The Architecture Of Photosynthesis

1
The Architecture Of Photosynthesis is Optimized
to
Cover the solar spectrum Protect against
photochemical damage
Separate energy and electron transfer
Transmit excitation to the reaction center with
near efficiency
Regulate the efficiency of light harvesting and
repair damage (PSII)
2
An Abstract Question How much Chl is in
Picassos tree ?
A Collection of Facts
A medium size tree has
100,000 leaves An average leaf has a
surface area of
2.8 x 10-3 m2 The average Chl content of
a C3 leaf is
5.6 x 10-4 mol m-2 The molecular weight of
Chl a is 894 g/ mol
A Pragmatic Answer 140 g Chl / tree
Pablo Picasso House and Trees Paris,
Winter 1908
http//www.hipernet.ufsc.br/wm/paint/auth/picasso/
landscapes/picasso.house-garden.jpg
3
How many special pair Chls are in Picassos
tree?
140 g Chl / tree 2 special pair Chls
initiate primary
photochemistry 0.5 g Chl special Chls
/ tree How can this be explained ?
Photosynthetic Reaction Center (RC)
4
Light Harvesting Timescales
5
Current Model of the PSU
Net Reaction of PSI and PSII ATP synthase Uses
electrochemical potential to synthesize ATP from
ADP
Net Reaction of the Calvin Cycle
6
Photosynthetic organisms experience excessive
light on a daily basis
excess light
rate of photosynthesis
rate of light absorption
incident light intensity
7
Pigments From a Portion of the LH2 Ring
RG2B
bB850B
RG2A
aB850A
B800B
B800A
RG1B
RG1A
8
Photosynthetic organisms experience frequent
short-term fluctuations in light intensity.
Külheim et al. (2002) Science 297 91-93
9
Photosystem II3.5 Å
D1 yellow D2 orange
K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J.
Barber and S. Iwata. Science. In Press. (2004)
10
Models for Repair of PSIID1 Protein
E. Baena-Gonzalez and E.-M. Aro. Phil . Trans.
R. Soc. Lond. B, 357, 1451-1460 (2002).
P. Silva et. al. Phil . Trans. R. Soc. Lond. B,
357, 1461-1468 (2002).
11
Photoprotection involves regulation of light
harvesting
heat (nonphotochemical quenching)
light
short term regulation
PS II
peripheral LHC
inner LHC
photochemistry
COO-
COO-
H
LHC protonation
zeaxanthin synthesis
and
lumen
long term regulation
thylakoid membrane
PS II
inner LHC
PQH2
stroma
regulation of nuclear LHC gene expression
12
Photosystem II Supercomplex
13
What is NPQ?
? Nonradiative energy dissipation in PSII
Purpose protects PSII from
photochemical damage Main Component
qe - high energy state quenching
High Light (10-20 min.) Decrease in ?F (50)
14
Necessary components for qE a. ?pH
b. Zeaxanthin c. PsbS
a.
b.
15
Molecular genetic analysis of npq4-1 showed that
PsbS is necessary for qE.
wild type
npq4-1
4.4 kb
genomic DNA hybridization with psbS
16
qE is more than two times greater in the
transgenic plants
wild type (2 copies of psbS)
wtone psbS gene 5 (4 copies of psbS)
npq4-1 (no psbS)
wtone psbS gene 17 (4 copies of psbS)
transgenic plants qE 2.9
wild type qE 1.3
17
Transient Absorption Experiment
Sn
Probe
Soret
( 1Bu) S2
Energy (cm-1)
S2 ( 1Bu)
Qx
(1Ag) S1
kET
Qy
S1 (1Ag)
Pump
So (1Ag)
(1Ag) So
So
Chl a
Car
Car
Absorption
NPQ
No NPQ
18
Transient Absorption Measurements
on Arabidopsis Mutants
3
3
wild type PsbS
wild type


more PsbS and
regular qE
2
2
more qE than wt
1
1
0
0
Quenched 1.9
Quenched 1.3
Light ON
Amplitude (a.u.)
0
10
20
30
40
50
0
10
20
30
40
50
Light OFF
3
3
npq4- E122Q E226Q
npq4-1


more PsbS
,

but a
no PsbS
2
2
?pump 664 nm ?probe 540 nm
nonfunctional
no qE

version no qE
1
1
0
0
Amplitude (a.u.)
0
10
20
30
40
50
0
10
20
30
40
50
Time (ps)
Time (ps)
19
Excited States of Xanthophyll-Chlorophyll Dimers
cofacial arrangement
Qy
Qx
Qy
S2
S1
S1
CT
CT
Energy in eV
Energy in eV
Anthera-Chl
Zea-Chl
ground state
ground state
Zea-Chl distance in Angstrom
Anthera-Chl distance in Angstrom
S1
Qx
Qy
CT
Energy in eV
Vio-Chl
Andreas Dreuw Martin Head-Gordon
ground state
Vio-Chl distance in Angstrom
20
Zeaxanthin-Chlorophyll Dimer
HOMO
LUMO
Andreas Dreuw Martin Head-Gordon
21
TA studies in the near-IR Formation of the
carotenoid radical cation.
Spinach thylakoids ?Pump 664 nm
?Probe 1000 nm
Near-IR spectra
a In PS II complexes from Synechocystis PCC
6803 (Tracewell, C. A. et al. (2003)
Biochemistry, 42, 9127).
Difference kinetic indicates charge separations
quenching during qE.
22
Near-IR Arabidopsis thaliana Studies
(?pump 664 nm ?probe 1000 nm)
Time (ps)
Difference Kinetic Fits trise(ps)
tdecay (ps) Detect 1000 nm WT
7.3 210 WT PsbS
6.7 136 Detect 540
nm WT -
7 WT PsbS -
7
Car formation is correlated with qE
23
One proposed quenching mechanism
Gives rise to the positive signal at 1000 nm.
Corresponds to the negative (bleaching) signal in
the 540 nm region.
Assigned time constants
kCS 1/(100-300 fs). kRec 1/(150 ps),
corresponds to the recovery dynamics in visible
and near-IR regions. gAnn 1/(10 ps)n0, n0
number of initial excitations in the complex.
kTr, kAdQ 1/(100-300 ps). 1/(10 ps) -
corresponds to the net Chl pool decay rate in the
vicinity of the charge transfer complex.