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OXIDATIVE PHOSPHORYLATION
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Oxidative Phosphorylation The process in which ATP is formed as a result of the transfer of electrons from NADH or FADH 2 to oxygen by a series of electron carriers Takes place in the mitochondria Electron flow proton flow pH gradient and transmembrane electrical potential proton motive force
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Mitochondria 2 µm in length; 0.5 µm in diameter Outer membrane is permeable to small molecules and ions because of the porins (VDAC) Inner membrane impermeable 2 faces: matrix (neg) cytosol (pos)
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REDOX CONCEPTS A strong reducing agent donates electrons and has negative reduction potential while a strong oxidizing agent accepts electrons and has positive reduction potential Standard reduction potential (Eo) How much energy will be produced from the reduction of oxygen with NADH?
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Electron carriers Flavins Iron-sulfur clusters Quinones Hemes Copper ions
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Flavins The isoalloxazine ring can undergo reversible reduction accepting either 1 or 2 electrons in the form of either 1 or 2 hydrogen atoms Variability in standard reduction potential is also an important feature
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Iron – Sulfur Clusters
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Iron – Sulfur Proteins Iron is not present in the heme but in association with inorganic sulfur atoms or the sulfur of cysteine. Rieske iron-sulfur proteins are a variation in which 1 iron atom is coordinated with 2 His residues All iron-sulfur proteins participate in 1 electron transfer There are at least 8 Fe-S clusters in the respiratory chain
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Quinones Ubiquinone or Coenzyme Q Can accept 1 or 2 electrons Can act at the junction between 2-electron donor and 1-electron acceptor because it is freely diffusable Plays a central role in coupling electron flow and proton movement because it carries both electrons and protons
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Hemes (cytochromes)
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Hemes (cytochrome) 3 classes: a, b, c (difference in light absorption spectra) Of the three, the heme of cytochrome c is covalently bonded to the protein The standard reduction potential of the hemes depends on its interaction with the protein side chains
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The Four Complexes of the Respiratory Chain NADH – Q oxidoreductase (Complex I) Succinate – Q reductase (Complex II) Q – cytochrome c oxidoreductase (Complex III) Cytochrome c oxidase (Complex IV)
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NADH – Q oxidoreductase Aka NADH dehydrogenase MW: 880 kDa Consists of at least 34 polypeptide chains Prosthtic groups: FMN and Fe-S clusters Catalyzes 2 simultaneous and obligately coupled processes
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NADH-Q oxidoreductase
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NADH – Q oxidoreductase 1. Exergonic transfer to ubiquinone of a hydride ion from NADH and a proton from the matrix 2. Endergonic transfer of four protons from the matrix to the intermembrane space
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Succinate – Q reductase Composed of 4 subunits Prosthetic groups: FAD and Fe-S No transport of protons for enzymes that transport electrons from FADH 2. Hence, less ATP is produced for the oxidation of FADH 2
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Cytochrome An electron transferring protein that contains a heme prosthetic group The iron alternates between reduced and oxidized forms during electron transport Q- cytochrome c oxidoreductase catalyzes the transfer of electrons from QH 2 to oxidized cytochrome c and concommitantly pump protons out of the mitochondrial matrix
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Q – Cytochrome c oxidoreductase (Cytochrome bc 1 complex)
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Cytochrome bc 1 complex A dimer with each monomer containing 11 subunits Contains 3 hemes 2 b-types (b H and b L ) 1 c-type The enzyme also contains Rieske center It also has 2 binding sites : Q 0 and Q i Q -cycle
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Q - cycle
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Cytochrome c oxidase Catalyzes the reduction of molecular oxygen to water Oxidation of the reduced Cyt c generated in complex III w/c is coupled w/ reduction of oxygen to 2 molecules of water
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Cytochrome c oxidase The enzyme contains 2 heme A groups and 3 copper ions arranged as 2 copper centers, A (Cu A /Cu A ) and B (Cu B ) heme A (yellow) is composed of heme a and heme a3 Cu A (blue) contains 2 copper ions linked by bridging cysteine residues
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Cytochrome c oxidase Heme a and a3 are located in different environments within the enzyme Heme a carries electrons from Cu A /Cu A Heme a3 passes electrons to Cu B Heme a3 and Cu B form the active center at which the oxygen is reduced to water
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Cytochrome c oxidase mechanism
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ATP synthesis ΔG˚’ = -52.6 kcal / mol ΔG˚’ = +7.3 kcal / mol
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ATP synthase Membrane embedded enzyme 2 subunits: F 1 and F o F 1 : protrudes from the mitochondrial matrix and contains the catalytic activity : α 3 β 3 γ δ ε : alpha and beta units are arranged hexamerically : beta subunit participates in catalysis : gamma subunit breaks the symmetry of the alpha and beta hexamer.
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ATP synthase Fo : hydrophobic segment that spans the inner mitochondrial membrane : contains the proton channel of the complex : consists of a ring comprising 10 – 14 c subunits embedded in the membrane : a single a subunit binds outside the ring * The role of the proton gradient is not to form ATP but to release it from the synthase
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Binding –Change Mechanism The changes in the properties of the three β subunits allows sequential ADP and Pi binding, ATP synthesis and ATP release Three conformations for the β subunit: T (tight) – binds ATP with great avidity but cannot release the ATP L (loose) – bind ADP and Pi but cannot release ADP and Pi O (open) – can exist with a bound nucleotide like T and L but it can also convert to form a more open conformation and release bound molecules The interconvertion of these three forms can be driven by the rotation of the γ subunit
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Proton flow around the c ring The mechanism depends on the structures of a and c subunit of Fo Each polypeptide chain forms a pair of α –helices that span the membrane An aspartic acid (Asp61) is found in the middle of the second helix The a subunit consists of two proton half channels that do not span the membrane The a subunit directly abuts the ring comprising the c subunits, with each half channel directly interacting with one c subunit
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a and c subunits of Fo
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INHIBITORS OF THE ETC Rotenone - blocks complex I Amytal – blocks complex I Antimycin A – blocks complex III Cyanide – blocks complex IV
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