13' Photosynthesis: electron transfer continued and photophosphorylation

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13. Photosynthesis electron transfer
(continued) and photophosphorylation
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Plants have two photosystems that work in series
to move electrons from water to NADP
P700
Chl
QK1
P680
Fe-S centers
Phe
Ferredoxin
Light
NADPH
PQA, PQB
NADP
Light
Cyt b6f complex
Photosystem I
O2
plastocyanin
H2O
P700
PQA, PQB plastoquinone QK1 phyloquinone plastocy
anin a Cu protein ferredoxin a soluble Fe-S
protein
Photosystem II
Mn center
P680
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(1) The efficiency of O2 evolution drops at
wavelengths above 680 nm, but can be increased by
green background light
Experimental evidence for the Z scheme
with background light
O2 released per photon absorbed
without background light
The green background light has this effect even
if it is turned off just before the red light is
turned on.
adapted from R. Emerson et al. Proc. Natl. Acad.
Sci. USA 43 133 (1957)
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The Z-scheme can explain these results if only
one of the two photosystems absorbs far red light
P700
P680
NADP
Photosystem I
Photosystem II
P700
H2O
Measurements of partial reactions (e.g., P700
photooxidation) showed that both photosystems
absorb below 680 nm, but only PS I absorbs at
longer wavelengths.
P680
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(2) Green light can either oxidize or reduce
components between PS II PS I, but far red
light only oxidizes them
P700
P680
NADP
Cyt f
PS I
H2O
P700
PS II
adapted from L. Duysens J. Amesz Biochim.
Biophys. Acta 64 243 (1962)
P680
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(3) Far-red background light decreases
fluorescence from PS II
P700
Chl
P680
Phe
NADP
energy transfer
PS I
P700
LIGHT
PS II
Chl
P680
PS II antenna
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PS II complexes are located mainly in stacked
regions of the thylakoid membrane PS I complexes
are mainly in unstacked lamellae
Cyt b6f
ATP synthase
stacked membranes (granal membranes)
unstacked membranes (stromal lamellae)
PS I PS II LHC-II
The LHC-II antenna complex can move between these
two regions.
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Phosphorylation causes the LHC-II antenna complex
to move to unstacked regions bind to PS I
This increases the fraction of the absorbed light
energy that goes to PS I
PS I
kinase
ADP
ATP
phosphatase
Pi
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The kinase that phosphorylates LHC-II is
activated when the ratio PQH2/PQ is high,
reflecting overexcitation of photosystem II
P700
P680
Light
PQH2
NADP
PQ
Light
Photosystem I
P700
H2O
Photosystem II
LHC II then moves to PS I, improving the balance
of excitation of the two systems.
P680
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O2 evolution requires accumulation of four
oxidizing equivalents
O2 evolved per flash
???? step removes one electron from the Mn4O4Ca
complex.
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The chloroplast cytochrome b6f complex is
homologous to the bc1 complexes of mitochondria
and photosynthetic bacteria
thylakoid membrane
Cytochrome b6f is a homodimer with multiple
subunits
H. Zhang et al. Proc. Natl. Acad. Sci. 100 5160
(2003) 1vf5.pdb
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Oxidation of PQH2 by the cytochrome b6f complex
pumps protons across the thylakoid membrane
Proton pumping generates an electrochemical
potential gradient that is used to make ATP.
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The Q cycle enables the cytb6f complex to pump
4 protons for each QH2 that is oxidized
4 H
PQH2
2 PQH2
2 PQ
PQ
PQ PQH2
2 H
The mitochondrial bacterial cyt-bc1 complexes
also carry out a Q cycle.
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Proton and electron circuits in chloroplasts
The electron-transfer reactions driven by light
acidify the thylkoid lumen relative to the stroma
NADP H NADPH
light
light
Fd
4 H
4 H
thylakoid lumen
2 H
H2O 1/2 O2
thylakoid membrane
6 H
stroma
ATP synthase
2 ADP 2 Pi H 2 ATP
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The ATP synthases of chloroplasts and
photosynthetic bacteria are very similar to those
of mitochondria and nonphotosynthetic bacteria
inner membrane
matrix
Mitochondrion
E. coli
Chloroplast
lumen
ATP synthase
inter-membrane space
stroma
thylakoid membrane
ATP synthase
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The orientation of the ATP synthase relative to
the electrochemical potential gradient generated
by electron transfer is the same in all cases
H