Redox Changes of Cytochrome b559 in
the Presence of Plastoquinones*
Jerzy
Kruk and
Kazimierz
Strzalka
From the Department of Plant Physiology and Biochemistry, The Jan
Zurzycki Institute of Molecular Biology, Jagiellonian University,
Aleja Mickiewicza 3, 31-120 Kraków, Poland
Received for publication, April 26, 2000, and in revised form, October 5, 2000
 |
ABSTRACT |
We have found that short chain plastoquinones
effectively stimulated photoreduction of the low potential form of
cytochrome b559 and were also active in dark
oxidation of this cytochrome under anaerobic conditions in Triton
X-100-solubilized photosystem II (PSII) particles. It is also shown
that molecular oxygen competes considerably with the prenylquinones in
cytochrome b559 oxidation under aerobic
conditions, indicating that both molecular oxygen and plastoquinones
could be electron acceptors from cytochrome b559 in PSII preparations.
-Tocopherol
quinone was not active in the stimulation of cytochrome photoreduction
but efficiently oxidized it in the dark. Both the observed
photoreduction and dark oxidation of the cytochrome were not sensitive
to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. It was concluded that
both quinone-binding sites responsible for the redox changes of
cytochrome b559 are different from either the
QA or QB site in PSII and represent new
quinone-binding sites in PSII.
 |
INTRODUCTION |
Cytochrome b559 (cyt
b559)1
is an integral component of the photosystem II (PSII) reaction center
(RC) complex, whose function has been the subject of very extensive
studies within recent years (1, 2). It was found that it has a crucial
role in PSII assembly, and its electron transport properties are
attributed to protection of PSII against photoinhibition (1, 3-5). It is found in thylakoid membranes and in PSII preparations in high potential (HP) and low potential (LP) forms differing in redox potentials (Em = 330-400 and 20-80 mV,
respectively) (2), and both these forms are believed to be at least
partially interconvertible (1, 4, 6). In native, fully functional PSII
complexes, the LP form is probably a minor component whose levels
increase under photoinhibitory conditions. Under strong light
illumination (acceptor side inhibition), the LP form of cyt
b559 was found to accept electrons from the
photoreduced pheophytin, preventing PSII from photoinhibition (3, 5,
7). However, the natural electron acceptor from the cyt
b559 LP form is unknown. Among the potential
candidates, plastoquinone and molecular oxygen have been suggested (4).
The second possibility is supported by autoxidation of the LP form of
cyt b559 (8). Under the impairment of the
oxygen-evolving complex, when electron donation from the oxygen-evolving complex to P680 is inhibited (donor side inhibition), the electrons from the HP form of cyt b559 are
transferred to P680 via the redox active chlorophyll Z
(Chlz) (1). Moreover, it was suggested (1) that under such
conditions the HP form is converted by an unknown molecular switch to
the LP cyt b559 form, which after its
autoxidation cannot donate electrons to P680. This state, in turn,
causes oxidation of Chlz, which in a radical form
efficiently quenches excitation energy, preventing RC from
overexcitation (1). It is known that the electron transport via cyt
b559 is relatively slow (1), and to fulfill its
photoprotective function, it should be relatively fast. Therefore, it
is of interest to investigate the factors accelerating reduction and
oxidation of the cyt b559 LP form. It was found
that in isolated reaction centers, where only the LP cyt
b559 form is present (9), the photoreduction of
cyt b559 was strongly stimulated by artificial quinones, such as DBMIB (10) or decyl-PQ (9). It was suggested that
these quinones substitute for the QA site and mediate
cytochrome reduction via the semiquinone form. However, it is not
certain whether the interactive site of the quinones is in fact the
QA site (11). Therefore, it is of interest to investigate
in more detail the specificity of cyt b559
photoreduction by different natural and artificial quinones, to
investigate whether the quinone-binding site(s) are different from the
QA site, and to consider whether this quinone-mediated
cytochrome reduction could take place in vivo.
In the present study, we have performed measurements of the
photoreduction and dark oxidation of the cyt
b559 LP form in Triton X-100-solubilized
PSII particles under the influence of different plastoquinone
homologues in anaerobic and aerobic conditions. We have also included
for these studies
-tocopherol quinone (
-TQ), a natural thylakoid
prenylquinone of unknown function in plants (12) that is an effective
quencher of PSII fluorescence (13) and also affects the cyclic electron
flow around PSII (14).
 |
MATERIALS AND METHODS |
PSII particles were isolated according to the method of Berthold
et al. (15). PQ-1-PQ-4 homologues were a kind gift of H. Koike, PQ-9 was a gift from Hoffmann-La Roche, PQ-decyl was from Sigma, and vitamin K3 (menadione) was from Aldrich.
-TQ
was prepared from
-tocopherol as described by Kruk (16). Menadiol
(K3H2, MD) was obtained by reduction of
menadione with NaBH4 in methanol, and its concentration was
determined using the millimolar extinction coefficient
= 20.1 at 245 nm in ethanol. Plastoquinone concentrations were calculated
using
255 = 17.94 mM
cm
1 (17) in absolute ethanol. All
measurements of cyt b559 redox changes on PSII
particles were made in 50 mM Hepes buffer, pH 7.5, 10 mM KCl, and 5 mM MgCl2 at a
chlorophyll concentration of 50 µg/ml and in the presence of 0.2%
(w/v) Triton X-100. Additionally, photoreduction of cyt
b559 was measured in the presence of 1 mM MnCl2 as an electron donor to PSII. The cyt
b559 concentration was determined using

559-570 = 20 mM
cm
1 (18). Unless otherwise indicated, all
kinetic measurements were performed under anaerobic conditions using an
oxygen trap composed of glucose oxidase (50 units/ml = 1.5 mg/ml),
catalase (500 units/ml = 23.8 µg/ml), and 1 mM
glucose in a tightly closed cuvette at a volume of 2.5 ml. Both aerobic
and anaerobic samples were incubated for 10 min before the measurements
under continuous stirring. That time was sufficient for oxygen
consumption by the oxygen trap and for the stabilization of the base
line after Triton X-100 addition. The presence of 0.2% Triton
X-100 had no influence on the efficiency of the oxygen trap when
checked by the Clark oxygen electrode. The absorption spectra were
measured in a split beam mode, and the kinetic measurements were made
in a dual wavelength mode at 559-570 nm using an SLM Aminco
DW2000 spectrophotometer. The prenylquinones were added to the reaction
mixtures as solutions in ethanol of which the final concentration did
not exceed 1%. Photoreduction experiments were performed using light
from a halogen projector passed through a 630-nm cutoff filter.
To protect the photomultiplier from the scattered light, an
interference filter (
max = 564 nm,
Tmax = 30%, full width at
half-maximum = 12 nm) was placed in front of the photomultiplier.
All the measurements were performed at room temperature.
 |
RESULTS |
Absorption Spectra of Different cyt b559 Forms--
To
observe only redox changes of the LP form of cyt
b559 in PSII particles and to avoid possible
absorption changes due to the HP form, we have performed our
measurements in the presence of Triton X-100, which is known to convert
the HP form into the LP form of cyt b559 (19).
This detergent was also found to increase the ascorbate-reducible
fraction of the cyt b559 as compared with the
dithionite-reducible cyt b559 in PSII RC (20),
probably by disaggregation of the RC preparations, which in turn
improves penetration of added redox compounds to the cytochrome. This
detergent should also increase the solubility of the added
hydrophobic prenylquinones. The difference absorption spectra in the
-band region of cyt b559 absorption show
(Fig. 1) that our PSII preparation
solubilized in 0.2% Triton X-100 does not contain any
detectable cyt b559 HP redox form, whereas the
ascorbate reduces the cytochrome to practically the same level as that
produced by MD, which is a stronger reductant than ascorbate.
Dithionite reduced the cytochrome to the same level as that produced by
MD and ascorbate (data not shown), indicating that Triton
X-100-solubilized PSII particles contain only the low-potential,
totally ascorbate-reducible cyt b559 form. The
presence of Triton X-100 also improves the optical properties of
the sample.

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Fig. 1.
Absorption difference spectra of different
forms of cyt b559 in PSII particles in the
presence of 0.2% Triton X-100 obtained as follows:
cyt b559 HP reduced
(red.) (control-ferricyanide), cyt
b559 HP oxidized (ox.)
(hydroquinone-control), cyt
b559 LP ascorbate
(Asc)-reducible (ascorbate-ferricyanide), cyt
b559 LP-MD-reducible
(MD-ferricyanide). Conditions: chlorophyll concentration, 50 µg/ml; 50 mM Hepes buffer, pH 7.5, 10 mM KCl,
5 mM MgCl2.
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|
Photoreduction of cyt b559 in the Presence of
Plastoquinones--
Because the Triton X-100 treatment
inactivates the oxygen-evolving complex, 1 mM
MnCl2 was added to all light-treated samples as an electron
donor to PSII particles. Fig. 2 shows
that under anaerobic conditions in the presence of even very low PQ-2
concentration (1.73 mol of PQ-2/mol of cyt
b559), the cytochrome is photoreduced at a fast
rate and oxidized back to the original level after turning off the
light. The second illumination of the same sample gives both faster
photoreduction and photooxidation rates. Interestingly, the same sample
in the presence of ambient oxygen shows no detectable redox changes of
the cytochrome. This might indicate that the cytochrome is oxidized so
quickly by molecular oxygen that there is no detectable steady-state
level of the reduced cytochrome. The control sample, with no added
quinone, shows almost no redox changes of the cytochrome. PQ-9 was also
active in catalyzing the photoreduction and subsequent dark oxidation
of the cytochrome. However, much higher amounts of PQ-9 are required to
obtain significant levels of the cytochrome photoreduction. Even though
the photoreduction is considerably less sensitive to PQ-9 than to PQ-2
in our measurements, it is much more sensitive than in analogous
experiments performed on PSII RCs (20), where the cytochrome could be
photoreduced to not more than 62% of the total cytochrome present in
the preparation and where this saturation was reached at over 300 mol
of PQ-9/mol of cyt b559. In our case, the
reduction level of cyt b559 is 65 and 40% for
110 PQ-9 and 55 PQ-9 samples (Fig. 2), respectively. It is also
evident for samples with low PQ-9 content (55 PQ-9 and 28 PQ-9)
that there is a residual fast photoreduction and dark oxidation
absorbance change that is still present in the control sample (with no
added quinone) but is not observed under aerobic conditions. These
cytochrome redox changes could originate from the residual
plastoquinone present in PSII preparations or a low efficient electron
transport via cyt b559 without the participation of the plastoquinone. In contrast to plastoquinones,
-TQ was not
active in stimulation of the cytochrome photoreduction even at high
concentrations (Fig. 2). None of the cytochrome redox changes shown in
Fig. 2 was inhibited by DCMU, indicating that the QB site
is not involved in the electron transport to and from cyt
b559 under our conditions.

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Fig. 2.
Light-dark redox changes of cyt
b559 in PSII-Triton X-100
particles in the presence of different prenylquinones measured at
559-570 nm. The numbers refer to the molar quinone/cyt
b559 ratio. The measurements were performed
under anaerobic conditions except for the 1.73 PQ-2 + O2
sample. The arrows ( , ) denote light on and off,
respectively. Red light (>630 nm) was applied at the intensity of 900 µmol/m2/s. Other conditions were as described in
the legend to Fig. 1.
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|
Fig. 3 shows that the cytochrome
photoreduction rate is proportional to the PQ-2 content and is already
observed at very low (substoichiometric) amounts of PQ-2 in relation to
cyt b559, indicating a very tight binding site
of PQ-2 in the vicinity of the cytochrome. The cytochrome is totally
reduced already at 7 molecules of PQ-2 per heme of the cytochrome
within 20 s of the measurements (compare Figs. 1 and 3). The
photoreduction of cyt b559 in the presence of 14 mol of PQ-2/mol of cyt b559 at different light
intensities under anaerobic conditions is presented in Fig.
4. It is evident that the photoreduction
rate is proportional to the light intensity but is not saturated even
at the highest light intensity applied. On the other hand, full
reduction is already reached at 100 µmol/m2/s
within the time of measurement. The time-dependent cyt
b559 photoreduction in the presence of
plastoquinones with the different side chain length and DBMIB at 1.73 mol of quinone/mol of cyt b559 under
anaerobic conditions shows (Fig. 5) that
with decreasing the side chain length of PQ the photoreduction rate
increases. PQ-9 was not active in the photoreduction at this
concentration. DBMIB, which was shown in some studies to catalyze the
cyt b559 photoreduction (10, 11, 21), was nearly
inactive at the concentration applied in Fig. 5 but showed stimulation
of the reaction at higher DBMIB/cyt b559
proportions (data not shown). It was suggested that the unusually high
potential of the cyt b559 HP form is caused by
the localization of the cyt b559 heme ring
within the very hydrophobic interior of the membrane and that the
transformation of the HP form to the LP form is the result of heme
exposure to the more polar surroundings (2, 22). The results shown in
Fig. 5 confirm that the heme of the cytochrome LP form is localized in
polar surroundings, because the photoreduction is catalyzed most
efficiently by relatively polar, short chain plastoquinones.

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Fig. 3.
Light-induced cyt
b559 reduction in the presence of
different PQ-2 amounts under anaerobic conditions. Other
conditions were as described in the legends to Figs. 1 and 2.
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Fig. 4.
Photoreduction of cyt
b559 in the presence of 14 mol of PQ-2/mol
of cyt b559 at different light intensities
under anaerobic conditions. Other conditions were as described in
the legends to Figs. 1 and 2.
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Fig. 5.
Photoreduction of cyt
b559 in the presence of plastoquinones
with the different length of the side chain and DBMIB at 1.73 mol of
quinone/mol of cyt b559
under anaerobic conditions. Other conditions were as described in
the legends to Fig. 1 and 2.
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Dark Autoxidation of cyt b559--
To analyze the
possible oxidation routes of the photoreduced cyt
b559 (Fig. 2), we used chemical reduction of the
cytochrome with a small excess of MD and followed the cytochrome
oxidation under molecular oxygen (in air-saturated buffer) and
exogenously added prenylquinones. Addition of MD under anaerobic
conditions results in fast dark reduction of the cytochrome,
which remains reduced for minutes (Fig.
6A). If MD is added under
aerobic conditions, after transient reduction, the cytochrome is
gradually oxidized by molecular oxygen (Fig. 6B). When the
cytochrome is reduced under anaerobic conditions and
H2O2 is added, which immediately releases
molecular oxygen on contact with the catalase, which is a
component of the oxygen trap, the cytochrome oxidation is observed
(Fig. 6C). The second H2O2 addition,
which should give a 200 µM oxygen concentration in the
reaction mixture, causes a higher cytochrome oxidation rate than that
under aerobic conditions, where the oxygen concentration should be
about 250 µM. The reason for this could be that under the
conditions illustrated in Fig. 6B there is an excess of MD
left after the cytochrome reduction that slows down its autoxidation.
In the case of the reaction illustrated in Fig. 6C, the
first H2O2 addition probably at least partially
removes the MD excess.

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Fig. 6.
Dark cyt
b559 reduction under the addition of
3.6 µM MD and the cytochrome
oxidation under anaerobic conditions (A), aerobic
conditions (B), or anaerobic conditions
(C) after additions of H2O2
( ) giving final H2O2 concentrations of 0.4, 0.8, and 0.4 mM. Other conditions were as described in
the legends to Figs. 1 and 2.
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|
Dark Prenylquinone-mediated Oxidation of cyt
b559--
Fig. 7 illustrates
PQ-2-induced dark cytochrome oxidation after its prereduction with MD.
PQ-2 induces cytochrome oxidation at rates proportional to the quinone
concentration, but under anaerobic conditions the extent of the
cytochrome reduction decreases with increasing PQ-2 concentration
(Table I). In the presence of oxygen, the
cytochrome oxidation rate is considerably increased by the simultaneous
cytochrome autoxidation. In this case, the cytochrome is completely
oxidized. PQ-9 and
-TQ were also active in the oxidation of cyt
b559 (Fig. 8), but
the oxidation rates in the presence of PQ-9 and
-TQ are considerably
lower than those caused by PQ-2. In contrast to PQ-2, the oxidation
extent is considerably higher for PQ-9 and
-TQ (Table I). The
oxidation rates under aerobic conditions are evidently higher, and the
rates correspond approximately to the oxidation rates under anaerobic
conditions increased by the cytochrome autoxidation rate (Table I).
Comparing the oxidation rates during dark periods in the photoreduced
samples (Fig. 2) and the rates obtained from Figs. 7 and 8, it can be seen that the rates for the dark cytochrome oxidation obtained from
experiments on cyt b559 prereduced by MD would
not be high enough to account for the observed cytochrome oxidation
rates prereduced by light at the same quinone concentrations. The
reason for this could be partially connected with the excess of MD that remains after the cytochrome reduction. This probably inhibits the
oxidation of the cytochrome by the added quinones, which can be
deduced from comparison of the observed rates of the cytochrome autoxidation by ambient oxygen and the oxygen released from
H2O2 (Fig. 6, B and C).
Nevertheless, the above experiments show that cyt
b559 can be effectively oxidized by
prenylquinones or molecular oxygen in our system. DCMU had no influence
on the observed dark oxidation of the cyt
b559.

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Fig. 7.
Dark reduction of cyt
b559 under the addition of 3.6 µM MD and the cytochrome oxidation after
the addition of different amounts of PQ-2 ( ) at the given PQ-2/cyt
b559 molar ratios. The measurements
were performed under anaerobic conditions except for the 28 PQ-2 + O2 sample. Other conditions were as described in the
legends to Figs. 1 and 2.
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Table I
Dark oxidation rates of cyt b559 LP form and its extent of
oxidation in PSII, 0.2% Triton X-100 preparations in the presence of
different prenylquinones (at given quinone/cyt b559 molar
ratios) under anaerobic or aerobic conditions (+ O2)
L-D stands for the cyt b559 oxidation in the
presence of plastoquinones after its photoreduction. The given range of
oxidation rates in L-D samples depends on the illumination sequence. In
other samples the cyt b559 was prereduced by the
addition of 3.6 µM menadiol. The maximal error in these
samples was not higher than 8%. (n.s.) indicates that the oxidation
extent was not saturated at the end of the measurement (7 min);
(H2O2) stands for the oxygen released from the 0.8 mM H2O2 added to the sample. For other
conditions see the legends to Figs. 1, 2, and 8.
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Fig. 8.
Dark reduction of cyt
b559 under the addition of 3.6 µM MD and the cytochrome oxidation after
the addition of different amounts of PQ-9 or
-TQ at the given quinone/cyt
b559 molar ratios. The measurements
were performed under anaerobic conditions unless otherwise indicated
(+ O2). Other conditions were as described in the
legends to Figs. 1 and 2.
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 |
DISCUSSION |
The observed effects described above of stimulation of cyt
b559 photoreduction by different plastoquinones
are similar to those observed with DBMIB, PQ-decyl, or PQ-9 on the
isolated PSII reaction centers (9, 10, 20), but in our case the
photoreduction proceeds with higher quinone sensitivity and to a higher
cytochrome reduction extent. The efficient photoreduction of the
cytochrome at very low PQ-2 concentrations (Fig. 3) suggests a very
tight binding site close to the cytochrome heme. The question arises as
to whether it is one of the known quinone-binding sites in PSII
(QA or QB) or another high affinity
quinone-binding site in PSII. The lack of DCMU sensitivity of the
cytochrome redox changes and the fact that the QB site is
detergent-sensitive (23) and easily damaged indicate that the binding
site is not QB. Moreover, to efficiently reduce the
cyt b559 LP form, quinone molecules must operate
via the semiquinone form, similarly to QA, and it is known
that the QB site operates via a two-electron gate. However, involvement of QA also seems unlikely. It has been
suggested that in isolated reaction centers, where the QA
and QB plastoquinone molecules are lost (21, 24), the added
quinones substitute for the QA site (10, 11, 20). However,
in our case this seems unlikely, because we used PSII membranes. It was
shown that it is very difficult to remove or substitute the
QA molecule in isolated PSII core preparations (25). It
requires high (1%) Triton X-100 concentrations in the presence
of phenanthroline and hours of incubation time (25). In our experiments
the samples were incubated only with 0.2% Triton X-100 for 10 min before the measurements. Moreover, we have found that the
photoreduction changes were already observed within a shorter time but
that the base-line stability and signal to noise ratio were poor in
that case (data not shown). Interestingly, the QA
reconstitution experiments in PQ-9-depleted PSII core preparations (26)
showed that this site shows a high structural specificity, and only
PQ-9 or UQ-10 efficiently substituted for QA function, in
contrast to PQ-2, DBMIB, or other short chain quinones. A similar
specificity, only for PQ-9 or UQ-9, was observed in PSII core complexes
where QA was exchanged by 1% Triton
X-100-phenanthroline incubation (25). On the other hand, rather
low structural quinone specificity is observed for cyt
b559 quinone-catalyzed photoreduction (9). These
data favor the view that the quinone-binding site active in cyt
b559 LP photoreduction is different from the
QA and QB sites. It is also evident that the
QA site shows specificity for hydrophobic quinones (PQ-9,
UQ-9 or UQ-10), whereas the cyt b559 quinone-binding site shows preference for polar plastoquinones or other
artificial quinones, which is connected with the exposure of the heme
of the cyt b559 LP form to the polar surroundings.
The question arises as to whether this cyt b559
LP quinone-binding site is occupied and functional in vivo
in the native PSII. Some indication may come from analyses of the PQ-9
content of PSII membranes and PSII core complexes. Such analyses
usually give about three or more PQ-9 molecules per RC in PSII
membranes (27, 28); two of these molecules are strongly bound
(non-extractable by hexane) (27), and about two are tightly bound PQ-9
molecules for PSII core complexes (28-30). In both preparations, one
molecule is certainly the QA molecule, but the identity of
the second PQ-9 in PSII core complexes is not certain (29, 30).
Probably it is not a QB molecule, because this site is
easily damaged by detergent treatment during the PSII core isolation
procedure, and the QB molecule is lost (23, 28-30). This
also would explain poor or no DCMU sensitivity of the electron
transport in PSII core complexes (29, 30). It was originally thought
that the additional, tightly bound PQ-9 molecule to PSII membranes is
Z (27), the primary electron donor to P680, but it turned out
that it was tyrosine. This data may suggest that the second, tightly
bound PQ-9 molecule in PSII core complexes is the PQ-9 molecule
participating in cyt b559 photoreduction via the
semiquinone form and is different from the QA molecule.
The site of quinone interaction with the cyt
b559 causing its oxidation seems to be a low
affinity quinone-binding site that is also DCMU-insensitive and is
probably in contact and exchange with the PQ pool in thylakoid
membranes. An argument supporting the concept of different
photoreduction and dark oxidation sites of cyt
b559 LP is the inefficiency of
-TQ in the
stimulation of the cytochrome photoreduction; but at the same
concentration
-TQ efficiently oxidizes the reduced cytochrome.
The model of electron transport pathways in PSII showing the PQ-binding
sites (QA and QB) together with the high
(QC) and low (Q) affinity PQ-binding sites involved
in cyt b559 redox changes is shown in Fig.
9. The possible physiological role of the
quinone-stimulated cyt b559 photoreduction (via
the QC site) and its reoxidation (via the Q site) could be
the acceleration of the electron transport from pheophytin to the PQ
pool or to molecular oxygen via cyt b559 LP upon
acceptor side inhibition (Fig. 9) when the electron route via
QA
QB
PQ pool is saturated or inhibited and
electron donation to P680 from the oxygen-evolving complex is active.
When PSII is fully functional, electron transport via
QA
QB dominates, and the cyt
b559 exists mainly in the HP form, which is kept
reduced by the plastoquinol of the PQ pool. Under donor side
inhibition, P680 is reduced by the cyt b559 HP
form via the redox active ChlZ molecule (1). Moreover, a
-carotene molecule probably mediates electron transfer between
ChlZ and P680 (31, 32). The hypothetical molecular switch
transforms the HP to the LP form, which is further oxidized by
molecular oxygen or a neighboring PQ molecule (Q in Fig. 9),
and the oxidized LP form is no longer able to donate electrons to P680.
Then, ChlZ is oxidized, giving
ChlZ+, which effectively quenches the
excitation energy, preventing the photosynthetic apparatus against
overexcitation (1). The electron route via QC upon
acceptor side inhibition also requires the HP
LP molecular switch to
give a sufficient cyt b559 LP level through
which electrons could outflow from pheophytin via QC and the cyt b559 LP form to O2 or PQ
pool, preventing charge recombination in the RC, leading to P680
triplet state formation and consequently RC degradation
(2, 21).

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Fig. 9.
A model of the electron transport reactions
within PSII showing the main and alternative electron transport
pathways protecting PSII against photoinhibition. Bold
arrows denote the electron transport route under non-inhibitory
conditions in PSII; non-bold arrows indicate the electron
transport reactions in isolated and quinone-reconstituted PSII RC
complexes and occurring possibly in vivo under acceptor side
inhibition (|); dashed arrows indicate electron transport
routes probably not occurring in isolated PSII RC complexes but taking
place in vivo at low yield under non-inhibitory conditions
and at higher yield under donor side photoinhibition (dashed
line); ( ) denotes interconversion between the cyt
b559 HP and LP forms; and QC and Q
are the quinone-binding sites to the cyt b559 LP
form, responsible for its photoreduction and oxidation.
Pheo, pheophytin; OEC, oxygen-evolving
complex.
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 |
ACKNOWLEDGEMENTS |
We are indebted to Dr. H. Koike (Himeji
Institute of Technology, Hyogo, Japan) for a kind gift of short chain
plastoquinones. We thank Dr. Wim Vermaas for helpful discussions.
 |
FOOTNOTES |
*
This work was supported by Grant 6 P04A 028 19 from the
Committee for Scientific Research (KBN) of Poland.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.: 48 12 634 13 05 ext. 237; Fax: 48 12 633 69 07; E-mail: strzalka@mol.uj.edu.pl.
Published, JBC Papers in Press, October 5, 2000, DOI 10.1074/jbc.M003602200
 |
ABBREVIATIONS |
The abbreviations used are:
cyt
b559, cytochrome b559;
PSII, photosystem II;
RC, reaction center;
HP, high potential;
LP, low
potential;
ChlZ, chlorophyll Z;
DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone;
PQ, plastoquinone;
-TQ,
-tocopherol quinone;
MD, menadiol;
DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea;
UQ, ubiquinone.
 |
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, and Yocum, C. F., eds)
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