OXIDATIVE PHOSPHORYLATION. Oxidative Phosphorylation The process in which ATP is formed as a result...
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Transcript of OXIDATIVE PHOSPHORYLATION. Oxidative Phosphorylation The process in which ATP is formed as a result...
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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 FADH2 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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EnFG
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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
PN HQHNADQHNADH 45 2
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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 FADH2. Hence, less ATP is produced for the oxidation of FADH2
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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 QH2 to oxidized cytochrome c and concommitantly pump protons out of the mitochondrial matrix
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Q – Cytochrome c oxidoreductase (Cytochrome bc1 complex)
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Cytochrome bc1 complex
A dimer with each monomer containing 11 subunits
Contains 3 hemes 2 b-types (bH and bL) 1 c-type
The enzyme also contains Rieske center It also has 2 binding sites : Q0 and Qi 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 (CuA/CuA ) and B (CuB)
heme A (yellow) is composed of heme a and heme a3
CuA (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 CuA
/CuA Heme a3 passes electrons to CuB Heme a3 and CuB 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
NADOHHONADH 222
1
OHATPHPADP i 2
ΔG˚’ = -52.6 kcal / mol
ΔG˚’ = +7.3 kcal / mol
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ATP synthase
Membrane embedded enzyme 2 subunits: F 1 and Fo
F1 : 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