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Quantification of photosystem I and II in different parts of the thylakoid
membrane from spinach
Ravi Danielsson, Per-A
˚ke Albertsson, Fikret Mamedov, Stenbjo¨rn Styring*
Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Received 30 June 2003; received in revised form 23 September 2003; accepted 17 October 2003
Abstract
Electron paramagnetic resonance (EPR) was used to quantify Photosystem I (PSI) and PSII in vesicles originating from a series of well-
defined but different domains of the thylakoid membrane in spinach prepared by non-detergent techniques. Thylakoids from spinach were
fragmented by sonication and separated by aqueous polymer two-phase partitioning into vesicles originating from grana and stroma lamellae.
The grana vesicles were further sonicated and separated into two vesicle preparations originating from the grana margins and the appressed
domains of grana (the grana core), respectively. PSI and PSII were determined in the same samples from the maximal size of the EPR signal
from P700
+
and Y
D
S
, respectively. The following PSI/PSII ratios were found: thylakoids, 1.13; grana vesicles, 0.43; grana core, 0.25; grana
margins, 1.28; stroma lamellae 3.10. In a sub-fraction of the stroma lamellae, denoted Y-100, PSI was highly enriched and the PSI/PSII ratio
was 13. The antenna size of the respective photosystems was calculated from the experimental data and the assumption that a PSII center in
the stroma lamellae (PSIIh) has an antenna size of 100 Chl. This gave the following results: PSI in grana margins (PSIa) 300, PSI (PSIh)in
stroma lamellae 214, PSII in grana core (PSIIa) 280. The results suggest that PSI in grana margins have two additional light-harvesting
complex II (LHCII) trimers per reaction center compared to PSI in stroma lamellae, and that PSII in grana has four LHCII trimers per
monomer compared to PSII in stroma lamellae. Calculation of the total chlorophyll associated with PSI and PSII, respectively, suggests that
more chlorophyll (about 10%) is associated with PSI than with PSII.
D2003 Elsevier B.V. All rights reserved.
Keywords: Photosystem I; Photosystem II; Thylakoid membrane; Photosystem antennae; EPR; Phase partition
1. Introduction
The light reaction of photosynthesis in higher plants is
driven by the cooperation of two photosystems (PS), PSI
and PSII, which, together with a chain of electron carriers,
are localized in the photosynthetic membrane, the thylakoid
[1]. This is differentiated into domains with characteristic
biochemical composition and specialized function. From the
geometry observed by electron microscopy, one can distin-
guish between appressed, planar domains of the grana and
single paired membranes of the stroma lamellae connecting
the grana; between the planar grana end membranes and the
curved membranes of the grana margins. The quantification
of the two photosystems in the different domains of the
thylakoid membrane is of particular importance for our
understanding of the function of the thylakoid membrane,
how the two photosystems are co-coordinated during elec-
tron transport and how this is regulated.
Earlier studies, using fragmentation and separation anal-
ysis or immune electron microscopy, have shown that the
two photosystems are segregated such that PSII is mainly
localized in the appressed grana domain and PSI in the
stroma exposed membranes, i.e. the stroma lamellae, end
membranes and the grana margins (Fig. 1) [2 – 4]. The gross
organization of PSII also differs and PSII dimers dominate
in the grana and PSII monomers in the stroma-exposed
membranes [5]. The two photosystems are also heteroge-
neous with respect to the size of their Chl-antenna system.
One can distinguish between so-called PSIIa(large Chl
antenna) in the grana and PSIIh(small Chl antenna) in the
stroma lamellae [6] and between PSIa(larger antenna) in
0005-2728/$ - see front matter D2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.bbabio.2003.10.005
Abbreviations: Chl, chlorophyll (a+b); EPR, electron paramagnetic
resonance; LHCII, light harvesting complex II; MES, 2-(N-morpholino)
ethanesulfonic acid; P700, primary electron donor in PSI; PEG, poly-
ethylene glycol; PSI, Photosystem I; PSII, Photosystem II; PpBQ, phenyl-p-
benzoquinone; TEMPO, 2,2,6,6-tetramethyl-piperidinyloxy radical; Y
D
,
tyrosine D in PSII
* Corresponding author. Tel.: +46-46-222-01-08; fax: +46-46-222-
45-34.
E-mail address: stenbjorn.styring@biokem.lu.se (S. Styring).
www.bba-direct.com
Biochimica et Biophysica Acta 1608 (2004) 53 – 61
the margins and PSIh(smaller antenna) in the stroma
lamellae and end membranes [7,8]. In addition, PSII is
functionally very different in the different parts of the
thylakoid membrane, probably reflecting an activity gradi-
ent due to the repair of PSII after photoinhibition [9].
Thus, the photosystems are not only laterally segregated in
the thylakoid. They are also functionally segregated and
photosynthetic electron flow in the different membrane
compartments is likely to be very different. To a superficial
extent (division in grana and stroma lamellae), this difference
is quite well understood (see Refs. [2,4] and references
therein). However, the thylakoid has more functionally dis-
tinct domains than the grana and stroma lamellae as is
commonly thought. To fully understand the function of the
dynamic thylakoid membrane, it is necessary to have a
detailed knowledge both about the function of the two
photosystems and their relative abundance in the different
domains of the thylakoid. We have earlier presented a
functional analysis of PSII in the different thylakoid domains
[9]. The dominating part of PSII in the grana region has a
functional acceptor side and an active oxygen-evolving
complex. In contrast, large part of PSII in the stromal region
is inactive on either or both the acceptor and donor sides.
Several other pools of PSII with more inhomogeneous
function are also found both in the stroma lamellae and the
margins of the grana [9].
The aim of the present study is to quantify, with
precision, the two photosystems versus each other (deter-
mine the PSI/PSII ratio) not only in the whole thylakoid but
also in each of the different sub-thylakoid fractions. To this
end, we have fragmented the thylakoid membrane and
isolated vesicles originating from different parts of the
membrane. The two photosystems have then been quanti-
fied by using electron paramagnetic resonance (EPR) spec-
troscopy, which is a more robust and precise method for this
purpose than earlier applied methods.
2. Materials and methods
2.1. Preparation of thylakoid membranes
Spinach (Spinachia oleracia L.) was grown hydroponi-
cally under cool white fluorescent light at 20 jC with light –
dark periods of 12 h and with the light intensity of 300 AE
m
2
s
1
. Two-month-old plants were dark-adapted 24
h before harvesting to reduce the content of starch grains.
All preparation procedures were made in weak green light at
4jC, and the sample was kept on ice through the whole
process. The thylakoid membranes were prepared with a
medium containing 5 mM MgCl
2
,asinRef. [7],and
resuspended in 10 mM sodium phosphate buffer pH 7.4, 5
mM NaCl, 1 mM MgCl
2
and 100 mM sucrose to a
chlorophyll concentration of 3 – 4 mg/ml. This buffer was
used for the sub-fractionation purifications. For EPR meas-
urements, the thylakoids and the sub-thylakoid fractions
were washed once and then dissolved to high Chl concen-
trationin15mM2-(N-morpholino) ethanesulfonic acid
(MES) pH 6.5, 15 mM NaCl and 300 mM sucrose. All
sub-thylakoid fractions were frozen after preparation and
stored at 80 jC.
2.1.1. Preparation of grana and stroma lamella vesicles
Thylakoid membrane suspension (8.2 g, at 3 – 4 mg Chl/
ml) was added to 32.8 g of a polymer mixture to give the
final concentrations: 5.7% (w/w) Dextran
500
, 5.7% (w/w)
polyethylene glycol (PEG)
4000
, 10 mM sodium phosphate
buffer pH 7.4, 3 mM NaCl, 1 mM MgCl
2
and 20 mM
sucrose. The thylakoid-polymer suspension was then soni-
cated by a Vibra-cell ultrasonic processor Model VC 500
(Sonics and Materials, Danbury, CT, USA) equipped with a
3/4-in. horn. The sample was sonicated 8 30 s, with 1-min
resting intervals in a cylindrical aluminium tube which was
cooled in ice water. The temperature was controlled never to
exceed 6 jC. The ultrasonic exposure had an intensity
output setting of 7, with 20% duty pulse. This sonication
procedure breaks the thylakoids into two main domains,
grana vesicles and stroma lamellae vesicles (Fig. 1) [7].To
the sonicated mixture was added 10 g of pure bottom phase
and 10 g of pure top phase from an aqueous two-phase
system, composed of 5.7% (w/w) Dextran
500
, 5.7% (w/w)
PEG
4000
, 10 mM sodium phosphate buffer pH 7.4, 5 mM
NaCl and 20 mM sucrose. The two-phase system (at 4 jC)
was shaken, mixed and centrifuged at 2000 gfor 6 min to
separate the phases. The top phase (T1) was carefully
collected and a thin layer above the interface was left. This
leftover, together with the interface and the bottom phase, is
here named the bottom phase fraction (B1). Fresh bottom
phase (23 ml) was added to the T1 fraction and 23 ml fresh
top phase to the B1 fraction. Each phase system was treated
as above. In this way, a top phase fraction (T2) is obtained
from the T1 fraction and a bottom phase fraction (B2) from
the B1 fraction. This procedure is repeated, giving the final
T3 and B3 fractions.
Fig. 1. Schematic representation of the different fractions of the thylakoid
membrane isolated and analyzed in this study. The exact location of the Y-
100 fraction is unclear, but it clearly represents a sub-domain in the stroma
lamellae fraction.
R. Danielsson et al. / Biochimica et Biophysica Acta 1608 (2004) 53–6154
The resulting top phase fraction, denoted T3, originating
mainly from stroma lamella and end membranes, and the
bottom phase fraction denoted B3, originating from ap-
pressed regions and margins of the grana (Fig. 1) [7] were
diluted three times with 15 mM MES pH 6.5, 15 mM NaCl
and 300 mM sucrose. They were then centrifuged at
100 000 g for 120 min. The pellets were resuspended in
the same buffer to a chlorophyll concentration of approxi-
mately 1.5–2.5 mg/ml.
2.1.2. Preparation of grana core and margin vesicles
Isolation of grana core vesicles and margins was carried
out by sonicating (20 30 s with output 9) the bottom phase
fraction B3 (see above) containing the grana vesicles. The
resulting fragments were separated by two-phase partition-
ing, in exactly the same manner as described above, to yield
new top and bottom phase fractions containing vesicles
originating from the margin and the grana core, respectively
(Fig. 1) [10].
2.1.3. Preparation of Y-100 vesicles
For preparation of Y-100 particles (Fig. 1),isolated
thylakoid membranes (2 –3 mg Chl/ml) in a buffer solution
composed of 10 mM sodium phosphate pH 7.4, 5 mM
NaCl, 5 mM MgCl
2
and 100 mM sucrose were twice passed
through a Yeda press at 4 jC and at a nitrogen pressure of
10 MPa [3,11,12]. The Yeda press homogenate was diluted
five times with the same buffer but without MgCl
2
and
centrifuged at 40 000 gat 4 jC for 30 min. The membrane
vesicles in the supernatant were sedimented by centrifuga-
tion at 100 000 gat 4 jC for 60 min to yield the Y-100
vesicles. The last pellet was resuspended in 15 mM MES
pH 6.5, 15 mM NaCl and 300 mM sucrose to 1.5 – 2.5 mg
Chl/ml.
2.2. Characterization of the sub-thylakoid fractions
The chlorophyll concentration was determined according
to Arnon [13] to allow a comparison with earlier work
where the Arnon method was used. For values of total
chlorophyll (a+b) the values obtained by the Arnon method
could be multiplied by a factor of 0.895 to obtain
corresponding values by the method of Porra et al. [14].
Steady state oxygen evolution was measured with a Clark
electrode at 20 jC using saturating white light. The sample
concentration was 20 Ag Chl/ml, and 2 mM ferricyanide and
0.5 mM phenyl-p-benzoquinone (PpBQ) were used as
electron acceptors. The measurements were done in 15
mM MES pH 6.5, 15 mM NaCl and 300 mM sucrose.
2.3. EPR measurements and analysis
Room temperature EPR was measured with a Bruker
Elexys E500 spectrometer equipped with a standard Bruker
4102 cavity. A flat cell with 250 Al volume was used. The
relative amount of PSI in the different fractions was esti-
mated on the basis of the intensity of non-saturated EPR
spectra from chemically oxidized P700
+
.P700
+
amounts to
one radical per functional PSI center and is independent of
the antenna size. P700 was oxidized in the dark at 20 jC
using 5 mM ferricyanide for 10 min. This concentration and
incubation time was found to induce maximal size of P700
+
in all fractions (not shown). Lower concentrations of ferri-
cyanide and/or shorter incubation times gave incomplete
P700
+
formation.
The relative amounts of PSII in the different fractions
were estimated on the basis of the intensity of the non-
saturated EPR spectra from the dark stable radical from
Tyrosine
D
,Y
D
S
. When properly photoinduced, there is one
Y
D
S
radical in each functional PSII center and this does not
depend on the antenna size, the presence of the Mn-cluster
or the electron transfer between Q
A
and Q
B
[15–17]. Full
induction of Y
D
S
was accomplished by 30-s illumination of
the sample in room light at 20 jC, followed by 5 min dark
incubation prior to the measurement [18]. This procedure
was found to result in maximal signal from Y
D
S
in all
fractions investigated, despite the widely varying antenna
size of PSII.
The spectral intensities of the Y
D
S
and P700
+
spectra
were determined by double integration of the spectra.
2.4. Determinations of chlorophyll per reaction center
Exact quantification of concentrations of a radical species
by means of EPR was achieved by comparison with the
EPR spectrum from a spin probe with known concentra-
tions, for instance 2,2,6,6-tetramethyl-piperidinyloxy radical
(TEMPO). The area of the EPR spectrum from the known
spin probe was compared with the areas of the EPR spectra
from the radical species under investigation.
In our case, the calculations of concentrations of the
reaction centers for PSII (Y
D
S
) and PSI (P700
+
) were based
on a standard calibration curve made with the well-known,
often used radical standard TEMPO. The measurements for
TEMPO were performed with the same EPR parameters as
for the sub-thylakoid fractions (see Fig. 2) using eight
different concentrations of TEMPO between 0.391 and
100 AM. By plotting the double-integrated areas from the
spectra recorded at the different concentrations of TEMPO,
a standard curve was obtained (not shown). This standard
curve was used to determine the exact concentration of PSII
(Y
D
S
) and PSI (P700
+
) radicals in the different fractions by
using the double-integrated areas from their EPR spectra.
3. Results
3.1. Characterization of thylakoid membrane and sub-
thylakoid fractions
A general account of the procedure and characterization
of the different sub-thylakoid vesicle fractions has been
R. Danielsson et al. / Biochimica et Biophysica Acta 1608 (2004) 53–61 55
published [19].Table 1 summarizes the oxygen evolution
and the Chl a/bratios in the different fractions. In essence,
our results agree with the results in earlier studies [9,19].
The highest oxygen evolution (on Chl basis) was found in
the grana core vesicles. It was less in the grana and still less
in the margin and the stroma lamellae vesicles. There was
(on Chl basis) nearly no detectable oxygen evolution in the
Y-100 sub-fraction.
3.2. Determination of the relative amounts of PSII and PSI
in the thylakoid sub-fractions
Our target is to use highly resolving spectroscopic
techniques to quantify the large photosynthetic redox
enzymes in the different compartments of the highly dy-
namic thylakoid membrane. Here we study PSI and PSII, for
which EPR spectroscopy is a particularly useful technique
since both photosystems can be very accurately quantified
by extremely well-characterized radicals in their respective
reaction centers.
3.2.1. EPR measurement of Y
D
S
in the thylakoid and sub-
thylakoid fractions
We have chosen to quantify PSII by using the dark stable
radical EPR signal from Y
D
S
. The Y
D
S
radical originates
from the neutral radical form of Y-161 on the D2 protein in
spinach [20,21]. Since it originates from a defined amino
acid, the yield of the radical can never exceed one radical
per PSII reaction center. The Y
D
S
radical is also extremely
stable and the radical life time is tens of minutes to many
hours depending on S-state and material [17,18,22 – 24].
The radical is efficiently induced by illumination and can,
due to its long lifetime, be measured quite easily and very
accurately.
Fig. 2 shows the EPR spectra of Y
D
S
in the intact
thylakoid membranes and the different sub-fractions. All
spectra shown are normalized to the same Chl concentration
to allow easy comparison. The spectra are recorded in
samples which were first illuminated for 30 s, then dark-
adapted for 5 min. This treatment is known to result in
complete oxidation of Y
D
S
in active and photoactivating
PSII centers [16,22]. We also tested this (not shown) and
found this illumination region to allow quantitative induc-
tion of Y
D
S
in all fractions. Severely photoinhibited PSII
centers are inactive in forming Y
D
S
[25,26].
In the thylakoid and grana preparations, the shape of
the EPR spectrum is entirely dominated by Y
D
S
[17,22].
In the margins, stroma lamellae and the Y-100 fractions
the EPR spectra are different and the peak at 3473 G is
somewhat higher than the left shoulder at 3466 G. The
left shoulder originates entirely from Y
D
S
while the
middle peak contains both Y
D
S
and a non-structured,
narrow free radical spectrum from another species than
PSII. The radical that gives rise to the narrow unstruc-
tured spectrum in the margins, stroma lamellae and Y-100
fractions is very stable and has EPR properties similar to
P700
+
(7.5–8.5 G wide, g= 2.0026) [17]. It could conse-
quently reflect a minor, abnormally stable fraction of
P700
+
. However, it should also be made clear that the
exact origin of this non-PSII radical does not interfere
with our determination of the PSII content in the fraction.
In the margins and stroma lamellae, this non-PSII radical
only amounts to a few percent and could easily be
subtracted from the spectrum. In contrast, it is significant-
ly bigger in the spectrum from the Y-100 fraction, where
Table 1
Characterization of the different fractions of the thylakoid membrane
Fraction (Fig. 1) Number of
preparations
analyzed
Chl a/b
(mol/mol)
O
2
evolution
(Amol of O
2
(mg of Chl)
1
h
1
)
Thylakoid 5 3.08 F0.06 125 F5
Grana core vesicles 3 2.23 F0.06 271 F16
Grana vesicles 3 2.41 F0.04 249 F26
Margins 3 2.90 F0.11 90 F10
Stroma lamella
vesicles
3 4.61 F0.02 78 F4
Stroma lamella
vesicles (Y-100)
3 7.11 F0.25 25 F7
Fig. 2. EPR spectra (recorded in the dark after illumination that fully
oxidize Y
D
S
(see Section 2) of Y
D
S
from PSII in different fractions of the
thylakoid membrane. The spectra are normalized to the same Chl
concentration. The spectrum from the Y-100 fraction (dotted curve) is a
mixture of the signal from Y
D
S
and a radical of unknown origin (see text).
A pure spectrum of Y
D
S
was weighted out from the mixed spectrum to
allow quantification of Y
D
S
. The full drawn curve shows the pure Y
D
S
spectrum (multiplied by a factor of 3) that was weighted out from the Y-100
spectrum and that was used for quantification (see text). EPR conditions:
temperature 293 K; microwave frequency 9.76 GHz; microwave power 8
mW; modulation amplitude 5 G.
R. Danielsson et al. / Biochimica et Biophysica Acta 1608 (2004) 53–6156
it needed careful treatment in our analysis (see below).
Fig. 2 shows that the signal from Y
D
S
is largest (on a Chl
basis) in the grana preparation and smaller in the stroma
lamellae compared to the intact thylakoid membranes. The
signal in the margin preparation is also smaller than in the
grana fractions while the spectrum is very small in the Y-
100 fraction (Y
D
S
is the very small shoulder at 3466 G in
the dotted spectrum).
To obtain ‘‘clean’’ spectra of Y
D
S
in the margins and
stroma lamella fractions, we first had to subtract the
radical contamination from the spectra in Fig. 2. This
was done by subtractions of a suitable amount of a
narrow radical spectrum to arrive at a pure spectrum
from Y
D
S
. The subtraction never exceeded a few percent
(4–8%) of the total spectrum. The purity of the resulting
spectrum from Y
D
S
(not shown) was ascertained by
overlapping with the clean Y
D
S
spectrum recorded in
the grana core fraction.
Table 2 shows the quantitative analysis obtained from
double integration of the spectra. The relative PSII
concentration is almost three times higher in the core
of the grana stacks than in the stroma lamellae. The
margin preparations are clearly of an intermediate char-
acter, having a much lower PSII concentration than the
grana preparations. Consequently, the grana vesicles that
contain both the margins and the grana core (Fig. 1) are
intermediate in PSII concentration between these two
fractions.
The spectrum recorded from the Y-100 fraction (Fig. 2,
dotted line) was clearly a mixed spectrum from a small Y
D
S
radical (visible as a small shoulder at 3466 G) and a larger
(in relative size) radical spectrum in the middle part of the
spectrum. Therefore, Y
D
S
could not be quantified directly.
Since the spectrum from the unknown radical (may be
reflecting a tiny fraction of stable P700
+
, see above) in
these fractions was of equal size or even larger than the Y
D
S
spectrum, it was almost impossible to subtract this away
from the recorded spectrum (Fig. 2, dotted spectrum).
Instead, we used a pure spectrum from Y
D
S
(recorded in
the grana core fraction) to completely subtract away the
Y
D
S
-shoulder in the spectrum from the Y-100 fraction. In
that way, we obtained a pure spectrum from the unknown
radical. The concentration of Y
D
S
, in turn, was estimated
from the intensity of the Y
D
S
spectrum needed to completely
subtract Y
D
S
away from the original spectrum. This is
shown as the full drawn spectrum from the Y-100 fraction
in Fig. 2 (note that this spectrum is magnified three times).
This analysis reveals that the relative concentration of PSII
(on a Chl basis) in the Y-100 fraction is ca. 23% and 13% of
that in the starting thylakoid fraction and the grana core,
respectively (Table 2).
To conclude, most of the PSII is found in the granal part
of the membrane. The largest concentration of PSII on a Chl
basis is found in the grana core and then it decreases in the
other fractions. Compared with Y-100, there are 7.4 and 6.5
times stronger signals from PSII in the grana core and grana,
respectively (on a Chl basis). The signals are intermediate in
the thylakoid and the margins, 4.5 and 4.1 times stronger,
respectively, and 2.7 times stronger in stroma lamella than in
Y-100.
3.2.2. EPR measurements of P700
+
in the thylakoid and
sub-thylakoid fractions
The PSI concentration was analyzed in an analogous
manner as for PSII. In this case, we used the EPR signal
from the P700
+
radical, recorded in samples oxidized with
ferricyanide, to estimate PSI. Also, P700
+
maximally
amounts to one radical per PSI reaction center. However,
it is possible that oxidation with ferricyanide is incomplete
in some fractions. We therefore tested this, and 10 min
incubation with 5 mM ferricyanide was found to induce
maximal P700
+
signals in all fractions studied (not shown).
Shorter incubation or lower concentration of the oxidant
failed to be complete, while higher concentration of ferri-
cyanide and/or longer incubation times did not increase the
yield of P700
+
.
The spectra from the ferricyanide-oxidized samples are
shown in Fig. 3A. These EPR spectra contain a mixture of
the stable signal from Y
D
S
and the signal from P700
+
. The
Y
D
S
contribution to the spectra is clearly observed as a
shoulder at 3466 G in the spectra from the grana, margins,
and thylakoids (Fig. 3A) and it is also present in the other
spectra albeit to a lower, less obvious extent. In order to
determine the contribution from P700
+
only, the spectra of
Y
D
S
(Fig. 2) were subtracted from the spectra in Fig. 3A.
This resulted in the difference spectra displayed in Fig. 3B.
From these, the concentration of P700
+
could be estimated
and the results are presented in Table 2. The smallest signal
Table 2
Quantification of PSI and PSII in different fractions of the thylakoid
membrane
Fraction of
thylakoid
membrane (Fig. 1)
Number of
preparations
analyzed
P700
+a
(arb. units)
Y
D
S
b
(arb. units)
PSI/PSII
c
Entire thylakoid 5 35.3 F2.2 31.8 F2.6 1.13 F0.05
Grana core vesicles 3 15.4F1.4 53.9 F2.8 0.25F0.06
Grana vesicles 3 19.7 F1.0 47.3 F4.3 0.43 F0.05
Margins 3 38.1 F0.6 30.1 F3.4 1.28 F0.14
Stroma lamella
vesicles
3 61.3 F1.7 20.0 F0.4 3.10 F0.11
Stroma lamella
vesicles (Y-100)
3 87.8 F3.1 7.3 F1.1 12.75 F1.65
a
Determined from the maximal intensity of P700
+
determined by
double integration the P700
+
EPR spectra. The data are normalized to the
same Chl concentration as for PSII.
b
Determined from the maximal intensity of Y
D
S
determined by double
integration the Y
D
S
EPR spectra. The data are normalized to the same Chl
concentration as for PSI.
c
The values of PSI/PSII shown are not exactly the ratio of the numbers
from the P700
+
and Y
D
S
columns in the table. Instead, the reported values
represent the means of the ratio of P700
+
and Y
D
S
obtained from each
independent preparation analyzed.
R. Danielsson et al. / Biochimica et Biophysica Acta 1608 (2004) 53–61 57
from P700
+
(Fig. 3B) was found in the grana core (Table 2)
while PSI completely dominated the stromal fractions. The
signals are 5.7 and 4.4 times smaller (on a Chl basis) in
grana core and grana, respectively, compared to the Y-100
vesicles. In the thylakoids and the margins, the relative PSI
concentration is intermediate, 2.5 and 2.3 times smaller,
respectively, than in the Y-100 vesicles. The P700
+
signal in
stroma lamellae is 1.4 times smaller than in the Y-100
vesicles. It should also be noted that our analysis in the Y-
100 fraction provides a number for P700
+
which includes
both the dark stable radical (ca. 7 – 8% of total P700
+
) and
the ferricyanide-oxidized P700
+
.
3.3. PSI/PSII ratio in the thylakoid and sub-thylakoid
fractions
The data obtained from integration of the EPR spectra of
Y
D
S
(Fig. 2) and P700
+
(Fig. 3B) directly allow determina-
tion of the PSI/PSII ratio in the different fractions with high
accuracy, simply by comparing the double-integrated area
of the spectra. The results are shown in Table 2. In the
intact thylakoid membranes, we find a PSI/PSII ratio of
1.13 F0.05. Thus, in our spinach leaves, there are slightly
more PSI than PSII centers. However, they are very un-
evenly distributed in the membrane. PSI dominates in the
stroma lamellae (PSI/PSII is 3.10 F0.20) and dominates
even further in the Y-100 fraction (PSI/PSII is 12.75 F
1.65). Thus, patches seems to exist in the membrane where
PSII is almost absent. In contrast, PSII is more abundant
than PSI in the grana and PSI/PSII is 0.25 F0.06 in the
grana core fraction, which is the purest PSII fraction we ob-
tained with our fractionation procedure. In the margins, there
is a little more PSI than PSII (PSI/PSII is 1.28 F0.13).
It should be emphasized that the overall picture we find
here is not new. It has been known for decades that PSII is
enriched in the grana, PSI in the stroma lamellae [2,3], and
more recent studies have shown that both PSI and PSII are
present in margins preparations [10]. However, our quanti-
fication using EPR, which allows direct comparison of PSII
and PSI by the same measurement, is novel and the PSI/PSII
ratios we describe here are probably the most accurate so far
determined.
3.4. Concentration of PSI and PSII reaction center per
chlorophyll
With EPR spectroscopy, the concentration of a radical
species can be determined very accurately by comparison
with the EPR spectrum from a radical with known concen-
tration. Such spin quantification is not dependent on prop-
erties like light scattering, overlapping of optical absorption
from other species, difficult determination of extinction
coefficients, fluorescence yield, etc., which are inherent
for all optical spectroscopy and fluorescence measurements
in photosynthetic membranes.
We determined the concentration on a Chl basis of the
Y
D
S
(Fig. 2) and P700
+
(Fig. 3B) radicals in the different
preparations by comparison with a standard calibration
curve from TEMPO, an often used spin probe [27]. The
data are presented on a Chl basis in Table 3.Table 3 also
presents the number of Chl molecules per PSI or PSII
reaction center in each fraction. This is useful for the
Fig. 3. (A) EPR spectra recorded in the different thylakoid fractions after oxidation with 5 mM ferricyanide in the dark for 10 min. The spectra originate from a
mixture of P700
+
from PSI and Y
D
S
from PSII. The contribution from Y
D
S
is clearly seen as a shoulder at 3466 G in for example the thylakoid fraction. (B)
EPR difference spectra of 5 mM ferricyanide oxidized (spectra in panel A) minus dark (spectra in Fig. 2) showing the pure P700
+
spectrum from the different
fractions of the thylakoid membrane. Both panels show spectra taken from: (a) thylakoids; (b) the grana core fraction; (c) the grana fraction; (d) the margin
fraction; (e) the stroma lamellae fraction; (f) the Y-100 fraction. The EPR conditions are the same as in Fig. 2.
R. Danielsson et al. / Biochimica et Biophysica Acta 1608 (2004) 53–6158
calculation of the antenna size of the two photosystems, see
below, Section 4.2.
4. Discussion
4.1. PSI/PSII ratios
The overall relative distribution of PSI and PSII between
the different fractions agrees with what one would expect
from earlier studies using fragmentation and separation
analysis [2,19]. However, the absolute values from our
determination of PSII differ from other results obtained with
other techniques. Our data give lower PSII concentrations
and hence larger PSI/PSII ratios than previously reported in
the literature. Thus, we find for whole thylakoids a PSI/PSII
ratio of 1.13 while most other studies report values in the
range of 0.5–0.9 [11,28–35].
In the case of PSI, its reaction center, P700, has been
determined by difference absorption of the oxidized minus
reduced form at 705 nm using an absorption coefficient
determined by Hiyama and Ke [36]. There seems to be
consensus with regard to the determination of PSI. Our
results also agree with earlier published values for Chl/
P700. Thus, for the Y-100 fraction, we obtain a value of 222
Chl/P700 (Table 3) compared to 210– 240 obtained by using
light microscopy [11,37,38].
In the case of PSII, the situation is more complicated and
controversial. Optical spectroscopy has been used for quan-
tification of the reduced primary quinone acceptor Q
A
[11,28–30,35] and pheophytin [30]. Flash oxygen yield
[32,35], and flash-induced proton release measurements
[33] have also been used. The results obtained from these
different assays are variable, leading to reported PSI/PSII
ratios of 0.5–0.9. Binding studies of atrazin to thylakoids
have also given variable PSI/PSII ratios such as 0.6 [31,35]
or 1.0 [39]. The discrepancies can only partly be explained
by varying plant material and growth conditions. It is known
that light conditions influence the PSI/PSII ratio [31], but
even for spinach grown under fairly similar conditions and
for which the Chl a/bratios are the same, the results differ.
In sum, several earlier studies report varying PSI/PSII ratios
which are less than 1 as compared to our value which is
slightly larger than 1.
It is not unlikely that reported differences in the PSI/PSII
ratio also might reflect non-functional PSII centers, which do
not appear in the conventional assays [40]. We don’t wish to
speculate on the origin of discrepancies with earlier reports
but argue that EPR is a more robust method to determine the
concentrations of PSI and PSII, since the signals measured
are only dependent on the number of spins and not on the
complete activity of the centers. In addition, the signals from
both PSI and PSII are determined in the same sample. Our
data are supported by the calculation of the antenna size
which agrees with what one would expect from analysis of
protein composition of the two photosystems, see below.
4.2. Calculation of the antennae size
The fractionation scheme gives two types of vesicles,
which are highly enriched: Y-100 (stroma lamellae) in PSI
and another, grana core vesicles, in PSII. Therefore, our data
allowed calculation of the antenna size of the two photo-
systems in the following way.
For each vesicle preparation, the following balance
equation holds:
Ntot ¼nINIþnIINII ð1Þ
where n
I
and n
II
are the number of reaction centers of PSI
and PSII, respectively, and N
I
and N
II
are the average
number of Chls in their respective antennae. N
tot
is the total
number of Chls in the preparation.
For the antennae of PSI:
NI¼Ntot
nI
nII
nI
NII
ð2Þ
For the most PSI-enriched (stroma lamellae) preparation, Y-
100 N
tot
/n
I
= 222 (Table 3),n
II
/n
I
= 0.078 (Table 2). In this
preparation, PSII is in the form of the so-called h-centers
which have a small antennae [6,28]. The number of antenna
Chl per PSII center in PSIIhcan be assigned a value of
approximately 100 Chl/PSII center based on PSII lacking
light-harvesting complex II (LHCII) [40] (see also below).
Inserting these values in Eq. (2), a value of N
I
= 214 for the
average antennae size of PSI in the stroma lamellae (N
I
for
PSIh) is obtained.
1
1
It should be noted that the assigned antenna size of PSIIhdoes not
significantly affect the value of N
I
since the concentration of PSIIhis so
low in the Y-100 fraction. For example, if we instead assign a value of 50
Chl/PSIIhthe N
I
value would only change to 218.
Table 3
Chlorophyll molecules per reaction center on the basis of Y
D
S
(PSII) and
P700
+
(PSI) in different domains of the thylakoid membrane
Fraction (Fig. 1) Y
D
S
a
(mol/1000
mol Chl)
b
Chl/Y
D
S
c
P700
+a
(mol/1000
mol Chl)
b
Chl/P700
+c
(mol/mol)
Thylakoid 1.62 617 1.81 552
Grana core
vesicles
2.82 355 0.77 1300
Grana vesicles 2.45 408 1.02 980
Margins 1.50 667 1.97 508
Stroma lamella
vesicles
1.03 971 3.16 316
Stroma lamella
vesicles (Y-100)
0.36 2780 4.50 222
All data are average values of three or five measurements.
a
The absolute concentrations of Y
D
S
and P700
+
were determined from
a standard spin concentration curve using TEMPO as a spin standard.
b
Chl a+ Chl b.
c
Total number of Chl molecules (a+b) per PSII or PSI center. Note that
all these chlorophylls are not connected to PSII or PSI (see Section 4 for
explanation of the calculation of antenna size).
R. Danielsson et al. / Biochimica et Biophysica Acta 1608 (2004) 53–61 59
It is known from earlier studies [8,37] that the antennae
size of PSI in the grana (N
I
for PSIa) is about 40% larger,
i.e. 300. That the antennae of PSI in the grana (PSIa)is
larger than PSI in the stroma lamellae (PSIh) is a result of
attachment of LHCII trimers (Lhcb,1,2) to PSIh.Ifitis
assumed that one LHCII monomer contains 15 Chl, the data
fit a model where two LHCII trimers ( = 90 Chl) are attached
to PSI in the grana (PSIa)[41].
For the antennae size of PSII:
NII ¼Ntot
nII
nI
nII
NI
ð3Þ
For the most PSII-enriched preparation, the grana core
vesicles, N
tot
/n
II
=355 (Table 3),n
I
/n
II
=0.25 (Table 2),
N
I
= 300 (see above). Inserting these values in Eq. (3), gives
a value of N
II
= 280 for the antennae size of PSII in the grana
core (N
II
for PSIIa).
A similar calculation applied to the grana vesicles results
in an average antennae size of (N
II
) 279 Chls per PSII
monomer and for margins this value will be 283. This points
to a fairly homogeneous distribution of the antenna size of
PSII over the grana disc. The antenna size of the PSII core
without any attached LHC proteins is about 50 Chls [42,43].
Together with the minor light-harvesting polypeptides
(Lhcb,4,5,6) the antenna size of PSII monomer without
any LHCII trimers attached can be assigned a value of about
100 Chl [41]. Thus, our determined antenna size of 280 Chls/
PSII center indicates that there are about four LHC trimers
per PSII monomer in the granal part of the membrane. Since
PSII in grana is dimeric [5] it means eight LHCII trimers per
dimer. This supports earlier models presented for PSII based
on biochemical fractionation after mild detergent treatment
[41,44–46] and is also compatible with models suggested
from transmission electron microscopy [5,47].
4.3. The overall distribution of total chlorophyll between
PSI and PSII in the thylakoid membranes
It has been shown earlier that mild sonication followed
by counter-current distribution can fractionate the thylakoid
membrane quantitatively into two, well-separated popula-
tions of membrane vesicles denoted aand hvesicles [7].
The a-vesicles originate from the grana and the h-vesicles
from the stroma lamellae and the end membrane: 36% and
64% of P700 are found in the a- and h-vesicles, respec-
tively, and about 80% and 20% of PSII are found in the a-
and h-vesicles, respectively [2,7]. In the literature, values
between 15% and 35% of the total PSII centers are assigned
to PSIIh[6,28,29]. If we combine these data with the values
for the antennae of the respective photosystems calculated
above, the overall distribution of Chl associated with PSI
and PSII, respectively, can be calculated as follows:
NItot
NIItot
¼RðpINIaþqINIbÞ
ðpIINIIaþqII NIIbÞð4Þ
where Ris the PSI/PSII reaction center ratio of the thyla-
koid; p
I
and q
I
are the fractions of PSI reaction centers in the
(a) and (h) vesic les, respectively, ( p
I
+q
I
= 1); p
II
and q
II
are
the corresponding fractions for PSII. Inserting the values
R= 1.13, p
I
= 0.36 and q
I
= 0.64 and p
II
= 0.8 and q
II
= 0.2,
N
Ia
= 300, N
Ih
= 214, N
IIa
= 280 and N
IIh
= 100 in Eq. (4)
gives a value of 1.1 for N
Itot
/N
IItot
, i.e. in the thylakoid
membrane from our spinach more Chl is associated with PSI
than PSII. Since carotenoids distribute in a similar fashion
as Chl between different sub-thylakoid vesicles [48] one can
conclude that more pigments are associated with PSI. This
agrees with earlier fractionation studies [2,4,39] and with
action spectra which demonstrate that PSI captures more
quanta than PSII [49,50].
Acknowledgements
This work was supported by The Swedish Research
Council, The Swedish National Energy Administration,
DESS, and the Knut and Alice Wallenberg Foundation.
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