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ATP Formation by
Electron-Transport Chains
Mitochondrial Electron-Transport
Components of the Electron-Transport Chain
Oxidative Phosphorylation
Recycling of Cytoplasmic NADH
Photosynthetic Electron-Transport
Synthesis of Carbohydrates by the Calvin Cycle
1
Introduction
Up to this point, we have dealt with
• Oxidation of substrates.
• Collection of electrons by cofactors.
Energy from the cofactors is recovered
using O2 as the final electron acceptor.
This is accomplished using a series of
carriers in the inner mitochondrial
membrane .
2
Mitochondrial electron transport
Stage I and II of carbohydrate catabolism
converge at the mitochondria.
Stage I
Stage II
citric acid cycle
electron-transport
oxidative phosphorylation
3
Mitochondrial electron transport
• Extensive inner membrane folding in the
mitochondria provides a large surface area.
• There are many molecular systems on this
membrane for production of ATP.
• Electron-transport chain components are
arranged in packages called respiratory
assemblies.
• There are also knob-like spheres called F1
particles.
4
Electron-transport and
oxidative phosphorylation
Electrons obtained from nutrients and
metabolic intermediates are transferred
to NAD+ and FAD.
dehydrogenase
AH2 + NAD+
BH2 + FAD
A + NADH + H+
dehydrogenase
B + FADH2
Since NAD+ and FAD are in limited supply,
they must be recycled.
5
Electron-transport and
oxidative phosphorylation
Recycling is accomplished by oxidation and
transfer of electrons to oxygen.
ADP + Pi
ATP
NADH + H+ + 1/2 O2
ADP + Pi
FADH2 + 1/2 O2
NAD+ + H2O
ATP
FAD + H2O
NAD+ and FAD are then available for additional
oxidative metabolism. The energy released during
electron transport is coupled to ATP synthesis.
6
Electron-transport chain
Composed of four large protein complexes.
• Complex I - NADH-Coenzyme Q reductase
• Complex II - Succinate-Coenzyme Q reductase
• Complex III - Cytochrome c reductase
• Complex IV - Cytochrome c oxidase
Many of the components are integral
membrane proteins with prosthetic
groups to move electrons.
7
Electron-transport chain
Two important characteristics of the
electron-transport chain
• order of electron carriers
• quantity of energy produced
Electron carriers are arranged in order of
increasing electron affinity.
This results in the spontaneous flow of
electrons from carrier to carrier.
8
Flow of electrons
9
Energy produced
The amount of energy can be calculated in
terms of Go’ :
NADH + H+ + 1/2 O2
NAD+ + H2O
Go’ = - 220 kJ/mol
FADH2 + 1/2 O2
FAD + H2O
Go’ = - 152 kJ/mol
Note: ADP + Pi
ATP Go’ = +31 kJ/mol
10
Components of the
electron transport chain
Complex I
• Electrons flow from NADH to flavin
mononucleotide (FMN) - similar to FAD.
• Electrons then flow to a prosthetic group
on an iron-sulfur cluster - iron cycles
between 3+ and 2+ states.
• Complex I terminates at ubiquinone - also
called coenzyme Q or CoQ.
11
Components of the
electron transport chain
2 H+
Complex I
QH2
2 one-electron
FMNH2 2 one-electron Fe-S
transfers
transfers
FMN
Q
2 H+
NADH
H+
NAD +
12
Components of the
electron transport chain
flavoprotein
13
Iron-sulfur clusters
S
Cys
Fe
Cys
S
Fe
Cys
S
S
S
Cys
Cys
Fe
S
S
S
Fe
S
Cys
Cys
S
S
Fe
S
S
Fe
S
Cys
protein
14
Components of the
electron transport chain
CoQ - ubiquinone
Highlighted region serves as an anchor to
inner mitochondrial membrane.
O
H3CO
CH3
CH3
H3CO
(CH2
CH
C
CH2)10
H
O
15
Reduction of CoQ
Oxidized form
Ubiquinone (CoQ)
Reduced form
Ubiquinol (CoQH2)
O
OH
H3CO
CH3
H3CO
CH3
H3CO
R
H3CO
R
O
O
e- +
OH
H3CO
CH3
H3CO
R
H+
OH
intermediate,semiquinone
e- +
H+
16
Components of the
electron transport chain
Complex II
• Entry point for both FADH2 and Complex I.
• Succinate dehydrogenase
From the citric acid cycle. Directs transfer of
electrons from succinate to CoQ via FADH2.
• Acyl-CoA dehydrogenase
From -oxidation of fatty acids. It also transfers
electrons to CoQ via FADH2.
Both enzymes have iron-sulfur clusters as
prosthetic groups and are integral proteins.
17
Components of the
electron transport chain
All electrons from FADH2 and NADH must pass
through CoQ.
innermembrane
space
I
Fe-S
FMN
II
FAD
NADH
NAD +
CoQ
Fe-S
FAD
Succinate
Fatty acyl
CoA
matrix
18
Components of the
electron transport chain
Complex III
Electron transfer from ubiquinol to
cytochrome c.
cytochrome c
heme prosthetic group
19
Components of the
electron transport chain
Protein
S
H3C
CHCH 3
H3C
CH3
N
N
Fe
N
H3C
H3C
Structure of
cytochrome c
heme group.
N
CH2CH2COO-
CH2CH2COO20
Components of the
electron transport chain
Complex IV
• Combination of cytochromes a and a3 cytochrome c oxidase.
• Consists of 10 protein subunits, 2 types of
prosthetic groups - 2 heme and 2 Cu.
• Cytochromes a and a3 are the only
species capable of direct transfer of
electrons to oxygen.
21
Components of the
electron transport chain
Complex
I
CoQ
Complex
III
cyt
c1
cyt b
NADH
cyt
c
Complex
IV
(Cu)
cyt
a/a3
O
2
matrix
22
Oxidative phosphorylation
• The electron-transport chain moves
electrons from NADH and FADH2 to O2.
• The next step is the phosphorylation of
ADP to produce ATP.
Catalyzed by the inner membrane
enzyme ATP synthase.
• The steps are coupled - electrons do not
flow to oxygen unless ATP is needed.
Each NADH produces 3 ATP
Each FADH2 produces 2 ATP
23
Coupling of electron-transport
with ATP synthesis
Chemiosmotic coupling mechanism
• Electron-transport causes unidirectional
movement of H+ into the innermembrane
space.
• The results in a H+ gradient being
produced.
• The gradient then drives the synthesis of
ATP.
24
Coupling of electron-transport
with ATP synthase
Outer mitochondrial membrane
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
Inner mitochondrial membrane
H+ H + H+
Electron
Transport
Chain
F1-ATP
synthase
complex
H+
ADP + Pi
ATP
25
Components of ATP synthase
These are knob-like projections into the
matrix side of the inner membrane.
Two units
• F1 contains the catalytic site for ATP
synthesis.
• F0 serves as a transmembrane channel
for H+ flow.
F1-F0 complex serves as the molecular apparatus
for coupling H+ movement to ATP synthase.
26
Components of ATP synthase
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
F0





F
1
27
Regulation of oxidative
phosphorylation
• Electrons do not flow unless ADP is
present for phosphorylation
• Increased ADP levels cause an increase
in catabolic reactions of various enzymes
including:
glycogen phosphorylase
phosphofructokinase
citrate synthase
28
Uncoupling of electron-transport
and oxidative phosphorylation
• In some special cases, the coupling of the two
processes can be disrupted.
• Large amounts of O2 are consumed but no ATP is
produced.
• Used by newborn animals and hibernating
mammals.
• Occurs in ‘brown fat’- dark color due to high
levels of mitochondria which contain
thermogenin (uncoupling protein).
• Thermogenin allows the release of energy as
heat instead of ATP.
29
Energy production from glucose
Glycolysis
2 ATP
2 NADH
3 ATP/NADH
Citric Acid Cycle
2 GTP
1 ATP/GTP
6 NADH
3 ATP/NADH
2 FADH2
2 ATP/FADH2
* 4 ATP in muscle and brain.
36 ATP / glucose
2 ATP
6 ATP*
2 ATP
18 ATP
4 ATP
38 ATP
(in heart)
30
Energy production from glucose
Mitochondria
Glycolysis
Glucose
2 Pyruvate
2 NADH
2 Acetyl CoA
2 NADH
6 NADH+
2 FADH2
2 GTP
Oxidative
phosphorylation
2 ATP
32-34 ATP
2 ATP
31
Recycling of cytoplasmic NADH
Different methods are used to recycle
NADH. This accounts for the different
energy productions from glucose.
Glycerol-3-phosphate shuttle
Used by skeletal muscles and the brain
Malate-aspartate shuttle
Used by the heart and liver
32
Glucose-3-phosphate shuttle
NAD
+ +
NAD
cytoplasmic
cytosolic
NADH
+ H++H+
glycerol-3-phosphate
NADH
glycerol-3-phosphate
dehydrogenase
dehydrogenase
Glycerol-3-phosphate
Dihydroxyacetone
phosphate
mitochondrial
glycerol-3-phosphate
dehydrogenase
Cytoplasm
FAD
FADH2
II
Q
III
Mitochondrial matrix
33
Malate-aspartate shuttle
Matrix
L-aspartate
L-aspartate
-ketoglutarate
Glycolysis
cytoplasmic
aspartate
aminotransferase
L-glutamate
oxaloacetate
cytoplasmic malate
dehydrogenase
NADH + H+
L-malate
+
NAD
-ketoglutarate
mitochondrial
aspartate
aminotransferase
L-glutamate
mitochondrial
malate
dehydrogenase
L-malate
NAD+
oxaloacetate
3 ATP
NADH
+ H+
34
Photosynthetic electron transport
Heterotrophs
Obtain energy by ingestion of other plants
and animals.
Phototrophs
Absorb solar radiation and divert the
energy through the electron transport
chain.
They can produce their own
carbohydrates from CO2 and H2O
35
Photosynthetic electron transport
Two type of reactions.
Light reactions - photo phase
Absorb energy using chlorophyll and
other pigments.
Dark reactions - synthesis phase
Carbon metabolism to make
carbohydrates. Light is not directly
required.
36
Chloroplast
The apparatus for light absorption and carbon
fixing in eukaryotic photosynthetic cells.
Outer membrane
Innermembrane
space
Inner membrane
Thylakoid
Granum
Stroma
37
Chloroplast
Stroma
Gel-like, unstructured matrix within the
inner compartment. It contains the
enzymes for the dark reactions.
Thylakoids
Membranes folded into sacs that are the
sites for light receiving pigments, electron
carriers and ATP synthesis. They are
arranged into stacks called grana.
38
Biomolecules and light
Several types of light absorbing pigments
are used.
Green plants
Chlorophylls a and b.
Bacteria
Bacteriochlorophyll.
Accessory pigments
Carotenes and phycobilins - absorb light
outside the range of chlorophyll.
39
Chlorophyll a
H
O
C
saturated bond in
bacteriochlorophyll
in chlorophyll b
CH 2
CH 2
CH3
CH3
C
O
H 3C
in bacteriochlorophyll
I
N
II
CH2CH3
III
CH3
N
Mg
H3C
CH3
CH 3
CH3
CH 3
O
H3C
phytol side chain
N
IV
H
C
O
N
C
H2
C
H2
H
H
H3CO
O
C
O
40
Carotenes
-carotene
OH
HO
lutein
41
Phycoerytherin
CH3
CH2
in phycocyanin
COO - COO -
CH 3
H3C
O
CH
N
H
CH2
CH2
CH2
H3C
CH3 H3C
N
H
N
H
unsaturated bond
in phycocyanin
CH
N
H
O
42
Photosynthetic light reactions
Electrons flow through an electron
transport chain from water to an electron
acceptor.
NADP+ is the acceptor in green plants.
2 H2O + 2 NADP+
light
2 H+ + O2 + 2 NADPH
43
Photosystems
h
Chl
Chl
Cat
Chl
Chl
Cat
Chl
Chl
Chl
Chl
Chl a
Reaction
Center
Cat
Cat
Chl
Chl
Chl
Chl
Cat
Two types
Each contain one
primary acceptor
molecule - usually
chlorophyll
Chl
Chl
A set of accessory
molecules help
funnel additional
light.
Cat
44
Photosystems
Photosystem I - P700
• Chlorophyll a and accessory pigments
• Absorb in 600-700 nm range
Photosystem II - P680
• Chlorophyll a, b and accessory pigments
• Absorb light with a maximum at 680 nm
All photosynthetic cells have P700. Both
are present in O2 evolving organisms higher plants, algae and cyanobacteria.
45
Linkage of photosystems I and II
In green plants, the two systems are linked.
• Light is absorbed by Photosystem I.
• Energy is transmitted to the P700 center
and an electron is excited.
• Electron is passed via an electron
transport chain.
• The ‘electron hole’ is filled by another
electron transport chain driven by
Photosystem II.
46
Photosystem I
Reduction potential, V
Photosystem I
-1.0
P700*
A0
A1
-0.5
0.0
Fe-S
Complex
P700
Ferredoxin-NADP+
reductase
light
+0.5
Ferredoxin
NADP +
NADPH + H+
+ proton gradient
47
Photosystem II
-1.0
Reduction potential, V
Photosystem II
P680*
-0.5
0.0
Q
A
Watersplitting
complex
Q
B
2HO
2
+0.5
P680
+1.0
O2 + 4 H+ +
proton gradient
light
48
Linkage of photosystems I and II
light
49
Photosystems I and II
Net reaction
2 H2O + 2 NADP+
8 h
O2 + 2 NADPH + 2 H+
Eight photons are required to transfer four
electrons.
50
Photophosphorylation
• Converting light into chemical bonds- very
similar to oxidative phosphorylation.
• Photoinduced electron transfer from
water to NADP+ pumps H+ through
thylkaloid membrane - from stromal side
to inner compartment.
• Protein complexes CF0 and CF1 are the
ATP synthases of chloroplasts.
51
Photophosphorylation
proton pump within
the light-induced
electron transport
system.
H+
H
+
Low Mg2+ High H+
Lumen
thylakoid
membrane
H+
H+
+
High Mg2+ Low H
ADP
ATP
Stroma
52
Photophosphorylation
Process is non-cyclic
• Starts with H2O and ends with NADPH and
O 2.
• Products will accumulate as long as there
is light.
A cyclic process exists for photosystem I.
• No H2O is consumed and no NADPH or O2
is produced.
• ADP is phosphorylated.
53
Cyclic photophosphorylation
P700*
A0
A1
Fe-S
Complex
Cytochrome
bf complex
Ferredoxin
P700
light
proton gradient
Plastocyanin
54
Synthesis of carbohydrates
The Calvin Cycle
• The ‘dark’ reactions - fixation of carbon
from CO2.
• Four stages - fix one carbon at a time.
• Six cycles per glucose.
Overall reaction for one glucose
6 CO2 + 12 NADPH + 12H+ + 18 ATP + 12 H2O
C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi
55
Calvin cycle
Stage 1
• Addition of CO2 to an acceptor molecule.
• Ribulose-1,5-bisphosphate
carboxylase/oxygenase (rubisco)
catalyzes the addition of CO2
• The ribulose-1,5-bisphosphate that is
produced will immediately cleave into two
molecules of 3-phosphoglycerate.
56
Calvin cycle
Stage 1
O
O
O
O-
P
O
-
O
CH2
C
H
C
OH
H
C
OH
+ CO2
O
O
P
CH 2
O
C
C
C
H
CH2
O
O-
H+
O
O-
P
C
H2O
H
O-
2
O
OH
C
O-
C
OH
O
O
P
C
H2
O-
O-
H+
CH 2
O
O-
O-
O
P
O-
3-phosphoglycerate
O-
ribulose-1,5bisphosphate
-keto acid intermediate
57
Calvin cycle
Stage 2
• Phosphorylation of the C1 carboxyl group,
producing 1,3-bisphosphoglycerate.
• Stromal 1,3-bisphosphoglycerate (1,3-PBG) is
reduced to glyceraldehyde-3-phosphate.
ATP
COOH C OH
CH2OPO32-
ADP
O
COPO32H C OH
3-phosphoglycerate
2CH
OPO
2
3
kinase
NADPH
+ H+
NADP+
+ Pi
glyceraldehyde
3-phosphate
dehydrogenase
H
O
C
H C OH
CH2OPO32-
58
Calvin cycle
Stage 3
• Carbohydrates are formed from glyceraldehyde-3phosphate. The same gluconeogenesis pathways
used earlier are used.
glyceraldehyde-3-phosphate
isomerase
DHAP + glyceraldehyde-3-phosphate
fructose-1,6-bisphosphate + H2O
fructose-6-phosphate
glucose-6-phosphate
dihydroxyacetone phosphate
aldolase
phosphatase
isomerase
phosphoglucomutase
fructose-1,6-bisphosphate
fructose-6-phosphate + Pi
glucose-6-phosphate
glucose-1-phosphate
59
Calvin cycle
Stage 4
• Only one of each six cycles results in
carbohydrate production.
• The other passes through the cycle are used to
regenerate the ribulose-1,5-bisphosphate.
• The first step is the conversion of glyceraldehyde3-phosphate to dihydroxyacetone phosphate.
H
H
O
C
H
C
OH
H
C
H
isomerase
H
C
OH
H
C
O
H
C
H
O
-
O
P
O
O
O-
-
O
P
O
O60
3-phosphoglycerate
H 2O
CO
2
ADP
ATP
ADP
Calvin
cycle
Glycerate-1,3 bisphosphate
Ribulose-1,5bisphosphate
NADPH
NAD +
Pi
ATP
Dihydroxyacetone
phosphate (DHAP)
Ribulose-5phosphate
X5P
F6P
E4P
X5P
S7P
R5P
Pi
Glucose
Sucrose, starch,
cellulose, etc.
Glucose-6phosphate
FbisP
Glyceraldehyde-3
phosphate (G3P)
G3P
DHAP
G3P
DHAP
G3P
Fructose-6phosphate
Pi
Fructose-1,6bisphosphate
61
Photorespiration
Rubisco can act as an oxygenase by
substituting O2 for CO2.
CH2OPO32C
-
O
O
CHOH
CH2OPO32-
rubisco
+ O2
-
CHOH
O
C
+
C
O
O
CHOH
CH2OPO32-
CH2OPO32-
62
Photorespiration
• This appears to be a counter productive
path - oxygen is consumed.
• Some plants have adapted this process as
an optional pathway for carbon fixation.
(sugar cane, corn, sorghum, ...)
• This can be described by the Hatch-Slack
pathway - C4 pathway
63
Hatch-Slack pathway
NADPH + H+
HPO24
oxaloacetate
malate
dehydrogenase
PEP
carboxykinase
HCO 3
NADP+
malate
malic
enzyme
pyruvate
phosphate
dikinase
phosphoenolpyruvate
CO
2
malate
AMP
+ PP i
Mesophyll
cell
pyruvate
ATP +
H 2PO4-
pyruvate
NADPH
+ H+
NADP +
+ CO2
to Calvin Cycle
Bundle
sheath cell
64