Objectives
1. To know about the structural and biochemical
organizations of a mitochondrion
2. To understand the electrochemical reactions
through which the chemical energy in food
can be converted to chemical energy in ATP
3. To realize how the structural organizations of
mitochondria have allowed the above
electrochemical reactions to be carried out
effectively
1
Energy Conversion (1): Mitochondria
• Cellular respiration
- Flow of electrons from reduced coenzymes to an
electron acceptor; generation of ATP
- NADH and FADH2 from glycolysis, TCA cycle, boxidations, etc.
- Ultimate electron acceptor is oxygen; reduced
form as water (aerobic respiration); takes place
with mitochondria in eukaryotic cells
2
The Energy Powerhouse
•
Discrete sausage-shaped structures; the
second largest organelle in most animal cells
•
A double-membrane organelle; outer
membrane separated from inner membrane
by intermembrane space
A. Outer membrane
•
Not a significant permeability barrier for
ions and small molecules; transmembrane
proteins (porins)
3
B. Intermembrane space
•
Continuous with the cytosol
C. Inner membrane
•
A permeability barrier to most solutes
•
Locale of the protein complexes of electron
transport and ATP synthesis
•
Distinctive foldings (cristae); increase surface
area to accommodate more the protein
complexes
4
D. Matrix
•
Semi-fluid enclosed by
inner membrane;
-
Enzymes for
mitochondrial functions
-
A circular DNA molecule;
coding for its own rRNAs,
tRNAs, and a number of
polypeptide subunits of
inner-membrane proteins
(genetic competence)
5
Electron Transport System (ETS)
•
Transfer of electrons from NADH and
FADH2 is highly exergonic
•
Multistep process; a series of reversibly
oxidizable electron carriers; total free energy
difference is released in increments to
prevent excessive amount being released as
heat (energy conservation for ATP)
•
4 different kinds of carriers::
6
A. Flavoproteins
•
Membrane-bound proteins using either flavin
adenine dinucleotide (FAD) or flavin
mononucleotide (FMN) as prosthetic group
•
Transfer both electrons and protons
7
B. Iron-Sulfur Proteins
•
Proteins containing iron-sulfur (Fe/S) centers;
iron and sulfur atoms complexed with cysteine
groups of the protein
•
Alternates between
Fe2+(ferrous)
•
Do not pick up and release protons
the
Fe3+(ferric)
and
C. Cytochromes (Cyt)
•
Contain iron; part of a porphyrin prosthetic
group (heme)
8
•
One-electron carriers; transfer electrons only:
1. Cyt b, c1, a and a3 are integral membrane
proteins
2. Cyt c is relatively hydrophilic; loosely associated
with inner face of membrane; not a part of the
complexes; mobile electron carrier
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3. Cyt a and a3
•
Copper - containing - cytochromes
(bimetallic iron-copper (Fe/Cu) center)
•
Components of cytochrome c oxidase
•
Keeping an O2 molecule bound to the
oxidase complex; completely picked up
the four electrons and four protons
10
D. Coenzyme Q (CoQ)
•
Ubiquinone (a benzene
derivative); the only
nonprotein component
•
Carries both protons and
electrons
•
Not part of a respiratory
complex; a collection point
for electrons from FMN- and
FAD-linked dehydrogenases
•
Active transport of protons
across inner mitochondrial
membrane
11
• The electron carriers function in a
sequence determined by their relative
reducing power (reduction potentials)
- Two interconvertible molecules or ions by
the loss or gain of electrons (redox pair)
• With exceptions of CoQ and Cyt c, the
electron carriers are organized into four
large multiprotein complexes (respiratory
complexes)
12
13
A. Complex I
•
NADH-coenzyme Q oxidoreductase
-
Transfers electrons from NADH to coenzyme
Q
B. Complex II
•
Succinate-coenzyme Q oxidoreductase
-
Transfers electrons derived from succinate
oxidation in TCA
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C. Complex III
• Coenzyme Q – cytochrome c oxidoreductase
- Accepts electrons from coenzyme Q and passes
them to cytochrome c
D. Complex IV
• Cytochrome c oxidase
- A terminal oxidase; capable of direct transfer of
electrons to oxygen
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Properties of the Mitochondrial Respiratory Complexes
Respiratory Complex
Number
I
II
III
IV
Name
NADH
dehydrogenase
(NADH-coenzyme Q
oxidoreductase)
Succinate-coenzyme
Q oxidoreductase
(succinate
dehydrogenase)
Coenzyme Q
-cytochrome c
oxidoreductase
(cytochrome b-c1
complex)
Cytochrome c
oxidase
Electron Flow
Number of
Polypeptides
Prosthetic Groups
Accepted from
Passed to
Proton
Transport?
22-26
1 FMN
6-9 Fe/S centers
NADH
Coenzyme Q
Yes
4-5
1 FAD
3 Fe/S centers
Coenzyme Q
No
8-10
2 cytochrome b
1 cytochrome c1
1 Fe/S center
Coenzyme Q
Cytochrome c
Yes
Cytochrome c
Oxygen (O2)
Yes
9
1 cytochrome a
1 cytochrome a3
2 Cu centers
(as Fe/Cu centers
with cytochrome
a3)
Succinate
(via enzyme-bound
FAD)
16
ATP Generation / Electron Transport
•
ATP generation: ADP + Pi ATP
A. Photophosphorylation
B. Substrate level phosphorylation
•
Glycolysis: 1,3-bisphosphoglycerate  3phospho-glycerate; phosphoenolpyruvate 
pyruvate
•
TCA: succinyl CoA  succinate
-
4 ATP molecules/glucose: 2 from glycolysis + 2
from TCA
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C. Oxidative phosphorylation
•
6 different oxidations (12 pairs of electrons):
1. Glycolysis: glyceraldehyde-3-phosphate  1,3bisphosphoglycerate (+NADH)
2. Pyruvate  acetyl CoA (+NADH)
3. TCA: isocitrate  -ketoglutarate (+NADH); KG  succinyl CoA (+NADH); succinate 
fumarate (+FADH2); malate  oxaloacetate
(+NADH)
18
Chemiosmotic Model
• Electrochemical potential across a
membrane; the link between electron
transport and ATP formation
- Exergonic transfer of electrons between
and within respiratory complexes;
unidirectional pumping of protons
across the membrane where the
transport system is localized
19
20
The F0F1 Complex
•
A F-type ATPase; both ATPase and ATP
synthase activities
•
Converts electrochemical energy (proton
gradient) into potential chemical energy (ATP)
A. F1 complex
•
3 and 3b polypeptides; 3 b complexes
(catalytic hexagon)
21
-
b subunit: catalytic site for ATP
synthesis/hydrolysis;  subunit: ATP/ADPbinding site
•
Both ATP synthase and ATPase activities
•
Proton translocation through F0 drives ATP
synthesis by F1
B. Stalk
•
Composes of ,  and  subunits
•
Allows rotation of F1 complex about F0
complex
22
23
C. F0 complex
•
Consists of 1a, 2b and 9-12 c subunits
•
c subunits are organized in a circle;
proton channel
•
As a proton translocator: channel
through
which
protons
flow
(protonation and deprotonation of
aspartate)
24
25
Binding Change Model
•
To explain how exergonic flow of protons
through
F0
can
drive
endergonic
phosphorylation of ADP to ATP
•
Electrochemical–to–mechanical–to–chemical
transducer
•
Each of the three b subunits exists in 3
different conformations at any point in time:
26
-
(O)pen: little affinity; ADP and Pi are free to
enter (ATP is free to leave) the catalytic site
-
(L)oose: higher affinity; lose binding of ADP
and Pi
-
(T)ight: Packing the ADP and Pi together
tightly; facilitating the condensation
- O  L  T:
27
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1. Flowing of protons flow through a channel in
the a subunit of F0
2. Rotation of the ring of c subunits; rotation of
the attached  subunit
3. Asymmetry of the  subunit; different
interactions with the three b subunits at any
point in time
4. Each b subunit passes successively through
the O, L, and T conformations as the 
subunit rotates 360º
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