Electron Transport Chain
Thermodynamics of Glucose
Oxidation
Glucose + 6 O2 ——> 6 CO2 + 6 H2O
∆Go’ = -2866 kJ/mol
Half-Reactions of Glucose
Oxidation
Glucose + 6 H2O ——> 6 CO2 + 24 H+ + 24 e–
6 O2 + 24 H+ + 24 e– ——> 12 H2O
NADH and FADH2
Sites of NADH and FADH2 Formation
Sites of NADH and FADH2 Formation
Mitochondrial
Electron Transport Chain
System of Linked
Electron Carriers
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
Oxidative Phosphorylation
Synthesis of ATP driven by free
energy of electrochemical
gradient
Coupling of Electron Transport
and ATP Synthesis
NOTE: ATP Synthesis in the Mitochondrion
The Mitochondrion
• Prokaryotic origin
 Double membrane bound
 Genome
o Human: encodes 13 genes, all ETC subunits.
Mitochondrial Outer Membrane
Permeable to molecules smaller
than ~5 kD
X-Ray Structure of E. coli OmpF Porin
Figure 9-23a
X-Ray Structure of E. coli OmpF
Porin Trimer
Figure 9-23b
Mitochondrial
Intermembrane Space (IMS)
[Metabolites] = Cytosolic Concentration
Localized Compartmentation of H+
Mitochondrial Inner Membrane
(Permeability Barrier)
Transport
Types of Transport
• Nonmediated Transport (Diffusion)
– H2O; O2; CO2
• Mediated Transport
– Passive-mediated Transport
(facilitated diffusion)
– Active Transport
– Facilitated by Proteins:
• Carriers, Transporters,
Translocases, or Permeases.
Kinetic Properties of Mediated
Transport
• Saturation kinetics
• Speed and specificity
• Susceptibility to
competitive inhibition
• Susceptibility to
chemical inactivation
Vmax[A]
V =
Km + [A]
Stoichiometry of Mediated Transport
Entry of “NADH” into
Mitochondria
No NADH Transporter
Malate–Aspartate Shuttle
Malate–Aspartate Shuttle
Glycerophosphate Shuttle
Transport of ADP, ATP, and
Inorganic Phosphate (Pi)
ADP-ATP Translocator
ADP/ATP Exchanger
Driven by electrochemical gradient
Phosphate Transport
Driven by ∆pH
Phosphate Transporter
H+(out)
H+(in)
H2PO4–(out)
H2PO4–(in)
Electron Transport
Electron Transport is an
Exergonic Process
Standard Reduction Potentials
Standard Reduction Potential
Difference
∆Eo’ = Eo’(e–
o’ –
–
E
acceptor)
(e donor)
∆Go’ = –nF∆Eo’
For negative G need positive E
E(acceptor) > E(donor)
Note: reduction potential is extremely pH sensitive
E = Eo’ + 0.06V*(7-pH)
What is the ∆Eo’ and ∆Go’ for the
Oxidation of NADH by O2?
Electron Carriers Operate in
Sequence
Electron Transport Complexes
• Complex I: NADH–Coenzyme Q Oxidoreductase
• Complex II: Succinate–Coenzyme Q Oxidoreductase
• Complex III: Coenzyme Q–Cytochrome c Oxidoreductase
• Complex IV: Cytochrome c Oxidase
Overview of Electron Transport
in the Mitochondrion
Mobile Electron Carriers
Coenzyme Q
Cytochrome c
Coenzyme Q
Oxidation States of Coenzyme Q
Cytochromes
Electron Transport Heme Proteins
Fe3+ + e– ——> Fe2+
a
Hemes
b
Note:
Iron-Protoporphyrin IX
isoprene side chain
Like Mb and Hb
c
Note:
Thioether Links
Cytochrome Spectra
Complex I
(NADH–Coenzyme Q Oxidoreductase)
Accepts Electrons from NADH
NADH + CoQ(oxidized) ——> NAD+ + CoQ(reduced)
Protons translocated
4H+(Matrix) ——> 4H+(IMS)
Coenzymes of Complex I
(Flavin Mononucleotide, FMN)
Oxidation states like FAD
Coenzymes of Complex I
(Iron-Sulfur Clusters)
One-electron oxidation-reduction
Conjugated System (Fe between +2 and +3)
Thermodynamics of Complex I
Hydrophilic Domain of Complex I
from Thermus thermophilis
~ matrix
~ cytoplasm
Electrons follow a multistep path
Structure of Bacteriorhodopsin
Figure 9-22
Proton Wire
1) Deprotonation of Schiff base
and protonation of Asp 85
2) Proton release to the
extracellular surface
3) Reprotonation of the Schiff
base and deprotonation of Asp
96
4) Reprotonation of Asp 96 from
the cytoplasmic surface
5) Deprotonation of Asp 85 and
reprotonation of the proton
release site
Complex II
(Succinate–Coenzyme Q Oxidoreductase)
Contributes Electrons to Coenzyme Q
Succinate + CoQ(oxidized) ——> Fumarate + CoQ(reduced)
Does not pump protons
Composition of Complex II
• Succinate Dehydrogenase
– FAD
•
•
•
•
[4Fe-4S] cluster
[3Fe-4S] cluster
[2Fe-2S] cluster
Cytochrome b560
Thermodynamics of Complex II
E. coli Complex II
Cytoplasm
~matrix
Plasma Membrane
~IM
Periplasm
~cytoplasm
Complex II
(Linear Chain of Redox Cofactors)
Cytochrome b560
scavenges electrons
to prevent formation
of reactive oxygen
species
Complex III
(Coenzyme Q–Cytochrome c Oxidoreductase)
Translocates Protons via the Q Cycle
CoQ(reduced) + 2 Cytochrome c (oxidized) ——>
CoQ(oxidized) + 2 Cytochrome c (reduced)
Oxidation States of Coenzyme Q
Composition of Complex III
• Cytochrome b562 (bH – high potential)
• Cytochrome b566 (bL – low potential)
• Cytochrome c1
• [2Fe–2S] cluster (ISP)
Thermodynamics of Complex III
Yeast Complex III
The Q Cycle
(Electrons from CoQH2 follow two paths)
Cycle 1
IMS
Matrix
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
Summary of Cycle 1
CoQH2 + Cytochrome c1 (Fe3+) ——>
CoQ•– + Cytochrome c1 (Fe2+) + 2 H+ (IMS)
Cycle 2
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)
Summary of Cycle 2
CoQH2 + CoQ•– + Cytochrome c1 (Fe3+) + 2 H+ (matrix) ——>
CoQ + CoQH2 + Cytochrome c1 (Fe2+) + 2 H+ (IMS)
Overall Summary of Q Cycles
CoQH2 + 2 Cytochrome c1 (Fe3+) + 2 H+ (matrix) ——>
CoQ + 2 Cytochrome c1 (Fe2+) + 4 H+ (IMS)
Complex IV
(Cytochrome c Oxidase)
Reduces Oxygen to Water
4 Cytochrome c (reduced) + 4 H+ + O2 ——>
4 Cytochrome c (oxidized) + 2 H2O
Composition of Complex IV
Homodimer
(2x 13 subunits)
Subunits I, II, and III: encoded
by mitochondrial DNA
Subunits IV–XIII: encoded by
nuclear DNA
Bovine Heart Cytochrome c Oxidase
Redox Centers in Cytochrome c
Oxidase
• Cytochrome a
• Cytochrome a3
• CuB
• CuA center (two Cu-atoms)
Organization of Redox Centers in
Cytochrome c Oxidase
Above
Membrane
Surface
Membrane
Electron Transfer in Cytochrome c
Oxidase
Cytochrome c —> CuA Center —> Cytochrome a —>
Cytochrome a3–CuB Binuclear Complex —> O2
Cytochrome c Oxidase Catalyzes
a Four-Electron Redox Reaction
4 Cytochrome c (reduced) + 4 H+ + O2 ——>
4 Cytochrome c (oxidized) + 2 H2O
Source of Four Electrons
• Heme a3 (Fe2+ —> Fe4+): 2 electrons
• CuB (Cu1+ —> Cu2+): 1 electron
• Tyrosine 244: 1 electron
– Covalent link to His 240
– Tyr–OH —> Tyr–O•
Heme a3–CuB Binuclear Complex
in Cytochrome c Oxidase
Proposed Reaction Sequence for
Cytochrome c Oxidase
Protons in Cytochrome c Oxidase
• Chemical or Scalar Protons (4)
– From matrix
– Used in reduction of O2 —> 2 H2O
• Pumped or Vectorial Protons (4)
– Matrix —> IMS
Summary of Proton Utilization
in Cytochrome c Oxidase
8 H+ (matrix) + O2 + 4 Cytochrome c (Fe2+) ——>
4 Cytochrome c (Fe3+) + 2 H2O + 4 H+ (IMS)
Complex Proton Channels
in Cytochrome c Oxidase
K-channel (lysine)
H+ (matrix) —> Tyr 244 —> H2O
D-channel (aspartate)
H+ (matrix) —> Heme a3–CuB —> H+ (IMS)
[pumped protons]
Summary of Electron Transport
• Complex I  Complex IV
1NADH + 11H+(matrix) + ½O2 ——> NAD+ + 10H+(IMS) + H2O
• Complex II  Complex IV
FADH 2+ 6H+(matrix) + ½O2 ——> FAD + 6H+(IMS) + H2O
~3H+/ATP
Thermodynamics of Electron
Transport Complexes
Standard Reduction Potentials of
Electron Transport Chain Components
Mitochondrial Electron Transport
Chain
Complex I
(NADH–Coenzyme Q Oxidoreductase)
NADH + CoQ (oxidized) ——> NAD+ + CoQ (reduced)
∆Eo’ = + 0.360 V
∆Go’ = – 69.5 kJ/mol
Complex II
(Succinate–Coenzyme Q Oxidoreductase)
Succinate + E–FAD ——> Fumarate + E–FADH2
E–FADH2 + CoQ (oxidized) ——> E–FAD + CoQ (reduced)
∆Eo’ = + 0.085 V
∆Go’ = – 16.4 kJ/mol
Complex III
(Coenzyme Q–Cytochrome c Oxidoreductase)
CoQ (reduced) + 2 Cytochrome c (oxidized) ——>
CoQ (oxidized) + 2 Cytochrome c (reduced)
∆Eo’ = + 0.190 V
∆Go’ = – 36.7 kJ/mol
Complex IV
(Cytochrome c Oxidase)
4 Cytochrome c (reduced) + 4 H+ + O2 ——>
4 Cytochrome c (oxidized) + 2 H2O
∆Eo’ = + 0.580 V
∆Go’ = – 112 kJ/mol
Electron transport chain
Complex I  Complex IV
2NADH + 2 H+ + O2 ——> 2NAD+ + 2 H2O
∆Eo’ = + 1.130 V
∆Go’ = – 218 kJ/mol
Complex II  Complex IV
2FADH2 + O2 ——> 2FAD + 2 H2O
∆Eo’ = + 0.855 V
∆Go’ = – 165 kJ/mol
ATP Synthesis from NADH
ATP synthesis: ∆Go’ = 30.5 kJ/mol
Standard Biochemical Conditions:
30.5 x 2.5
Efficiency =
= ~35%
21 8
(FADH2 is ~30%)

Physiological conditions ~70% efficiency