Electron Transport and Oxidative Phosphorylation

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Electron TransportandOxidative Phosphorylation
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Sites of NADH and FADH2 Formation
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Sites of NADH and FADH2 Formation
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Mitochondrial Electron Transport Chain

• System of Linked
• Electron Carriers

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Components of Electron Transport Process

• Reoxidation of NADH and FADH2
• Sequential oxidation-reduction of multiple redox
  centers (four enzyme complexes)
• Production of proton gradient across the
  mitochondrial membrane

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Oxidative Phosphorylation

• Synthesis of ATP driven by free energy of
  electrochemical gradient

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Coupling of Electron Transport and ATP Synthesis
NOTE ATP Synthesis in the Mitochondrion
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The Mitochondrion
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X-Ray Structure of E. coli OmpF Porin
Figure 9-23a
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X-Ray Structure of E. coli OmpF Porin Trimer
Figure 9-23b
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Kinetics and MechanismsofTransport
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Types of Transport

• Nonmediated Transport (Diffusion)
• Mediated Transport (Transport Proteins)
• Passive-mediated Transport(facilitated
  diffusion)
• Active Transport

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Terminology

• Carriers
• Permeases
• Porters

• Translocases
• Translocators
• Transporters

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Non-mediated Transport(Permeability Coefficient)
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Mediated Transport(Transport Proteins)
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Mediated Transport Properties

• Saturation kinetics
• Speed and specificity
• Susceptibility to competitive inhibition
• Susceptibility to chemical inactivation

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Properties of Transporters
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Stoichiometry
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Electrical Character

• Electroneutral
• Electrogenic

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Entry of NADH into Mitochondria

• No NADH Transporter

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MalateAspartate Shuttle
Figure 16-20
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Glycerophosphate Shuttle
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ADP-ATP Translocator
ADP/ATP Exchanger Electrogenic Antiporter Driven
by electrochemical gradient
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Phosphate Transport

• Electroneutral PiH Symport
• Driven by ?pH

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Phosphate Transport
H(out) H2PO4(out)
H(in) H2PO4(in)
Electroneutral Symport
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Electron Transport is an Exergonic Process
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Standard Reduction Potentials
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Standard Reduction Potential Difference

• ?Eo Eo(e acceptor) Eo(e donor)
• ?Go nF?Eo

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Half Reactions of Oxidation of NADH by O2
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Overall Reaction(Oxidation of NADH by O2)
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ATP Synthesis
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Efficiency of Electron Transport and Oxidative
Phosphorylation
Efficiency 35 (standard
biochemical conditions)
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Efficiency of Electron Transport and Oxidative
Phosphorylation
70 (physiological conditions) (automobile
engine 30)
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Electron Carriers Operate in Sequence
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Overview of Electron Transport in the
Mitochondrion
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Electron Transport Complexes

• Complex I NADHCoenzyme Q Oxidoreductase
• Complex II SuccinateCoenzyme Q Oxidoreductase
• Complex III Coenzyme QCytochrome c
  Oxidoreductase
• Complex IV Cytochrome c Oxidase

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Mobile Electron Carriers

• Coenzyme Q
• Cytochrome c

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Coenzyme Q
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Oxidation States of Coenzyme Q
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Oxidation States of Coenzyme Q
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Cytochromes

• Electron Transport Heme Proteins

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Hemes

• Fe3 e gt Fe2

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Heme a
Note isoprene side chain
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Heme b
Iron-Protoporphyrin IX Myoglobin and Hemoglobin
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Heme c
NOTE Thioether Links
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Cytochrome Spectra
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Thermodynamics of Electron Transport Complexes
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ATP Synthesis
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Complex I(NADHCoenzyme Q Oxidoreductase)
NADH CoQ (oxidized) gt NAD CoQ
(reduced) ?Eo 0.360 V ?Go
69.5 kJ/mol
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Complex II(SuccinateCoenzyme Q Oxidoreductase)
Succinate EFAD gt Fumarate
EFADH2 EFADH2 CoQ (oxidized) gt EFAD
CoQ (reduced) ?Eo 0.085 V ?Go
16.4 kJ/mol
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Complex III(Coenzyme QCytochrome c
Oxidoreductase)
CoQ (reduced) 2 Cytochrome c (oxidized) gt
CoQ (oxidized) 2 Cytochrome c (reduced) ?Eo
0.190 V ?Go 36.7 kJ/mol
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Complex IVCytochrome c Oxidase)
4 Cytochrome c (reduced) 4 H O2 gt 4
Cytochrome c (oxidized) 2 H2O ?Eo 0.580 V
?Go 112 kJ/mol
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Reduction Potentials of Electron Transport Chain
Components
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Mitochondrial Electron Transport Chain
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Complex I

• Accepts Electrons from NADH
• NADH CoQ(oxidized) gt NAD CoQ(reduced)

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Coenzymes of Complex I(Flavin Mononucleotide,
FMN)
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Oxidation States of FMN
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Oxidation States of FMN
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Coenzymes of Complex I(Iron-Sulfur Clusters)
One-electron oxidation-reduction Conjugated
System (Fe between 2 and 3
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Thermodynamics of Complex I
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Hydrophilic Domain of Complex I from Thermus
thermophilis
Electrons follow a multistep path
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Structure of Bacteriorhodopsin
Figure 9-22
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Retinal Prosthetic Group
NOTE Schiff Base
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Proton Wire
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Proton Movement

• Deprotonation of Schiff base and protonation of
  Asp 85
• Proton release to the extracellular surface
• Reprotonation of the Schiff base and
  deprotonation of Asp 96
• Reprotonation of Asp 96 from the cytoplasmic
  surface
• Deprotonation of Asp 85 and reprotonation of the
  proton release site

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Complex II

• Contributes Electrons to Coenzyme Q
• Succinate CoQ(oxidized) gt Fumarate
  CoQ(reduced)

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Composition of Complex II

• Succinate Dehydrogenase
• FAD
• 4Fe-4S cluster
• 3Fe-4S cluster
• 2Fe-2S cluster
• Cytochrome b560

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Thermodynamics of Complex II
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E. coli Complex II
Mitochondrial Matrix
Mitochondrial Inner Membrane
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Complex II(Linear Chain of Redox Cofactors)
Cytochrome b560 scavenges electrons to prevent
formation of reactive oxygen species
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Mitochondrial Electron Transport Chain
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Complex III

• Translocates Protons via the Q Cycle
• CoQ(reduced) 2 Cytochrome c (oxidized) gt
• CoQ(oxidized) 2 Cytochrome c (reduced)

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Oxidation States of Coenzyme Q
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Composition of Complex III

• Cytochrome b562 (bH high potential)
• Cytochrome b566 (bL low potential)
• Cytochrome c1
• 2Fe2S cluster (ISP)

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Thermodynamics of Complex III
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Yeast Complex III
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The Q Cycle(Electrons from CoQH2 follow two
paths)
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Cycle 1
Matrix
IMS
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Steps in Cycle 1

• CoQH2 supplied by Complex I from matrix side
• CoQH2 diffuses to IMS side and binds to Qo site
• CoQH2 transfers one electron to ISP and releases
  2 H into IMS yielding CoQ ISP reduces
  cytochrome c1
• CoQ transfers electron to cytochrome bL
  yielding CoQ
• CoQ diffuses to the matrix side and binds to Qi
  site
• Cytochrome bL transfers electron to cytochrome bH
• CoQ in Qi site reduced to CoQ by cytochrome bH

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Summary of Cycle 1
CoQH2 Cytochrome c1 (Fe3) gt CoQ
Cytochrome c1 (Fe2) 2 H (IMS)
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Cycle 2
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Steps in Cycle 2

• CoQH2 supplied by Complex I from matrix side
• CoQH2 diffuses to IMS side and binds to Qo site
• CoQH2 transfers one electron to ISP and releases
  2 H into IMS yielding CoQ ISP reduces
  cytochrome c1
• CoQ transfers electron to cytochrome bL
  yielding CoQ
• CoQ diffuses to the matrix side (to Complex I)
• Cytochrome bL transfers electron to cytochrome bH
• CoQ in Qi site reduced to CoQH2 by cytochrome
  bH (2 H from Matrix side)

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Summary of Cycle 2
CoQH2 CoQ Cytochrome c1 (Fe3) 2 H
(matrix) gt CoQ CoQH2 Cytochrome c1 (Fe2)
2 H (IMS)
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Overall Summary of Q Cycles
CoQH2 2 Cytochrome c1 (Fe3) 2 H (matrix)
gt CoQ 2 Cytochrome c1 (Fe2) 4 H (IMS)
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Proton Transfer

• By Redox Center/Proton Carrier
• CoQH2

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Cytochrome c

• Soluble Electron Carrier

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Mitochondrial Electron Transport Chain
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Complex IV

• Reduces Oxygen to Water
• 4 Cytochrome c (reduced) 4 H O2 gt
• 4 Cytochrome c (oxidized) 2 H2O

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Composition of Complex IV Homodimer(2x 13
subunits)

• Subunits I, II, and III encoded by mitochondrial
  DNA
• Subunits IVXIII encoded by nuclear DNA

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Bovine Heart Cytochrome c Oxidase
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Redox Centers in Cytochrome c Oxidase

• Cytochrome a
• Cytochrome a3
• CuB
• CuA center (two Cu-atoms)

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Organization of Redox Centers in Cytochrome c
Oxidase
Above Membrane Surface
Membrane
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Electron Transfer in Cytochrome c Oxidase
Cytochrome c gt CuA Center gt Cytochrome a
gt Cytochrome a3CuB Binuclear Complex gt O2
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Cytochrome c Oxidase Catalyzes a Four-Electron
Redox Reaction

• 4 Cytochrome c (reduced) 4 H O2 gt
• 4 Cytochrome c (oxidized) 2 H2O

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Source of Four Electrons

• Heme a3 (Fe2 gt Fe4) 2 electrons
• CuB (Cu1 gt Cu2) 1 electron
• Tyrosine 244 1 electron
• Covalent link to His 240
• TyrOH gt TyrO

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Heme a3CuB Binuclear Complexin Cytochrome c
Oxidase
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Proposed Reaction Sequence for Cytochrome c
Oxidase
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Protons in Cytochrome c Oxidase

• Chemical or Scalar Protons (4)
• From matrix
• Used in reduction of O2 gt 2 H2O
• Pumped or Vectorial Protons (4)
• Matrix gt IMS

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Summary of Proton Utilizationin Cytochrome c
Oxidase
8 H (matrix) O2 4 Cytochrome c (Fe2) gt 4
Cytochrome c (Fe3) 2 H2O 4 H (IMS)
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Complex Proton Channelsin Cytochrome c Oxidase

• K-channel (lysine)H (matrix) gt Tyr 244 gt H2O
• D-channel (aspartate)H (matrix) gt Heme a3CuB
  gt H (IMS) pumped protons