Chapter 19
Oxidative phosphorylation
and photophosphorylation
Generation of ATP by using a across-
membrane proton gradient,which is
generated from electron flowing
through a chain of carriers.
1,ATP is synthesized using the same
strategy in oxidative phosphorylation
and photophosphorylation
? Oxidative phosphorylation is the process in which
ATP is generated as a result of electron flow from
NADH or FADH2 to O2 via a series of membrane-
bound electron carriers,called the respiratory chain
(reducing O2 to H2O at the end).
? Photophosphorylation is the process in which ATP
(and NADPH) is synthesized as a result of electron
flow from H2O to NADP+ via a series of membrane-
bound electron carriers (oxidizing H2O to O2 at the
beginning).
? Oxidative phosphorylation and
photophosphorylation are mechanistically similar:
– Both involve the flow of electrons through a chain
of membrane-bound carriers.
– The energy released from,downhill” electron
flow is first used for,uphill” pumping of protons
to produce a proton gradient (thus a
transmembrane electrochemical potential) cross a
biomembrane.
– ATP is then synthesized by a,downhill”
transmembrane flow of protons through a specific
protein machinery,
Cristae (the convoluted
inner membrane of
mitochondria) is
where the respiratory
chain is located.
Oxidative
Phosphorylation
(0n inner membrane
of mitochondria)
Photophosphorylation
(on thylakoid of chloroplasts)
2,Electrons collected in NADH and
FADH2 are released and transported
to O2 via the respiratory chain
? The chain is located on the convoluted inner
membrane (cristae) of mitochondria in eukaryotic
cells (revealed by Eugene Kennedy and Albert
Lehninger in 1948) or on the plasma membrane in
prokaryotic cells.
? A 1.14-volt potential difference (?E`0) between
NADH (-0.320 V) and O2 (0.816 V) drives electron
flow through the chain.
? The respiratory chain consists of four large multi-
protein complexes (I,II,III,and IV; three being
proton pumps) and two mobile electron carriers,
ubiquinone (Q or coenzyme Q) and cytochrome c.
? Prosthetic groups acting in the proteins of
respiratory chain include flavins (FMN,FAD),
hemes (heme A,iron protoporphyrin IX,heme C),
iron-sulfur clusters (2Fe-2S,4Fe-4S),and copper.
Four multi-protein
Complexes (I,II,
III,and IV)
Two mobile
Electron carriers
I II
III
IV
FMN can accept one electron
( and FMNH2 can donate one
electron) to form a semiquinone
radical intermediate.
Heme groups
of cytochrome
proteins
Heme groups
Of cytochromes
Different types of
iron-sulfur centers
?Iron atoms cycle between Fe2+
(reduced) and Fe3+(oxidized).
4Fe-4S2Fe-2S
A ferredoxin
Reduced cytochromes
has three absorption
bands in the visible
wavelengthsCyt a,~600 nm;
Cyt b,~560 nm;
Cyt c,~550 nm
3,NADH enters the chain at NADH
dehydrogenase (complex I)
? Also named as NADH:ubiquinone oxidoreductase or
NADH-Q reductase.
? A,L” shaped 850 kD multimeric protein complex
of 42 different subunits (larger than a ribosome!).
? Polypeptides encoded by both genomes.
? FMN,Fe-S centers act as prosthetic groups.
? Exergonic electron transferring is coupled to
endergonic proton pumping ( with 4 H+ pumped
from the matrix side to intermembrane space per
electron pair transferred),with mechanism unknown.
? Final electron acceptor is ubiquinone (coenzyme Q).
NADH Dehydrogenase
(complex I)
?Fat soluble
benzoquinone with a
very long isoprenoid side
chain; can accept one or
two electrons,forming
radical semiquinone or
ubiquinol (QH2); QH2
diffuses to the next
complex (III); the only
electron carrier not
bound to a protein.
Ubiquinone is a mobile
electron/proton carrier
4,FADH2 of flavoproteins also
transfer their electrons to ubiquinone
? Flavoproteins like succinate dehydrogenase
(complex II),fatty acyl-CoA dehydrogenase,and
glycerol 3-phosphate dehydrogenase are associated
to the inner membrane of mitochondria and transfers
their electrons collected on FADH2 to Q to form
QH2.
? The energy released from these electron transferring
is not high enough to promote proton pumping.
Ubiquinone (Q)
accepts electrons
from both NADH
and FADH2 in the
respiratory chain
5,Electrons of QH2 is transferred to
cytochrome c via ubiquinone:cytochrome
c oxidoreductase (complex III)
? Also called cytochrome c reductase or cytochrome
bc1 complex.
? A 250 kD multiprotein complex of 11 subunits.
? Complete 3-D structure was determined in 1997!
? The functional core consists of three subunits,
cytochrome b (with two hemes,bH and bL); an Fe-S
protein; and cytochrome c1 (with the heme group
covalently bound to protein via two thioether bonds).
? Two-electron carrier QH2 passes one electron to the
one-electron carrier Fe-S center,then to the heme C
group in Cyt c1,and finally to the heme C group of
Cyt c; the other electron to bL,bH,and finally to an
Q or Q.- via a so-called,Q cycle”.
? Cytochrome c,a soluble protein located in the
intermembrane space,will move to complex IV.
Cytochrome bc1 complex
(complex III)
The three
core subunits
Electron path
in complex III
The Q
cycle
The Q
cycle
The 1st QH2 The 2nd QH2
6,Electrons of Cyt c are transferred
to O2 on cytochrome oxidase
(complex IV)
? A 204 kD 13-subunit protein complex,with
structure determined in 1996.
? Three subunits are probably critical to the function.
? Three copper ions (2 CuA,1CuB),two heme A
groups (a and a3) act as electron carriers in complex
IV.
? Four electrons need to be transferred to reduce one
O2 molecule at the,Fe-CuB center” (via peroxy
intermediates) of complex IV to form 2 H2O.
? Four,substrate” protons are consumed from
the N side for every four electrons transferred
to one O2 molecule.
? One proton is pumped out from the N to P side
for each electron to be transferred by an yet
defined mechanism.
The three critical subunits of
cytochrome oxidase (complex IV)
2CuA
CuB
Heme a
Heme a3 CuA
CuA
The electron
path in
complex IV
7,A proton gradient across the inner
membrane of mitochondria is generated
using the electron motive force
? An estimate of 10 protons are pumped for oxidizing
one NADH and 6 for one FADH2 accompanying the
electron flow through complexes I,III and IV.
? Conformational changes induced by electron
transferring is believed to be coupled to proton
pumping (however,the actual mechanisms is yet
revealed!).
? In actively respiring mitochondria,the measured
?pH is about 0.75 and difference in electrical
potential (??) is about 0.15-0.2 V.
? The energy stored in such an H+ gradient can
be used to synthesize ATP or to do other work.
A H+ gradient across the inner membrane
of mitochondria (or plasma membrane of
bacteria) is generated by,uphill” H+
pumping using energy released by the
“downhill” flow of electrons.
8,The order of the many electron
carriers on the respiratory chain have
been elucidated via various studies
? Measurement of the standard reduction potential
(?E`0)),Electrons tend to transfer from low ?E`0
carriers to high ?E`0 carriers (but may deviate from
this in real cells).
? Oxidation kinetics studies,Full reduction followed
by sudden O2 introduction; earlier oxidation,closer
to the end of the respiratory chain; using rapid and
sensitive spectrophotometric techniques to follow
the oxidation of the cytochromes,which have
different wavelength of maximal absorption).
? Effects of various specific inhibitors,those
before the blocked step should be reduced and
those after be oxidized.
? Isolation and characterization of each of the
multiprotein complexes,specific electron
donors and acceptors can be determined for
portions of the chain.
Electron carriers may have an order of increasing E`0
Various inhibitors generate various
patterns of reduced/oxidized carriers
Reduced Oxidized
Reduced Oxidized
Reduced
9,Electron transfer to O2 was found
to be coupled to ATP synthesis from
ADP + Pi in isolated mitochondria
? ATP would not be synthesized when only ADP and
Pi are added in isolated mitochondria suspensions.
? O2 consumption,an indication of electron flow,was
detected when a reductant (e.g.,succinate) is added,
accompanied by an increase of ATP synthesis.
? Both O2 consumption and ATP synthesis were
suppressed when inhibitors of respiratory chain (e.g.,
cyanide,CO,or antimycin A) was added.
? ATP synthesis depends on the occurrence of
electron flow in mitochondria.
? O2 consumption was neither observed if ADP
was not added to the suspension,although a
reductant is provided.
? The O2 consumption was also not observed in the
presence of inhibitors of ATP synthase (e.g.,
oligomycin or venturicidin).
? Electron flow also depends on ATP synthesis!
Electron transfer was found to be
obligatorily coupled to ATP Synthesis
in isolated mitochondria suspensions,
neither occurs without the other.
10,It was widely believed that ATP
synthesis occurs by chemical coupling
? High energy intermediates similar to 1,3-
bisphophoglycerate (which is formed in the
glycolytic pathway and transfers an phosphoryl
group to ADP to form ATP) was proposed to be
produced first from the electron flows on both the
mitochondrial and chloroplast membranes.
? Phophorylated protein intermediates were also
hypothesized.
? But neither were ever revealed despite intense
efforts by a large number of investigators over many
years.
11,The chemiosmotic model was
proposed to explain the coupling of
electron flow and ATP synthesis
? First proposed in 1961 by Peter Michell (a British).
? Energy released from electron transferring is
hypothesized to be first used to pump protons from
the mitochondrial matrix to the intermembrane
space (or from stroma to thylakoid lumen in
chloroplasts),thus generating a proton gradient
across the inner membrane; such a proton-motive
force then drives ATP synthesis by moving protons
back into the matrix via the ATP synthase.
? The model was initially opposed by virtually
all researchers working in oxidative
phosphorylation and photosynthesis.
The chemiosmotic
model by Mitchell
12,The supporting evidences for the
chemiosmotic coupling was collected
? A closed membrane system is essential for ATP
synthesis but not for the electron flow (tested with
detergent or physical shearing.
? Hydrophobic weak acids (DNP and FCCP) and
ionophores (valinomycin) were found to be able to
uncouple ATP synthesis from electron transferring.
? The transmembrane proton pumping has been
experimentally detected,pH in the intermembrane
space was found to decrease when electron flow
occurs (more protons are pumped when NADH,
rather than succinate,is utilized as reductant).
? An artificially imposed electrochemical gradient
across the chloroplast thylakoid membrane and inner
mitochondrial membrane alone (both were
performed using sub-organelle vesicles) were found
to drive ATP synthesis (with the ATP synthase
present).
? The across-membrane proton gradient was thus
finally accepted as the driving force for ATP
synthesis,the chemiosmotic model was accepted as
a theory!
? The chemiosmotic theory unified the apparently
disparate energy transduction processes as oxidative
phosphorylation,photophosphorylation,active
transport across membrane and the motion of
bacterial flagella.
ATP synthesized
DNP,a hydrophobic weak acid,uncouples
ATP synthesis from electron flow
DNP and CCCP
are able to
dissipate the
proton gradient
The artificially
imposed proton
gradient alone
was found to drive
ATP synthesis!
13,ATP synthase was first identified
by dissociation and reconstitution
studies
? Abundant knoblike protruding structures were
observed on the matrix side of the inner
mitochondrial membrane by EM (Racker in 1960).
? The inside-out submitochondrial particles with the
“knobs” are capable of both electron transferring
and ATP synthesis.
? When the protruding F1 part was removed by
agitation,electron transferring could still occur,but
without proton gradient and ATP synthesis.
? ATP synthesis reappeared when F1 was
reconstituted back (the solubilized F1 alone can
catalyze ATP hydrolysis,thus was originally named
as F1ATPase).
? F1 was the first essential factor identified for
oxidative phosphorylation.
14,Isotope exchange experiments
revealed that the ?G`0 for ATP
synthesis on purified F1 is close to zero!
? When solubilized F1 (act as a ATPase) was
incubated with ATP in the presence of 18O-labeled
H2O,three or four 18O atoms were incorporated into
the Pi,indicating multiple rounds of ATP formation
and hydrolysis occurred in the enzyme active site(s).
? The measurement of Kd values,ATP has much
higher affinity to the enzyme.
? The proton gradient was proposed to drive the
release of ATP from the enzyme surface.
The 18O experiment:
The ?G`0 for ATP
synthesis on
purified F1
is close to zero!
(Paul Boyer)
Release of ATP from ATP synthase was proposed to be the
major energy barrier for ATP synthesis
15,ATP synthase comprises a proton
channel (Fo) and a ATPase (F1)
? The F1 part consists of nine subunits of five types,a3b3gde.
? The knoblike F1 portion is a hexamer of alternating a and b
subunits,which sits atop the single rod-shaped g subunit.
? The Fo portion consists three types of subunits,ab2c10-12.
? The c subunits,each forming two transmembrane helices,
form a donut-shaped ring in the plane of the membrane.
? The leg-and-foot-shaped ge subunits stands firmly on the
ring of c subunits.
? The two b subunits of Fo seem to connect to the ab hexamer
via the d subunit of F1.
? The proton channel is believed to lie between the a subunit
and the ring of c subunits.
? X-ray crystallography revealed that the three b subunits of
F1 assumes three different conformations,with bound ADP,
ATP analog,or empty respectively.
The ATP synthase
comprises a proton
channel (Fo) and a
ATPase (F1)
The ten c subunits of Fo
The g subunit of F1
a
b
Rod-shaped g subunit.
ADP App(NH)p
Empty
Each b sununit of ATP synthase can
assume three different conformations!
16,The binding-change model was
proposed to explain the action
mechanism of ATP synthase
? The model was proposed based on kinetic and binding
studies in (before the 3-D structure of bovine F1 or yeast
FoF1 was determined).
? Downhill proton movement through Fo will drive the
rotation of the c-subunit ring and the asymmetrical g
subunits,which will cause each of the three b subunits to
interconvert between the three conformations,as a result,
each of them take turns to take up ADP + Pi,synthesize
ATP,and release ATP.
? Rotation of the g subunit of the F1 unit in discrete
120o (powered by ATP hydrolysis catalyzed by the
b subunits) has been observed using a fluorescence
microscope.
? The estimation of H+ consumption for each ATP
formed is 4 (among which one is consumed for Pi
transport).
b-ATP
b-ADPb-empty
The binding-change
Model proposed by
Paul Boyer
g
gg
Rotation of the g
subunit and the
ring of c subunits
in the FoF1 complex
was observed by
in vitro studies
using fluorescence
microscopy
Rotation of the g subunit and the ring of c subunits
in the FoF1 complex was directely observed by in vitro
studies using fluorescence microscopy
17,The energy stored in the proton
gradient can be used to do other work
? The ADP,Pi,and pyruvate are believed to be
transported into,and ATP out the mitochondrial
matrix by using the proton gradient.
? The rotary motion of the bacterial flagella is
energized directly by the proton gradient across the
cytoplasmic membrane.
? The thermogenin on the inner mitochondrial
membrane of brown fat tissue cells uses the proton-
gradient to produce heat to maintain body
temperature for hibernating animals,newborn
animals and mammals adapted to cold
(thermogenesis).
The proton-
motive force
is used for
active transport
through the
inner
membrane of
the mitochondria
The rotary motion of the bacterial flagella
is energized directly by the proton
gradient across the cytoplasmic
Membrane.
18,Electrons in NADH generated in
cytosol is shuttled into mitochondria
to enter the respiratory chain
? This is usually fulfilled by the malate-aspartate
shuttle system in liver,kidney and heart,using the
Malate-a-ketoglutarate and the glutamate-aspartate
transporters.
? Electrons of NADH in the cytosols of skeletal
muscle and brain are often shuttled into the matrix
by using the glycerol 3-phosphate shuttle,which
delivers the electrons to complex III,thus releasing
less amount of energy for proton gradient generation.
? Electrons of NADH generated in plant cytosol enter
the respiratory chain directly with no need of
shuttling due to the presence of an externally
oriented NADH dehydrogenase.
19,The pathways leading to ATP
sysnthesis is coordinately regulated
? The rate of the respiration is generally controlled by
the availability of ADP,since ADP acts as the
acceptor of Pi,this way of regulation is thus called
“acceptor control”.
? ATP,ADP,NADH,NAD+ regulates rate of fuel
oxidation at further upstream steps (in stage I and II
of fuel oxidation pathway).
? The ratio [ATP]/([ADP][Pi]) fluctuates only slightly
in most tissues due to a coordinated regulation of all
the pathways leading to ATP production.
20,Photosynthetic organisms generate
ATPs (and NADPH) via
photophosphorylation.
? The molecular mechanism of photophosphorylation
is remarkably similar to that of oxidative
phosphorylation,also mediated via a across-
membrane proton gradient generated using energy
released from stepwise electron flow through a
series of electron carriers,located on the thylakoid
membranes of chloroplasts or plasma membrane of
the bacteria.
? The electron donor in photophosphorylation,H2O,is
first charged by using light energy to provide
electrons of high potential energy.
? The excess energy-rich ATP and NADPH generated
by photophosphorylation is further stored in stable
energy-rich carbohydrates through the carbon-
assimilation (fixation) reactions occurring in the
stroma of chloroplasts.
Stage I
Stage II
Stage III
Stage I
Stroma
Thylakoid
Stage IV
Stroma
Thylakoid
21,It took a long time for humans to
understand the chemical process of
photosynthesis
? O2 is produced by plants (1780).
? Light is needed for plants to produce O2 (1786).
? CO2 is taken up by plants (1790s).
? H2O is taken up during CO2 fixation because the
sum of weights of organic matter and O2 is much
more than the weight of CO2 consumed,water is the
only other substance present (1790s).
? Plants convert solar energy into chemical free
energy (1842).
? Experiment with leaf extract containing
chloroplasts revealed that absorbed light energy
causes electrons to flow from H2O to an artificial
electron acceptor (NADP+ was found to be the
acceptor in chloroplasts later); CO2 is not required
for this process; therefore O2 could not be
produced from CO2 (1930s,Hill);
? Radio isotope tracer experiments revealed that CO2
is added to ribulose-1,5-bisphosphate in a cyclic
pathway before it is used for glucose synthesis
(1950s,Calvin).
22,The major light absorbing
pigments on thylakoid membrane
was revealed to be chlorophylls
? Chlorophylls (a and b) were found to resemble the
heme group of hemoglobin,being polycyclic planar
polytenes,except that the central Fe2+ is replaced by
a Mg 2+; a 21-carbon alcohol called phytol is
attached to a carboxyl group on the protoporphyrin
ring; there exists an extra non-pyrrole ring.
? The light absorbing pigments in algae and
photosynthetic bacteria (named as
bacteriochlorophyll) are very similar to that of
higher plants.
? Carotenoids,absorbing light at wavelengths distinct
from chlorophylls,act as accessory pigments on
thylakoid membranes,
? Chlorophyll is always associated with specific
proteins to form light-harvesting complexes (LHCs).
? The absorption spectra of chlorophyll a and b
overlap with the action spectrum of photosynthesis
in chloroplasts.
? Cyanobacteria and red algae use open-chain
tetrapyrroles,called phycobilins,to absorb light at
wavelengths between 520-630 nm.
The light absorbing
pigments in higher
plants,algae,and
photosynthetic
bacteria are all
heme-like molecules.
Carotenoids,act as accessory pigments on
thylakoid membranes
Phycobilins
A light-harvesting
complexe (LHCII)
Chlorophyll a
Chlorophyll b
Lutein
The action spectrum
of photosynthesis in alga
overlaps with the
absorption spectra
of chlorophyll a and b.
Light absorbing pigments in cyanobacteria
and red algae,phycobilins,are open-chain
tetraparrole polytenes
23,Photons absorbed by many
chlorophylls funnel into one
reaction center via exciton transfer
? A saturating light flash was found to lead to the
production of only one O2 per 2500 chlorophyll
molecules (chlorella cells,1932).
? The photosynthetic unit concept was thus proposed,
photons absorbed by many antenna pigments funnel
to one reaction center having a specially localized
pair of chlorophyll a molecules via exciton transfer.
? Charge separation would occur at the reaction center,
where electron flow,in turn,would be initiated.
Possible way of
pigment arrangement
in a photosysntem
Charge separation at the reaction center
may be caused by the absorption of one
photon from one chlorophyll molecule
24,Two types of photochemical
reaction centers have been revealed
in bacteria
? Type II in purple bacteria,a cyclic electron flow
pathway; electrons activated from the reaction
center chlorophylls (P870) are first accepted by
pheophytins (chlorophylls lacking the central Mg2+)
causing charge separation; then to a quinone,before
being transferred back to P870 via cytochrome bc1
complex and Cyt c2.
? Type I in green sulfur bacteria,both a cyclic
electron pathway corresponding to the one in purple
bacteria and a linear pathway leading to NADH
formation using ferredoxin (a 2Fe-2S) and Fd-NAD
reductase.
? The cytochrome bc complexes,being similar to the
complex III in mitochondria,pumps protons across
the plasma membranes.
Cyt c2
A deduced path of
electron flow on
purple bacterial
plasma membrane
Cytochrome
bc1 complex
(exciton)
Cyt c2
QH2
The two protein
complexes involved
in electron transferring
in purple bacteria
Cytochrome
bc1 complex
Phtoreaction
center
Cyt c2
The cyclic and noncyclic electron transferring path
found in photosynthetic bacteria.
24,Two photosystems (II and I) work
in tandem to move electrons from
H2O to NADP+ in higher plants
? PSI and PSII were revealed by quantum efficiency
studies (“red drop” and,enhancement” phenomena
for chloroplasts) and bleaching studies (a temporary
decrease in absorption of light at a specific
wavelength).
? The electrons are charged twice (at P680 and P700).
? Pheophytin (脱镁叶绿素 ) acts as the first electron
acceptor for the excited chlorophyll molecules (the
“special pair”) in PSII resulting in charge separation.
? Plastoquinone,structurally similar to ubiquinone,
carries electrons from PSII to cytochrome b6f
complex.
? The cytochrome b6f complex (similar to the
cytochrome bc1 complex for oxidative
phosphorylation) pumps H+ across the thylakoid
membrane.
? Plastocyanin (质体蓝素 ),a Cu-containing soluble
protein,carries electrons from the cytochrome b6f
complex to P700 of PSII (playing a similar role as
cytochrome c in oxidative phosphorylation).
? The cytochrome b6f complex and cytochrome c act
in both oxidative phosphorylation and
phtophosphorylation in cyanobacteria.
? PSII are often found only in the stacked regions of
the thylakoid membrane,with PSI and ATP
synthase often only in the unstacked region.
Proton
gradient
The, Z scheme” to
show the electron
flow from PSII to
PSI
Flow of electrons from QH2 to
plastocyanin via cytochrome b6f
complex
The cytochrome b6f complex
and cytochrome c act in both
oxidative phosphorylation
and phtophosphorylation in
cyanobacteria.
H+
H+ H+
H+ H+
H+
H+
25,P680+ in PSII extracts electrons
from H2O to form O2 via a Mn-
containing oxygen-evolving complex
? P680+ first accepts electrons from a Tyr residue
(often designated as Z) of the D1 subunits of PSII,
producing a Tyrosyl radical (Tyr*).
? Tyr* then accepts electrons from the Mn complex in
the oxygen-evolving complex.
? The Mn complex is believe to serve as a charge
accumulator that enables O2 to be formed (from 2
splitting H2O) without generating hazardous partly
reduced intermediates,however with mechanism yet
to be elucidated.
The Mn complex releases H+ to the thylakoid lumen
while transferring electrons from H2O to Tyr via a
mechanism yet to be revealed
2 H2O?
Summary
? ATP is synthesized using the same strategy in
oxidative phosphorylation and photophosphorylation.
? Electrons collected in NADH and FADH2 are
released (at different entering points) and
transported to O2 via the respiratory chain,which
consists of four multiprotein complexes (I,II,III,
and IV) and two mobile electron carriers
(ubiquinone and cytochrome c).
? A proton gradient across the inner membrane of
mitochondria is generated using the electron motive
force generated by electron transferring through the
respiratory chain.
? The order of the many electron carriers on the
respiratory chain have been elucidated via various
studies,including measurements of the standard
reduction potential,oxidation kinetics of the electron
carriers,and effects of various respiratory chain
inhibitors.
? Electron transfer to O2 was found to be coupled to
ATP synthesis from ADP + Pi in isolated
mitochondria.
? The chemiosmotic theory explains the coupling of
electron flow and ATP synthesis.
? Isotope exchange experiments revealed that the
?G`0 for ATP synthesis on purified F1 is close to
zero!
? ATP synthase comprises a proton channel (Fo) and a
ATPase (F1).
? The binding-change model was proposed to explain the
action mechanism of ATP synthase.
? The energy stored in the proton gradient can be used to do
other work.
? Electrons in NADH generated in cytosol is shuttled into
mitochondria to enter the respiratory chain.
? The pathways leading to ATP sysnthesis is coordinately
regulated.
? Photosynthetic organisms generate ATPs (and NADPH) via
photophosphorylation.
? It took a long time for humans to understand the
chemical process of photosynthesis.
? The major light absorbing pigments on thylakoid
membrane was revealed to be chlorophylls.
? Photons absorbed by many chlorophylls funnel into
one reaction center via exciton transfer.
? Two types of photochemical reaction centers have
been revealed in bacteria.
? Two photosystems (II and I) work in tandem to
move electrons from H2O to NADP+ in higher plants.
? P680+ in PSII extracts electrons from H2O to form
O2 via a Mn-containing oxygen-evolving complex.
Oxidative phosphorylation
and photophosphorylation
Generation of ATP by using a across-
membrane proton gradient,which is
generated from electron flowing
through a chain of carriers.
1,ATP is synthesized using the same
strategy in oxidative phosphorylation
and photophosphorylation
? Oxidative phosphorylation is the process in which
ATP is generated as a result of electron flow from
NADH or FADH2 to O2 via a series of membrane-
bound electron carriers,called the respiratory chain
(reducing O2 to H2O at the end).
? Photophosphorylation is the process in which ATP
(and NADPH) is synthesized as a result of electron
flow from H2O to NADP+ via a series of membrane-
bound electron carriers (oxidizing H2O to O2 at the
beginning).
? Oxidative phosphorylation and
photophosphorylation are mechanistically similar:
– Both involve the flow of electrons through a chain
of membrane-bound carriers.
– The energy released from,downhill” electron
flow is first used for,uphill” pumping of protons
to produce a proton gradient (thus a
transmembrane electrochemical potential) cross a
biomembrane.
– ATP is then synthesized by a,downhill”
transmembrane flow of protons through a specific
protein machinery,
Cristae (the convoluted
inner membrane of
mitochondria) is
where the respiratory
chain is located.
Oxidative
Phosphorylation
(0n inner membrane
of mitochondria)
Photophosphorylation
(on thylakoid of chloroplasts)
2,Electrons collected in NADH and
FADH2 are released and transported
to O2 via the respiratory chain
? The chain is located on the convoluted inner
membrane (cristae) of mitochondria in eukaryotic
cells (revealed by Eugene Kennedy and Albert
Lehninger in 1948) or on the plasma membrane in
prokaryotic cells.
? A 1.14-volt potential difference (?E`0) between
NADH (-0.320 V) and O2 (0.816 V) drives electron
flow through the chain.
? The respiratory chain consists of four large multi-
protein complexes (I,II,III,and IV; three being
proton pumps) and two mobile electron carriers,
ubiquinone (Q or coenzyme Q) and cytochrome c.
? Prosthetic groups acting in the proteins of
respiratory chain include flavins (FMN,FAD),
hemes (heme A,iron protoporphyrin IX,heme C),
iron-sulfur clusters (2Fe-2S,4Fe-4S),and copper.
Four multi-protein
Complexes (I,II,
III,and IV)
Two mobile
Electron carriers
I II
III
IV
FMN can accept one electron
( and FMNH2 can donate one
electron) to form a semiquinone
radical intermediate.
Heme groups
of cytochrome
proteins
Heme groups
Of cytochromes
Different types of
iron-sulfur centers
?Iron atoms cycle between Fe2+
(reduced) and Fe3+(oxidized).
4Fe-4S2Fe-2S
A ferredoxin
Reduced cytochromes
has three absorption
bands in the visible
wavelengthsCyt a,~600 nm;
Cyt b,~560 nm;
Cyt c,~550 nm
3,NADH enters the chain at NADH
dehydrogenase (complex I)
? Also named as NADH:ubiquinone oxidoreductase or
NADH-Q reductase.
? A,L” shaped 850 kD multimeric protein complex
of 42 different subunits (larger than a ribosome!).
? Polypeptides encoded by both genomes.
? FMN,Fe-S centers act as prosthetic groups.
? Exergonic electron transferring is coupled to
endergonic proton pumping ( with 4 H+ pumped
from the matrix side to intermembrane space per
electron pair transferred),with mechanism unknown.
? Final electron acceptor is ubiquinone (coenzyme Q).
NADH Dehydrogenase
(complex I)
?Fat soluble
benzoquinone with a
very long isoprenoid side
chain; can accept one or
two electrons,forming
radical semiquinone or
ubiquinol (QH2); QH2
diffuses to the next
complex (III); the only
electron carrier not
bound to a protein.
Ubiquinone is a mobile
electron/proton carrier
4,FADH2 of flavoproteins also
transfer their electrons to ubiquinone
? Flavoproteins like succinate dehydrogenase
(complex II),fatty acyl-CoA dehydrogenase,and
glycerol 3-phosphate dehydrogenase are associated
to the inner membrane of mitochondria and transfers
their electrons collected on FADH2 to Q to form
QH2.
? The energy released from these electron transferring
is not high enough to promote proton pumping.
Ubiquinone (Q)
accepts electrons
from both NADH
and FADH2 in the
respiratory chain
5,Electrons of QH2 is transferred to
cytochrome c via ubiquinone:cytochrome
c oxidoreductase (complex III)
? Also called cytochrome c reductase or cytochrome
bc1 complex.
? A 250 kD multiprotein complex of 11 subunits.
? Complete 3-D structure was determined in 1997!
? The functional core consists of three subunits,
cytochrome b (with two hemes,bH and bL); an Fe-S
protein; and cytochrome c1 (with the heme group
covalently bound to protein via two thioether bonds).
? Two-electron carrier QH2 passes one electron to the
one-electron carrier Fe-S center,then to the heme C
group in Cyt c1,and finally to the heme C group of
Cyt c; the other electron to bL,bH,and finally to an
Q or Q.- via a so-called,Q cycle”.
? Cytochrome c,a soluble protein located in the
intermembrane space,will move to complex IV.
Cytochrome bc1 complex
(complex III)
The three
core subunits
Electron path
in complex III
The Q
cycle
The Q
cycle
The 1st QH2 The 2nd QH2
6,Electrons of Cyt c are transferred
to O2 on cytochrome oxidase
(complex IV)
? A 204 kD 13-subunit protein complex,with
structure determined in 1996.
? Three subunits are probably critical to the function.
? Three copper ions (2 CuA,1CuB),two heme A
groups (a and a3) act as electron carriers in complex
IV.
? Four electrons need to be transferred to reduce one
O2 molecule at the,Fe-CuB center” (via peroxy
intermediates) of complex IV to form 2 H2O.
? Four,substrate” protons are consumed from
the N side for every four electrons transferred
to one O2 molecule.
? One proton is pumped out from the N to P side
for each electron to be transferred by an yet
defined mechanism.
The three critical subunits of
cytochrome oxidase (complex IV)
2CuA
CuB
Heme a
Heme a3 CuA
CuA
The electron
path in
complex IV
7,A proton gradient across the inner
membrane of mitochondria is generated
using the electron motive force
? An estimate of 10 protons are pumped for oxidizing
one NADH and 6 for one FADH2 accompanying the
electron flow through complexes I,III and IV.
? Conformational changes induced by electron
transferring is believed to be coupled to proton
pumping (however,the actual mechanisms is yet
revealed!).
? In actively respiring mitochondria,the measured
?pH is about 0.75 and difference in electrical
potential (??) is about 0.15-0.2 V.
? The energy stored in such an H+ gradient can
be used to synthesize ATP or to do other work.
A H+ gradient across the inner membrane
of mitochondria (or plasma membrane of
bacteria) is generated by,uphill” H+
pumping using energy released by the
“downhill” flow of electrons.
8,The order of the many electron
carriers on the respiratory chain have
been elucidated via various studies
? Measurement of the standard reduction potential
(?E`0)),Electrons tend to transfer from low ?E`0
carriers to high ?E`0 carriers (but may deviate from
this in real cells).
? Oxidation kinetics studies,Full reduction followed
by sudden O2 introduction; earlier oxidation,closer
to the end of the respiratory chain; using rapid and
sensitive spectrophotometric techniques to follow
the oxidation of the cytochromes,which have
different wavelength of maximal absorption).
? Effects of various specific inhibitors,those
before the blocked step should be reduced and
those after be oxidized.
? Isolation and characterization of each of the
multiprotein complexes,specific electron
donors and acceptors can be determined for
portions of the chain.
Electron carriers may have an order of increasing E`0
Various inhibitors generate various
patterns of reduced/oxidized carriers
Reduced Oxidized
Reduced Oxidized
Reduced
9,Electron transfer to O2 was found
to be coupled to ATP synthesis from
ADP + Pi in isolated mitochondria
? ATP would not be synthesized when only ADP and
Pi are added in isolated mitochondria suspensions.
? O2 consumption,an indication of electron flow,was
detected when a reductant (e.g.,succinate) is added,
accompanied by an increase of ATP synthesis.
? Both O2 consumption and ATP synthesis were
suppressed when inhibitors of respiratory chain (e.g.,
cyanide,CO,or antimycin A) was added.
? ATP synthesis depends on the occurrence of
electron flow in mitochondria.
? O2 consumption was neither observed if ADP
was not added to the suspension,although a
reductant is provided.
? The O2 consumption was also not observed in the
presence of inhibitors of ATP synthase (e.g.,
oligomycin or venturicidin).
? Electron flow also depends on ATP synthesis!
Electron transfer was found to be
obligatorily coupled to ATP Synthesis
in isolated mitochondria suspensions,
neither occurs without the other.
10,It was widely believed that ATP
synthesis occurs by chemical coupling
? High energy intermediates similar to 1,3-
bisphophoglycerate (which is formed in the
glycolytic pathway and transfers an phosphoryl
group to ADP to form ATP) was proposed to be
produced first from the electron flows on both the
mitochondrial and chloroplast membranes.
? Phophorylated protein intermediates were also
hypothesized.
? But neither were ever revealed despite intense
efforts by a large number of investigators over many
years.
11,The chemiosmotic model was
proposed to explain the coupling of
electron flow and ATP synthesis
? First proposed in 1961 by Peter Michell (a British).
? Energy released from electron transferring is
hypothesized to be first used to pump protons from
the mitochondrial matrix to the intermembrane
space (or from stroma to thylakoid lumen in
chloroplasts),thus generating a proton gradient
across the inner membrane; such a proton-motive
force then drives ATP synthesis by moving protons
back into the matrix via the ATP synthase.
? The model was initially opposed by virtually
all researchers working in oxidative
phosphorylation and photosynthesis.
The chemiosmotic
model by Mitchell
12,The supporting evidences for the
chemiosmotic coupling was collected
? A closed membrane system is essential for ATP
synthesis but not for the electron flow (tested with
detergent or physical shearing.
? Hydrophobic weak acids (DNP and FCCP) and
ionophores (valinomycin) were found to be able to
uncouple ATP synthesis from electron transferring.
? The transmembrane proton pumping has been
experimentally detected,pH in the intermembrane
space was found to decrease when electron flow
occurs (more protons are pumped when NADH,
rather than succinate,is utilized as reductant).
? An artificially imposed electrochemical gradient
across the chloroplast thylakoid membrane and inner
mitochondrial membrane alone (both were
performed using sub-organelle vesicles) were found
to drive ATP synthesis (with the ATP synthase
present).
? The across-membrane proton gradient was thus
finally accepted as the driving force for ATP
synthesis,the chemiosmotic model was accepted as
a theory!
? The chemiosmotic theory unified the apparently
disparate energy transduction processes as oxidative
phosphorylation,photophosphorylation,active
transport across membrane and the motion of
bacterial flagella.
ATP synthesized
DNP,a hydrophobic weak acid,uncouples
ATP synthesis from electron flow
DNP and CCCP
are able to
dissipate the
proton gradient
The artificially
imposed proton
gradient alone
was found to drive
ATP synthesis!
13,ATP synthase was first identified
by dissociation and reconstitution
studies
? Abundant knoblike protruding structures were
observed on the matrix side of the inner
mitochondrial membrane by EM (Racker in 1960).
? The inside-out submitochondrial particles with the
“knobs” are capable of both electron transferring
and ATP synthesis.
? When the protruding F1 part was removed by
agitation,electron transferring could still occur,but
without proton gradient and ATP synthesis.
? ATP synthesis reappeared when F1 was
reconstituted back (the solubilized F1 alone can
catalyze ATP hydrolysis,thus was originally named
as F1ATPase).
? F1 was the first essential factor identified for
oxidative phosphorylation.
14,Isotope exchange experiments
revealed that the ?G`0 for ATP
synthesis on purified F1 is close to zero!
? When solubilized F1 (act as a ATPase) was
incubated with ATP in the presence of 18O-labeled
H2O,three or four 18O atoms were incorporated into
the Pi,indicating multiple rounds of ATP formation
and hydrolysis occurred in the enzyme active site(s).
? The measurement of Kd values,ATP has much
higher affinity to the enzyme.
? The proton gradient was proposed to drive the
release of ATP from the enzyme surface.
The 18O experiment:
The ?G`0 for ATP
synthesis on
purified F1
is close to zero!
(Paul Boyer)
Release of ATP from ATP synthase was proposed to be the
major energy barrier for ATP synthesis
15,ATP synthase comprises a proton
channel (Fo) and a ATPase (F1)
? The F1 part consists of nine subunits of five types,a3b3gde.
? The knoblike F1 portion is a hexamer of alternating a and b
subunits,which sits atop the single rod-shaped g subunit.
? The Fo portion consists three types of subunits,ab2c10-12.
? The c subunits,each forming two transmembrane helices,
form a donut-shaped ring in the plane of the membrane.
? The leg-and-foot-shaped ge subunits stands firmly on the
ring of c subunits.
? The two b subunits of Fo seem to connect to the ab hexamer
via the d subunit of F1.
? The proton channel is believed to lie between the a subunit
and the ring of c subunits.
? X-ray crystallography revealed that the three b subunits of
F1 assumes three different conformations,with bound ADP,
ATP analog,or empty respectively.
The ATP synthase
comprises a proton
channel (Fo) and a
ATPase (F1)
The ten c subunits of Fo
The g subunit of F1
a
b
Rod-shaped g subunit.
ADP App(NH)p
Empty
Each b sununit of ATP synthase can
assume three different conformations!
16,The binding-change model was
proposed to explain the action
mechanism of ATP synthase
? The model was proposed based on kinetic and binding
studies in (before the 3-D structure of bovine F1 or yeast
FoF1 was determined).
? Downhill proton movement through Fo will drive the
rotation of the c-subunit ring and the asymmetrical g
subunits,which will cause each of the three b subunits to
interconvert between the three conformations,as a result,
each of them take turns to take up ADP + Pi,synthesize
ATP,and release ATP.
? Rotation of the g subunit of the F1 unit in discrete
120o (powered by ATP hydrolysis catalyzed by the
b subunits) has been observed using a fluorescence
microscope.
? The estimation of H+ consumption for each ATP
formed is 4 (among which one is consumed for Pi
transport).
b-ATP
b-ADPb-empty
The binding-change
Model proposed by
Paul Boyer
g
gg
Rotation of the g
subunit and the
ring of c subunits
in the FoF1 complex
was observed by
in vitro studies
using fluorescence
microscopy
Rotation of the g subunit and the ring of c subunits
in the FoF1 complex was directely observed by in vitro
studies using fluorescence microscopy
17,The energy stored in the proton
gradient can be used to do other work
? The ADP,Pi,and pyruvate are believed to be
transported into,and ATP out the mitochondrial
matrix by using the proton gradient.
? The rotary motion of the bacterial flagella is
energized directly by the proton gradient across the
cytoplasmic membrane.
? The thermogenin on the inner mitochondrial
membrane of brown fat tissue cells uses the proton-
gradient to produce heat to maintain body
temperature for hibernating animals,newborn
animals and mammals adapted to cold
(thermogenesis).
The proton-
motive force
is used for
active transport
through the
inner
membrane of
the mitochondria
The rotary motion of the bacterial flagella
is energized directly by the proton
gradient across the cytoplasmic
Membrane.
18,Electrons in NADH generated in
cytosol is shuttled into mitochondria
to enter the respiratory chain
? This is usually fulfilled by the malate-aspartate
shuttle system in liver,kidney and heart,using the
Malate-a-ketoglutarate and the glutamate-aspartate
transporters.
? Electrons of NADH in the cytosols of skeletal
muscle and brain are often shuttled into the matrix
by using the glycerol 3-phosphate shuttle,which
delivers the electrons to complex III,thus releasing
less amount of energy for proton gradient generation.
? Electrons of NADH generated in plant cytosol enter
the respiratory chain directly with no need of
shuttling due to the presence of an externally
oriented NADH dehydrogenase.
19,The pathways leading to ATP
sysnthesis is coordinately regulated
? The rate of the respiration is generally controlled by
the availability of ADP,since ADP acts as the
acceptor of Pi,this way of regulation is thus called
“acceptor control”.
? ATP,ADP,NADH,NAD+ regulates rate of fuel
oxidation at further upstream steps (in stage I and II
of fuel oxidation pathway).
? The ratio [ATP]/([ADP][Pi]) fluctuates only slightly
in most tissues due to a coordinated regulation of all
the pathways leading to ATP production.
20,Photosynthetic organisms generate
ATPs (and NADPH) via
photophosphorylation.
? The molecular mechanism of photophosphorylation
is remarkably similar to that of oxidative
phosphorylation,also mediated via a across-
membrane proton gradient generated using energy
released from stepwise electron flow through a
series of electron carriers,located on the thylakoid
membranes of chloroplasts or plasma membrane of
the bacteria.
? The electron donor in photophosphorylation,H2O,is
first charged by using light energy to provide
electrons of high potential energy.
? The excess energy-rich ATP and NADPH generated
by photophosphorylation is further stored in stable
energy-rich carbohydrates through the carbon-
assimilation (fixation) reactions occurring in the
stroma of chloroplasts.
Stage I
Stage II
Stage III
Stage I
Stroma
Thylakoid
Stage IV
Stroma
Thylakoid
21,It took a long time for humans to
understand the chemical process of
photosynthesis
? O2 is produced by plants (1780).
? Light is needed for plants to produce O2 (1786).
? CO2 is taken up by plants (1790s).
? H2O is taken up during CO2 fixation because the
sum of weights of organic matter and O2 is much
more than the weight of CO2 consumed,water is the
only other substance present (1790s).
? Plants convert solar energy into chemical free
energy (1842).
? Experiment with leaf extract containing
chloroplasts revealed that absorbed light energy
causes electrons to flow from H2O to an artificial
electron acceptor (NADP+ was found to be the
acceptor in chloroplasts later); CO2 is not required
for this process; therefore O2 could not be
produced from CO2 (1930s,Hill);
? Radio isotope tracer experiments revealed that CO2
is added to ribulose-1,5-bisphosphate in a cyclic
pathway before it is used for glucose synthesis
(1950s,Calvin).
22,The major light absorbing
pigments on thylakoid membrane
was revealed to be chlorophylls
? Chlorophylls (a and b) were found to resemble the
heme group of hemoglobin,being polycyclic planar
polytenes,except that the central Fe2+ is replaced by
a Mg 2+; a 21-carbon alcohol called phytol is
attached to a carboxyl group on the protoporphyrin
ring; there exists an extra non-pyrrole ring.
? The light absorbing pigments in algae and
photosynthetic bacteria (named as
bacteriochlorophyll) are very similar to that of
higher plants.
? Carotenoids,absorbing light at wavelengths distinct
from chlorophylls,act as accessory pigments on
thylakoid membranes,
? Chlorophyll is always associated with specific
proteins to form light-harvesting complexes (LHCs).
? The absorption spectra of chlorophyll a and b
overlap with the action spectrum of photosynthesis
in chloroplasts.
? Cyanobacteria and red algae use open-chain
tetrapyrroles,called phycobilins,to absorb light at
wavelengths between 520-630 nm.
The light absorbing
pigments in higher
plants,algae,and
photosynthetic
bacteria are all
heme-like molecules.
Carotenoids,act as accessory pigments on
thylakoid membranes
Phycobilins
A light-harvesting
complexe (LHCII)
Chlorophyll a
Chlorophyll b
Lutein
The action spectrum
of photosynthesis in alga
overlaps with the
absorption spectra
of chlorophyll a and b.
Light absorbing pigments in cyanobacteria
and red algae,phycobilins,are open-chain
tetraparrole polytenes
23,Photons absorbed by many
chlorophylls funnel into one
reaction center via exciton transfer
? A saturating light flash was found to lead to the
production of only one O2 per 2500 chlorophyll
molecules (chlorella cells,1932).
? The photosynthetic unit concept was thus proposed,
photons absorbed by many antenna pigments funnel
to one reaction center having a specially localized
pair of chlorophyll a molecules via exciton transfer.
? Charge separation would occur at the reaction center,
where electron flow,in turn,would be initiated.
Possible way of
pigment arrangement
in a photosysntem
Charge separation at the reaction center
may be caused by the absorption of one
photon from one chlorophyll molecule
24,Two types of photochemical
reaction centers have been revealed
in bacteria
? Type II in purple bacteria,a cyclic electron flow
pathway; electrons activated from the reaction
center chlorophylls (P870) are first accepted by
pheophytins (chlorophylls lacking the central Mg2+)
causing charge separation; then to a quinone,before
being transferred back to P870 via cytochrome bc1
complex and Cyt c2.
? Type I in green sulfur bacteria,both a cyclic
electron pathway corresponding to the one in purple
bacteria and a linear pathway leading to NADH
formation using ferredoxin (a 2Fe-2S) and Fd-NAD
reductase.
? The cytochrome bc complexes,being similar to the
complex III in mitochondria,pumps protons across
the plasma membranes.
Cyt c2
A deduced path of
electron flow on
purple bacterial
plasma membrane
Cytochrome
bc1 complex
(exciton)
Cyt c2
QH2
The two protein
complexes involved
in electron transferring
in purple bacteria
Cytochrome
bc1 complex
Phtoreaction
center
Cyt c2
The cyclic and noncyclic electron transferring path
found in photosynthetic bacteria.
24,Two photosystems (II and I) work
in tandem to move electrons from
H2O to NADP+ in higher plants
? PSI and PSII were revealed by quantum efficiency
studies (“red drop” and,enhancement” phenomena
for chloroplasts) and bleaching studies (a temporary
decrease in absorption of light at a specific
wavelength).
? The electrons are charged twice (at P680 and P700).
? Pheophytin (脱镁叶绿素 ) acts as the first electron
acceptor for the excited chlorophyll molecules (the
“special pair”) in PSII resulting in charge separation.
? Plastoquinone,structurally similar to ubiquinone,
carries electrons from PSII to cytochrome b6f
complex.
? The cytochrome b6f complex (similar to the
cytochrome bc1 complex for oxidative
phosphorylation) pumps H+ across the thylakoid
membrane.
? Plastocyanin (质体蓝素 ),a Cu-containing soluble
protein,carries electrons from the cytochrome b6f
complex to P700 of PSII (playing a similar role as
cytochrome c in oxidative phosphorylation).
? The cytochrome b6f complex and cytochrome c act
in both oxidative phosphorylation and
phtophosphorylation in cyanobacteria.
? PSII are often found only in the stacked regions of
the thylakoid membrane,with PSI and ATP
synthase often only in the unstacked region.
Proton
gradient
The, Z scheme” to
show the electron
flow from PSII to
PSI
Flow of electrons from QH2 to
plastocyanin via cytochrome b6f
complex
The cytochrome b6f complex
and cytochrome c act in both
oxidative phosphorylation
and phtophosphorylation in
cyanobacteria.
H+
H+ H+
H+ H+
H+
H+
25,P680+ in PSII extracts electrons
from H2O to form O2 via a Mn-
containing oxygen-evolving complex
? P680+ first accepts electrons from a Tyr residue
(often designated as Z) of the D1 subunits of PSII,
producing a Tyrosyl radical (Tyr*).
? Tyr* then accepts electrons from the Mn complex in
the oxygen-evolving complex.
? The Mn complex is believe to serve as a charge
accumulator that enables O2 to be formed (from 2
splitting H2O) without generating hazardous partly
reduced intermediates,however with mechanism yet
to be elucidated.
The Mn complex releases H+ to the thylakoid lumen
while transferring electrons from H2O to Tyr via a
mechanism yet to be revealed
2 H2O?
Summary
? ATP is synthesized using the same strategy in
oxidative phosphorylation and photophosphorylation.
? Electrons collected in NADH and FADH2 are
released (at different entering points) and
transported to O2 via the respiratory chain,which
consists of four multiprotein complexes (I,II,III,
and IV) and two mobile electron carriers
(ubiquinone and cytochrome c).
? A proton gradient across the inner membrane of
mitochondria is generated using the electron motive
force generated by electron transferring through the
respiratory chain.
? The order of the many electron carriers on the
respiratory chain have been elucidated via various
studies,including measurements of the standard
reduction potential,oxidation kinetics of the electron
carriers,and effects of various respiratory chain
inhibitors.
? Electron transfer to O2 was found to be coupled to
ATP synthesis from ADP + Pi in isolated
mitochondria.
? The chemiosmotic theory explains the coupling of
electron flow and ATP synthesis.
? Isotope exchange experiments revealed that the
?G`0 for ATP synthesis on purified F1 is close to
zero!
? ATP synthase comprises a proton channel (Fo) and a
ATPase (F1).
? The binding-change model was proposed to explain the
action mechanism of ATP synthase.
? The energy stored in the proton gradient can be used to do
other work.
? Electrons in NADH generated in cytosol is shuttled into
mitochondria to enter the respiratory chain.
? The pathways leading to ATP sysnthesis is coordinately
regulated.
? Photosynthetic organisms generate ATPs (and NADPH) via
photophosphorylation.
? It took a long time for humans to understand the
chemical process of photosynthesis.
? The major light absorbing pigments on thylakoid
membrane was revealed to be chlorophylls.
? Photons absorbed by many chlorophylls funnel into
one reaction center via exciton transfer.
? Two types of photochemical reaction centers have
been revealed in bacteria.
? Two photosystems (II and I) work in tandem to
move electrons from H2O to NADP+ in higher plants.
? P680+ in PSII extracts electrons from H2O to form
O2 via a Mn-containing oxygen-evolving complex.
