acid cycle, the fattyacid -oxidation pathway, the pathways of amino acid oxidation all the pathways of fuel oxidation except glycolysis, which takes place in the cytosol. Oxidation of 1 mole of NADH by the respiratory chain provides sufficient energy for synthesis of ~2.5 moles of ATP from ADP.

8 To understand oxidative phosphorylation, it is important to first review the hydrogen atom and the process of oxidation and reduction An atom of hydrogen contains only one proton (H+) and one electron (e-). Therefore, the term proton and the term hydrogen ion (H+) are interchangable. Also remember that electrons have stored energy, or potential energy, ready to do work and when an atom or molecule loses that electron (becomes oxidized) that energy is released and able to do cellular work. An oxidation reaction during which both a proton (H+) and an electron are lost is called dehydrogenation A reduction reaction during which both a proton and an electron are gained is called hydrogenation.

9 The Mitochondrion: Scene of the Action Localization of respiratory processes in the mitochondrion: a) A mitochondrion from a pancreatic cell. b) Overview of oxidative phosphorylation: Reduced electron carriers, produced by cytosolic dehydrogenases and mitochondrial oxidative pathways, become reoxidized by enzyme complexes bound in the inner membrane. These complexes actively pump protons outward, creating an energy gradient whose discharge through complex V drives ATP synthesis.

10 Respiratory chain Oxidative phosphorylation begins with the entry of electrons into the chain of electron carriers called the respiratory chain. Cells use specific molecules to carry the electrons that are removed during the oxidation of an energy source. These molecules are called electron carriers and they alternately become oxidized and reduced during electron and proton transfer Most of these electrons arise from the action of dehydrogenases that collect electrons from catabolic pathways and funnel them into universal electron acceptors -These include three freely diffusible coenzymes known as NAD+, FAD, and NADP+. The reduced forms of these coenzymes (NADH, FADH2, and NADPH) have reducing power because their bonds contain a form of usable energy

11 Electron Transport Embedded within the inner membrane are the protein carriers that constitute the respiratory chain. Complex I and complex II receive electrons from the oxidation of NADH and succinate, respectively, and pass them along to a lipid-soluble electron carrier, coenzyme Q, which moves freely in the membrane. Complex III catalyzes the transfer of electrons from the reduced form of coenzyme Q to cytochrome c, a protein electron carrier that is also mobile within the intermembrane space. Complex IV catalyzes the oxidation of cytochrome c, reducing O 2 to water. The energy released by these exergonic reactions creates a proton gradient across the inner membrane, with protons being pumped from the matrix into the intermembrane space. complex V Protons then re-enter the matrix through a specific channel in.the energy released by this exergonic process drives the endergonic synthesis of ATP from ADP and inorganic phosphate.

12 Electron Transport Respiratory electron carriers in the mitochondrion: The sequence of electron carriers that oxidize succinate and NAD-linked substrates in the inner membrane. The respiratory chain catalyzes the transport of electrons from low-potential carriers to high-potential carriers.

13 Electron Transport Electron Carriers in the Respiratory Chain: Flavoproteins o Contain tightly bound FMN or FAD as redox cofactors. o Each flavoprotein provides a different microenvironment for the isoalloxazine ring, conferring a unique standard reduction potential on the flavin. o Flavin nucleotides can act as transformers between two-electron and oneelectron processes due to their ability to exist as stable one-electron reduced semiquinone intermediates. Iron sulfur proteins o Iron sulfur clusters consist of nonheme iron complexed to sulfur in four known ways. Coenzyme Q

14 Electron Transport Structures of iron sulfur clusters: The top panel illustrates the covalent attachment of a pair of clusters in a subunit of complex I from Thermus thermophilus

15 Electron Transport The cytochrome Q carrier was found to be a benzoquinone linked to a number of isoprene units, usually 10 in mammalian cells and 6 in bacteria. Because the substance is ubiquitous in living cells, one group of researchers named it ubiquinone, while another called it coenzyme Q, or Q.

16 Electron Transport Standard reduction potentials of the major respiratory electron carriers: Three reactions in the respiratory chain have DG o values greater than kj/mol, the DG o for ATP hydrolysis: FMN Q; cyt b cyt c 1 ; and cyt a O 2.

17 Electron Transport Sites of action of some respiratory inhibitors and artificial electron acceptors: This schematic of the respiratory chain from NADH to O 2 shows the sites of action of some useful inhibitors (red) and some artificial electron acceptors (blue). Each acceptor is positioned according to its value (in parentheses), identifying the most likely site at which an acceptor will withdraw electrons from the respiratory chain when added to mitochondria.

18 Proposed model for coupling of electron transport to proton pumping by complex I. Electron Transport Structure and function of complex I (NADH coenzyme Q reductase): Model of complex I from the yeast Yarrowia lipolytica X-ray structure of the entire complex I from the archaea Thermus thermophilus. Proposed model of proton translocation by complex I. NADH binds near FMN.

19 Electron Transport Structure of complex II (succinate dehydrogenase): The enzyme is composed of two hydrophilic subunits extending into the matrix, the FADbinding subunit (blue) and the iron sulfur subunit (yellow), and two transmembrane subunits (pink and gold). The path of electron transport from FAD through the three iron sulfur clusters to coenzyme Q (UQ) is shown on the right. Succinate binds near FAD in the blue subunit.

20 Electron Transport Structure of Complex III (coenzyme Q:cytochrome c oxidoreductase): a) X-ray structure of the dimeric complex b) Cartoon of the complex III dimer illustrating the arrangement of the subunits and redox carriers. The approximate location of the complex in the inner membrane is indicated.

21 Electron Transport Structure of cytochrome c oxidase (complex IV): Four electrons are donated, one at a time, by reduced cyt c, to the CuA center, through heme a, and on to the catalytic site (binuclear a 3 CuB site) where one molecule of O 2 is reduced, yielding two molecules of H 2 O. Protons are pumped from the matrix side to the intermembrane space (IMS) side of the membrane.

22 ATP Synthase ATP synthase, also called Complex V, has two distinct components: Fo and F1, a peripheral membrane protein, which is integral to the membrane.

23 Oxidative Phosphorylation Structure of the F 0 F 1 complex: The F 0 F 1 complex, also called ATP synthase or complex V, contains an F 1 knob projecting into the mitochondrial matrix and connected by a central stalk to the F 0 base. The globular F1 knob contains three ab dimers, arranged about the central stalk, which is made up of g, d and e subunits. The F 0 base is composed of c subunits (the c-ring) and one subunit a. The peripheral stalk (subunits b, d, F6, and OSCP) is attached to the F 0 base via subunit a and at least four other minor membrane-embedded subunits that are not shown. The central stalk and the c-ring compose the rotor of ATP synthase. The remainder of the subunits make up the stator, a structure that prevents the rotation of the three dimers of F 1.

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25 Complexes; Complexes I and II catalyze electron transfer to ubiquinone from two different electron donors: NADH (Complex I) and succinate (Complex II). Complex III carries electrons from reduced ubiquinone to cytochrome c, Complex IV completes the sequence by transferring electrons from cytochrome c to O2

26 Electron Transport Chain Succinate dehydrogenase Electrons flow from carriers with low to high reduction potential. This is energetically downhill.

27 Per electron pair: 4 H + in 4 H + out Electron Transport Chain Succinate dehydrogenase 4 H + in 4 H + out 2 H + in 2 H + out

28 Outline of ETS Electrons are transferred from protein to protein in the electron transport chain. Each successive protein in the transport chain can accept a lowerenergy electron. As electrons travel from a high-energy state to a low-energy state, energy is released. This energy is used to pump protons across the membrane to set up a gradient. The final electron acceptor is oxygen (O2). Oxygen has a high electronegativity; thus, oxygen s high affinity for electrons makes it an ideal acceptor for low-energy electrons. With the electrons, hydrogen is added to oxygen forming water as the final product.

29 ATP formation Fo has a proton pore through which protons leak as fast as they are pumped by electron transfer A terminal oxygen atom of ADP attacks the phosphorus atom of P i to form a pentacovalent intermediate, which then dissociates into ATP and H 2 O.

30 Malate-aspartate shuttle

31 Brain and muscles use a different NADH shutle called glycerol 3-phosphate shuttle. This shuttle transfer its protons and electrons not to Complex 1, but to ubiquinone through FAD- FADH2 to «complex III» So 1.5 ATP is produced.

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