18' Photosynthesis: antennas and reaction centers

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18. Photosynthesis antennas and reaction centers
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Synthesizing carbohydrates from CO2 and water
presents a formidable thermodynamic problem
6 CO2 6 H2O C6H12O6 6 O2
DGo 679 kcal/mol (2480 kJ/mol). Keq 10-496
Photosynthetic organisms use the energy of light
to drive carbohydrate synthesis against this
enormous gradient.
The energy of red light (700 nm) is E Nhn 41
kcal/einstein (172 kJ/einstein)
An einstein is a mol of photons. N Avogadros
number (6x1023) h Plancks constant
(6.63x10-34 J/s) n frequency (s-1).
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When light raises a molecule to an excited
electronic state, the molecule becomes a stronger
reductant
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The photochemically reactive pigments are
chlorins or bacteriochlorins, which are
structurally related to hemes
Hemes symmetrical p systems absorb blue light
Chlorophylls asymmetrical p systems absorb blue
red light
Bacteriochlorophylls more asymmetrical p
systems absorb blue, orange near-IR light
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The photochemical reactions of photosynthesis
take place in integral membrane proteins
thylakoid lumen
outer membrane
inner membrane
stroma
chloroplast
thylakoid membrane
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Most of the pigments in photosynthetic cells do
not participate in the electron-transfer
reactions of photosynthesis. Instead, they serve
as an antenna that increases the absorption of
light.
R. Emerson W. Arnold measured the amount of O2
formed when they excited algae with short flashes
of light.
At high light intensity, the maximum O2 released
per flash was about 1 O2 per 2400 Chls.

At low light intensity, 1 O2 is formed for each
8 photons absorbed (yellow dashed line).

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The antenna system of purple photosynthetic
bacteria has circular pigment-protein complexes
The LH1 complex, with 31 transmembrane a-helical
peptides (yellow) and 30 BChls (CPK colors),
surrounds the reaction center (red, orange
brown), where the electron-transfer reactions
occur.
view normal to the membrane
view in the plane of the membrane
A. W. Roszak et al. Science 302 1969 (2003)
1pyh.pdb
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When the antenna is excited with light,
excitations are transferred to the reaction
center within 40 ps
Smaller LH2 antenna complexes transfer energy
rapidly to LH1
LH2
LH2
LH2
35 ps
RC
view normal to the membrane 1 ps 10-12 s
antenna BChls are green and blue in this figure.
LH1
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The LHC-II (light-harvesting complex II) antenna
protein of plants is a trimer with 14
chlorophylls and 3 carotenoids per protein subunit
Chl-b Chl-a carotenoid
Z. Liu et al. Nature 428 287 (2004) 1rwt.pdb
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The reaction center of purple photosynthetic
bacteria has 3 to 4 subunits, depending on the
species
The two central subunits are integral membrane
proteins with homologous structures
A small number of bacteriochlorophylls and other
electron carriers are bound to the proteins
membrane phospholipid bilayer
view parallel to the membrane
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The electron carriers in the bacterial reaction
center are arranged around an axis of approximate
rotational symmetry
Two of the four BChls form a dimer (P870) that
acts as the initial electron donor
BChl bacteriochlorophyll BPh
bacteriopheophytin (BChl with 2 H in place of
Mg) Q ubiquinone
Axis of approximate rotational symmetry
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The sequence and kinetics of the initial
electron-transfer reactions can be studied by
exciting RCs with short pulses of light
beam splitter
train of short pulses
Laser
vary the path length for the probe pulses
probe pulses
excitation pulses
l1 and l2 different probe wavelengths detect
different electron carriers.
sample
measure the intensity of the transmitted probe
beam averaged over many pulses
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When the complex is excited with light, an
electron moves from the BChl dimer (P870) to a
BPh and then to a quinone
LIGHT
BChl dimer
3 ps
BChl
1 ps
BPh
200 ps
Q
Fe
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The photosynthetic electron-transfer system in
purple bacteria is cyclic
P870
BChl
BPh
QA
1. Reduced ubiquinone (QBH2) dissociates from
the RC and diffuses to the cytochrome bc1
complex, where it is reoxidized.
QB
Light
1.
cytochrome bc1
cytochrome c
2. Reduced cytochrome c diffuses back to the RC
to complete the cycle.
2.
P870
Electron flow pumps protons across the membrane
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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
Photosystem II
Mn center
PQA, PQB plastoquinone QK1 phyloquinone plastocy
anin a Cu protein ferredoxin an Fe-S protein
P680
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The reaction center of Photosystem II has 20
subunits
It binds 32 molecules of Chl-a (green). Most of
these are part of the antenna.
view parallel to the membrane
K.N. Ferreira et al. Science 303 1831 (2004)
1s5l.pdb. B. Loll et al. Nature 438 1040 (2005)
2axt.pdb.
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The core of the Photosystem II reaction center is
very similar to that of purple bacteria
1s5l.pdb, 2axt.pdb.
The polypeptide backbones of the two main
subunits are shown in red and orange,
chlorophylls in green, pheophytins in blue and
quinones in yellow.
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Electron carriers in the Photosystem II reaction
center
The cluster of 4 Mn atoms and a Ca is the
O2-evolution site and is unique to PS II. Note
the tyrosine residue between the Mn cluster and
P680.
Pushkar et al. Proc. Natl. Acad. Sci. USA 105
1879 (2008).
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The Photosystem I reaction center has 12 subunits
It binds about 100 molecules of Chl. Those shown
in yellow are part of the antenna.
The arrangement of the Chl electron carriers
(blue) again is very similar to that in bacterial
RCs.
view normal to the membrane
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Two of the Fe-S centers in Photosystem I are in
subunit C, on the stromal side of the membrane
view parallel to the membrane surface