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Oxidative Phosphorylation Part 1
electron transfer in mitochondria
20
Biochemistry
02/23/2011

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Term
Oxidative Phosphorylation- what is it?
Definition
  • all oxidative steps in degradation of carbs, fats and amino acids converge at this final stage of cellular respiration
  • energy of oxidation drives the synthesis of ATP
  • mitochondria and chloroplasts
  • involves the reduction of O2 to H2O with electrons donated by NADH and FADH2
  • oxidative and photophosphorylation both:involve the flow of electrons through membrane bound carriers, the free energy made available by this exergonic electron flow is coupled to uphill transport of protons against a membrane, and the transport of the protons down their [] gradients thru specific channels provides free energy for ATP synthesis
Term
What Occur where in the Mitochondria?
Definition
  • Mitochondrial matrix contains all oxidative pathways except glycolysis (cytosol)
  • specific transporters carry pyruvate, fatty acids, and amino acids or their a-keto derivatives from the inner membrane into the matrix for access into the citric acid cycle
  • [image]
Term
Electrons are Funneled to Universal Electron Acceptors: NAD(+)
Definition
  • dehydrogenases collect electrons in catabolic pathways and funnel them to electron acceptors-NAD/P(+) or FAD/FMN
  • nicotinamine linked dehydrogenases catalyze: reduced substrate + NAD/P(+) <--> oxidized substrate + NADH + H(+)
  • NAD linked dehydrogenases romove 2 hydrogens from substrate, one H:(-) goes to NAD(+) and one is released at H(+)
  • these are water soluble and associate reversibly with dehydrogenases
  • NADH carries electrons from catabolic reactions into resp. chain (NADH dehydrogenase complex)
  • NADPH supplies electrons to anabolic reactions
Term
Electrons are Funneled to Universal Electron Acceptors: FAD
Definition
  • dehydrogenases collect electrons in catabolic pathways and funnel them to electron acceptors-NAD/P(+) or FAD/FMN
  • flavoproteins contain tightly bound FMN or FAD
  • Oxidized flavin nucleotide can accept one electron(semiquinine) or two (FADH2)
  • electron transfer occurs because the flavoprotein has higher reduction potential than the compound oxidized
  • the std reduction potential of the flavin nucleotide, unlike NAD, depends on the protein with which it is associated(entire flavoprotein)- local interactions with functional groups in the protein distort electron orbits in flavin and relative stabilities of oxidized/reduced forms are changed
Term
Electrons Pass Through Membrane Bound Carriers
Definition
  • Mitochondrial respiratory chain consists of sequentially acting electron carriers, most of whihc are integral proteins with prosthetic groups which accept/donate one or two electrons
  • Three types of electron transfers occur in ox. phosphorylation: direct transfer of electrons, transfer as hydrogen atom(H(+) and e(-)) and transfer as two electron hydride ion (:H(-))
Term
Reducing Equivalent
Definition
  • designates a single eectron equivalent transferred in a redox reaction
Term
Types of Electron Carrying Molecules in Respiratory Chain
Definition
  • NAD, Flavoproteins, hydrophobic quinone, cytochromes and iron-sulfur proteins
Term
Types of Electron Carrying Molecules in Respiratory Chain: Ubiquinone
Definition
  • Called Q
  • lipid soluble
  • can accept one or two electrons and can act at the junction between a two electron donor and a one electron acceptor (like flavoprotein carriers)
  • because small and can freely diffuse within the inner mitochondrial membrane, it can shuttle electrons between other less mobile electron carriers
  • carries bothe protons and electrons and couples electron flow to proton movement
  • [image]
Term
Types of Electron Carrying Molecules in Respiratory Chain: Cytochromes
Definition
  • Strong absorption of visible light
  • three types: a,b,c
  • prosthetic groups have porphyrin with central Fe atom
  • std reduction potential depends on interaction w/ protein side chains
  • mostly inner mitochondrial membrane proteins
  • [image]
Term
Iron Sulfur Proteins
Definition
  • participate in one-electron transfers in which one iron atom of iron-sulfur cluster is oxidized or reduced
  • [image]
Term
Sequence of Electron Carriers
Definition
  • standard reducton potentials of individual electron carriers are known, and those with lower reduction potential transfer their electrons to those with higher electron potentials
  • order is: NADH-->Q-->cytochrome b-->cytochrome c1-->cytochrome c-->cytochome a-->cytochrome a3-->O2
  • this may/may not be cellular order because cells have varying [oxidzed]:[reduced] carriers
Term
Electron Carriers Function in Multienzyme Complexes
Definition
  • A membrane embedded supromelecular complex of all electron carriers can be separated into four complexes
  • complexes I and II transfer electrons to ubiquinone from two different electron donors: NADH(complexI) and Succinate (complex II)
  • complex III carries electrons from reduced ubiquinone to cytochrome c
  •  complex IVtransfers electrons from cytochrome c to O2
Term
Complex I: NADH to Ubiquinone
Definition
  • also called NADH dehydrogenase
  • catalyzes two simultaneous and coupled processes: 1) exergonic transfer to ubiquinone of a hydride ion from NADH and a proton from the matrix
  • NADH + H(+) + Q--> NAD(+) + QH2
  • and 2)endergonic transfer of four protons from the matrix to the intermembrane space
  • complex I is a proton pump driven by electron transfer( and intermembrane space becomes positively charged)
  • QH2(ubiquinol) diffuses from complex I to III where it is oxidized to Q in a process that involves the outward movement of H(+)
  • [image]
  • [image]
Term
Complex II: Succinate to Ubiquinone
Definition
  • succinate dehydrogenase
  • A transmembrane complex with 5 prosthetic groups and four subunits
  • [image]
Term
Complex III: Ubiquinone to Cytochrome C
Definition
  • couples transfer of electrons from QH2 to cytochrome C while transporting protons from the matrix to the intermembrane space
  • add Q cycle and complex III pictures
Term
Complex IV: Cytochrome C to O2
Definition
  • in the final step of the respiratory chain, complex IV carries electrons from cytochrome c to O2, forming water
  • for every four electrons passing through this complex, the enzme consumes four substrate H(+)'s from the matrix side of the complex
  • it uses the energy of this redox reaction to pump one proton outward into the intermembrane space for ea/ electron that goes through, adding to the electrochemical potential created by complexes I and III
  • overall rxn is 4 Cyt c(reduced) + 8 H(+) + O2 --> 4 Cyt c (oxidized) + 4 H(+) + 2 H2O
  • (show figure 19-14)
Term
The Energy of Electron Transfer is efficiently conserved in a proton gradient and summary of reaction
Definition
  • Transfer of 2 electrons from NADH in the resp. chain: NADH + H(+) + 1/2 O2-->NAD(+) + H2O
  • very exergonic, - 220kj/mol
  • high energy is used to pump protons out of the matrix
  • for each pair of electrons transferred to O2, 4 protons are pumped out by complex I, four by complex III and 2 by complex IV
  • NADH + 11H(+) + 1/2 O2--> NAD(+) + 10H(+) + H2O
Term
Proton Motive Force
Definition
  • electrochemical energy inherent in difference in proton concentration and separation of charge represents a stored energy - called proton motive force
  • 2 components: 1) chemical potential energy due to the differential concentration of a chemical species(H(+) separated by a membrane 2)electrical potential energy that results from the separation of charge when a proton moves across a membrane w/o counter ion
  • when protons spontaneously flow down their electrochemical gradient, energy is made available to do work. this drives the synthesis of ATP

 

Term
ATP synthesis
Definition
  • Chemiosmotic model:
  • electrochemical gradient due to proton concentration and separation of charge accross the inner mitochondrial membrane.
  • This drives the synthesis of ATP as protons flow down the [] gradient through a pore associated with ATP synthase
  • show figure 19-19
Term
Definition