Biochemistry: A Short Course

1
Electron Transport Chain/Respiratory Chain
Proton gradient formed Four large protein
complexes Mitochondria localized Energetically
favorable electron flow
2
Mitochondrion Inner Membrane
Respiration site Surface area for humans ca. 3
football fields Highly impermeable (no
mitochondrial porins) Matrix and cytoplasmic
sides
3
Mitochondria an Endosymbiotic Event
Semi- autonomous organelles All mitochondria
derived from one endosymbiotic event an
ancestor of louse-borne typhus (Rickettsia
prowazeki)
Why did mitochondria become ubiquitous in nature?
4
Redox Potential a Measurement of Electron Flow
Electrons flow through the wire ions flow
through the salt bridge X- H X
½H2 Half-reaction defined as zero H e-
½H2 e.g. O2 negative NADH positive
5
Standard Reduction Potentials
6
Favorable Electron Flow NADH to O2
Net electron flow through electron transport
chain ½O2 2H 2e- H2O ?E?
0.82V NAD H 2e- NADH ?E? -
0.32V Subtracting reaction B from A ½O2
NADH H H2O NAD ?E?
1.14V ?G? -220 kJ mol-1
?G? -nF ? E? F 96,480 J mol-1 V-1
7
Electron Transport Chain Components

• Protein complexes
• NADH-Q reductase
• Succinate dehydrogenase
• Cytochrome C reductase
• Cytochrome C oxidase

Bridging components Coenzyme Q and Cytochrome C
What is the driving force for this electron flow?
8
Coupled Electron-Proton Transfer Through NADH-Q
Oxidoreductase
FMN bridges NADH 2 e- donor with FeS 1 e-
acceptor L-shaped Complex I Overall
reaction NADH Q 5H NAD QH2 4H
9
Coupled Electron-Proton Transfer Through NADH-Q
Oxidoreductase
H movement with 1 NADH
Iron-sulfur clusters (a.k.a. nonheme-iron
proteins) 2Fe 2S or 4Fe 4S complexes
10
NADH-Q Oxidoreductase (Complex I) Structure
Largest of respiratory complexes Mammalian
system contains 45 polypeptide subunits 8 Fe-S
complexes 60 transmembrane helices
11
Different Quinone (Q) Oxidation States
QH2 generated by complex I II Membrane-bound
bridging molecule Overall reaction QH2 2Cyt
Cox 2H Q 2Cyt Cred 4H
X
12

• Oxaloacetate Enzyme Regeneration from Succinate
• Succinate Dehydrogenase
• Fumerase
• Malate Dehydrogenase

13
Pathways that Contribute to the Ubiquinol Pool
Without Utilizing Complex I
14
Alternative Q-Cycle Entry Points
Complex I Complex II (citric acid
cycle) Glycerol 3-phosphate shuttle Fatty acid
oxidation (electron-transferring-flavoprotein
dehydrogenase)
15
Electron-Transport Chain Reactions in the
Mitochondria
16
The Q Cycle
Electron transfer to Cytochrome c Reductase via 3
hemes and a Rieske iron-sulfur center Overall
reaction QH2 2Cyt Cox 2H Q 2Cyt
Cred 4H
17
The Q Cycle
Electron transfer to Cytochrome c Reductase via 3
hemes and a Rieske iron-sulfur center Overall
reaction QH2 2Cyt Cox 2H Q
2Cyt Cred 4H
ISP iron sulfur protein
18
The Q Cycle
19
The Q Cycle
20
Cytochrome c Oxidoreductase Structure
Intermembrane side

• Heme-containing homodimer with 11 subunit
  monomers
• Functions to
• Transfer e- to Cyt c
• Pump protons into the intermembrane space

Matrix side
21
Cytochrome c Oxidase Proton Pumping and O2
Reduction
CuA/CuA
CuB
22
Cytochrome c Oxidase O2 Reduction to H2O
Reaction shown 2Cyt Cred 2H ½ O2 2Cyt
Cox H2O Overall reaction 2Cyt Cred 4H
½ O2 2Cyt Cox H2O 2H
23
Cytochrome c Oxidase
Intermembrane space
Oxygen requiring step 13 subunits 10 encoded by
nuclear DNA CuA/CuA prosthetic group
positioned near intermembrane space
O2 to H2O reduction site
Matrix
24
Cytochrome c Oxidase
25
Electron-Transport Chain Reactions in the
Mitochondria
26
Electron-Transport Chain Reactions in the
Mitochondria
27
Mitochondrial Electron-Transport Chain Components
28
Superoxide Dismutase Deactivation Mechanism
Superoxide and Peroxide generation
Superoxide deactivation
Hydrogen peroxide
29
Conditions Associated with Free-Radical Exposure
30
Environmental Impact of Excessive Aquatic
Respiration
Dead Zone from Agricultural runoff Water sources
include Mississippi, Sabine, and Trinity Rivers
31
Chapter 19 Problems 1, 3, 4, 6, 7, 8, 9,10, and
12