Cyanobacteria have multiple psbA genes encoding PsbA, the D1 reaction center protein of the Photosystem II complex which bears together with PsbD, the D2 protein, most of the cofactors involved in ...electron transfer reactions. The thermophilic cyanobacterium Thermosynechococcus elongatus has three psbA genes differently expressed depending on the environmental conditions. Among the 344 residues constituting each of the 3 possible PsbA variants there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2 and 27 between PsbA2 and PsbA3. In this review, we summarize the changes already identified in the properties of the redox cofactors depending on the D1 variant constituting Photosystem II in T. elongatus. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.
The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH3-treated PSII from spinach in the S2-state are simulated with collinear and rhombic g- and Mn-hyperfine tensors. The ...obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn4 cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and 55Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4J-coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S=1/2 ground state with a low lying excited spin state (30 cm-1) and an easy conversion to a S=5/2 system responsible for the g=4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between -290 and -130 cm-1.
In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for the Photosystem II (PSII) D1 subunit that interacts with most of the main cofactors involved in the electron ...transfers. Recently, the 3D crystal structures of both PsbA2-PSII and PsbA3-PSII have been solved Nakajima et al., J. Biol. Chem. 298 (2022) 102668.. It was proposed that the loss of one hydrogen bond of PheD1 due to the D1-Y147F exchange in PsbA2-PSII resulted in a more negative Em of PheD1 in PsbA2-PSII when compared to PsbA3-PSII. In addition, the loss of two water molecules in the Cl-1 channel was attributed to the D1-P173M substitution in PsbA2-PSII. This exchange, by narrowing the Cl-1 proton channel, could be at the origin of a slowing down of the proton release. Here, we have continued the characterization of PsbA2-PSII by measuring the thermoluminescence from the S2QA−/DCMU charge recombination and by measuring proton release kinetics using time-resolved absorption changes of the dye bromocresol purple. It was found that i) the Em of PheD1–/PheD1 was decreased by ∼30 mV in PsbA2-PSII when compared to PsbA3-PSII and ii) the kinetics of the proton release into the bulk was significantly slowed down in PsbA2-PSII in the S2TyrZ• to S3TyrZ and S3TyrZ• → (S3TyrZ•)’ transitions. This slowing down was partially reversed by the PsbA2/M173P mutation and induced by the PsbA3/P173M mutation thus confirming a role of the D1-173 residue in the egress of protons trough the Cl-1 channel.
•The Em of PheD1–•/PheD1 is ∼30 mV lower in PsbA2-PSII than in PsbA3-PSII.•Proton release kinetics are slowed down in PsbA2-PSII compared to PsbA3-PSII.•This slowing down is partially reversed by the PsbA2/M173P mutation and induced by the PsbA3/P173M mutation.•The data confirm the role of the residue 173 of D1 in the function of the Cl-1 channel.
The stoichiometry and kinetics of the proton release were investigated during each transition of the S-state cycle in Photosystem II (PSII) from Thermosynechococcus elongatus containing either a Mn
...CaO
(PSII/Ca) or a Mn
SrO
(PSII/Sr) cluster. The measurements were done at pH 6.0 and pH 7.0 knowing that, in PSII/Ca at pH 6.0 and pH 7.0 and in PSII/Sr at pH 6.0, the flash-induced S
-state is in a low-spin configuration (S
) whereas in PSII/Sr at pH 7.0, the S
-state is in a high-spin configuration (S
) in half of the centers. Two measurements were done; the time-resolved flash dependent i) absorption of either bromocresol purple at pH 6.0 or neutral red at pH 7.0 and ii) electrochromism in the Soret band of P
at 440 nm. The fittings of the oscillations with a period of four indicate that one proton is released in the S
to S
transition in PSII/Sr at pH 7.0. It has previously been suggested that the proton released in the S
to S
transition would be released in a S
Tyr
→ S
Tyr
transition before the electron transfer from the cluster to Tyr
occurs. The release of a proton in the S
Tyr
→ S
Tyr
transition would logically imply that this proton release is missing in the S
Tyr
to S
Tyr
transition. Instead, the proton release in the S
to S
transition in PSII/Sr at pH 7.0 was mainly done at the expense of the proton release in the S
to S
and S
to S
transitions. However, at pH 7.0, the electrochromism of P
seems larger in PSII/Sr when compared to PSII/Ca in the S
state. This points to the complex link between proton movements in and immediately around the Mn
cluster and the mechanism leading to the release of protons into the bulk.
Photosystem II (PSII), the oxygen-evolving enzyme, consists of 17 trans-membrane and 3 extrinsic membrane proteins. Other subunits bind to PSII during assembly, like Psb27, Psb28, and Tsl0063. The ...presence of Psb27 has been proposed (Zabret et al. in Nat Plants 7:524–538, 2021; Huang et al. Proc Natl Acad Sci USA 118:e2018053118, 2021; Xiao et al. in Nat Plants 7:1132–1142, 2021) to prevent the binding of PsbJ, a single transmembrane α-helix close to the quinone
Q
B
binding site. Consequently, a PSII rid of Psb27, Psb28, and Tsl0034 prior to the binding of PsbJ would logically correspond to an assembly intermediate. The present work describes experiments aiming at further characterizing such a ∆PsbJ–PSII, purified from the thermophilic
Thermosynechococcus elongatus
, by means of MALDI-TOF spectroscopy, thermoluminescence, EPR spectroscopy, and UV–visible time-resolved spectroscopy. In the purified ∆PsbJ–PSII, an active Mn
4
CaO
5
cluster is present in 60–70% of the centers. In these centers, although the forward electron transfer seems not affected, the
Em
of the
Q
B
/
Q
B
−
couple increases by ≥ 120 mV , thus disfavoring the electron coming back on
Q
A
. The increase of the energy gap between
Q
A
/
Q
A
−
and
Q
B
/
Q
B
−
could contribute in a protection against the charge recombination between the donor side and
Q
B
−
, identified at the origin of photoinhibition under low light (Keren et al. in Proc Natl Acad Sci USA 94:1579–1584, 1997), and possibly during the slow photoactivation process.
Two symmetrically positioned redox active tyrosine residues are present in the photosystem II (PSII) reaction center. One of them, TyrZ, is oxidized in the ns-µs time scale by P680+and reduced ...rapidly (µs to ms) by electrons from the Mn complex. The other one, TyrD, is stable in its oxidized form and seems to play no direct role in enzyme function. Here, we have studied electron donation from these tyrosines to the chlorophyll cation (P680+)in Mn-depleted PSII from plants and cyanobacteria. In particular, a mutant lacking TyrZ was used to investigate electron donation from TyrD. By using EPR and time-resolved absorption spectroscopy, we show that reduced TyrD is capable of donating an electron to P680+with t1/2≈190 ns at pH 8.5 in approximately half of the centers. This rate is ≈105times faster than was previously thought and similar to the TyrZ donation rate in Mn-depleted wild-type PSII (pH 8.5). Some earlier arguments put forward to rationalize the supposedly slow electron donation from TyrD (compared with that from TyrZ) can be reassessed. At pH 6.5, TyrZ (t1/2= 2-10 µs) donates much faster to P680+than does TyrD (t1/2> 150 µs). These different rates may reflect the different fates of the proton released from the respective tyrosines upon oxidation. The rapid rate of electron donation from TyrD requires at least partial localization of P680+on the chlorophyll (PD2) that is located on the D2 side of the reaction center.
Ca2+ and Cl− ions are essential elements for the oxygen evolution activity of photosystem II (PSII). It has been demonstrated that these ions can be exchanged with Sr2+ and Br−, respectively, and ...that these ion exchanges modify the kinetics of some electron transfer reactions at the Mn4Ca cluster level (Ishida et al., J. Biol. Chem. 283 (2008) 13330–13340). It has been proposed from thermoluminescence experiments that the kinetic effects arise, at least in part, from a decrease in the free energy level of the Mn4Ca cluster in the S3 state though some changes on the acceptor side were also observed. Therefore, in the present work, by using thin-layer cell spectroelectrochemistry, the effects of the Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone electron acceptor QA, Em(QA/QA−), were investigated. Since the previous studies on the Ca2+/Sr2+ and Cl−/Br− exchanges were performed in PsbA3-containing PSII purified from the thermophilic cyanobacterium Thermosynechococcus elongatus, we first investigated the influences of the PsbA1/PsbA3 exchange on Em(QA/QA−). Here we show that i) the Em(QA/QA−) was up-shifted by ca. +38mV in PsbA3-PSII when compared to PsbA1-PSII and ii) the Ca2+/Sr2+ exchange up-shifted the Em(QA/QA−) by ca. +27mV, whereas the Cl−/Br− exchange hardly influenced Em(QA/QA−). On the basis of the results of Em(QA/QA−) together with previous thermoluminescence measurements, the ion-exchange effects on the energetics in PSII are discussed.
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► PsbA1/A3 exchange shifts the redox potential of QA of PSII by ca. +38mV. ► Biosynthetic Ca2+/Sr2+ exchange shifts the redox potential of QA by ca. +27mV. ► Cl–/Br– exchange hardly influences the redox potential of QA. ► The findings show that tuning of the cofactors' potentials is exquisitely subtle.
In Photosystem II (PSII) from
Thermosynechococcus elongatus, high-light intensity growth conditions induce the preferential expression of the
psbA
3 gene over the
psbA
1 gene. These genes encode for ...the D1 protein variants labeled D1:3 and D1:1, respectively. We have compared steady state absorption and photo-induced difference spectra at <
10 K of PSII containing either D1:1 or D1:3. The following differences were observed. (i) The pheophytin Q
x band was red-shifted in D1:3 (547.3 nm) compared to D1:1 (544.3 nm). (ii) The electrochromism on the Pheo
D1 Q
x band induced by Q
A
− (the C550 shift) was more asymmetric in D1:3. (iii) The two variants differed in their responses to excitation with far red (704 nm) light. When green light was used there was little difference between the two variants. With far red light the stable (
t
1/2
>
50 ms) Q
A
− yield was ∼
95% in D1:3, and ∼
60% in D1:1, relative to green light excitation. (iv) For the D1:1 variant, the quantum efficiency of photo-induced oxidation of side-pathway donors was lower. These effects can be correlated with amino acid changes between the two D1 variants. The effects on the pheophytin Q
x band can be attributed to the hydrogen bond from Glu130 in D1:3 to the 13
1-keto of Pheo
D1, which is absent for Gln130 in D1:1. The reduced yield with red light in the D1:1 variant could be associated with either the Glu130Gln change, and/or the four changes near the binding site of P
D1, in particular Ser153Ala. Photo-induced Q
A
− formation with far red light is assigned to the direct optical excitation of a weakly absorbing charge transfer state of the reaction centre. We suggest that this state is blue-shifted in the D1:1 variant. A reduced efficiency for the oxidation of side-pathway donors in the D1:1 variant could be explained by a variation in the location and/or redox potential of P
+.
A kinetic-LED-array-spectrophotometer (Klas) was recently developed for measuring in vivo redox changes of P700, plastocyanin (PCy), and ferredoxin (Fd) in the near-infrared (NIR). This ...spectrophotometer is used in the present work for in vitro light-induced measurements with various combinations of photosystem I (PSI) from tobacco and two different cyanobacteria, spinach plastocyanin, cyanobacterial cytochrome
c
6
(cyt.
c
6
), and Fd. It is shown that cyt.
c
6
oxidation contributes to the NIR absorption changes. The reduction of (
F
A
F
B
), the terminal electron acceptor of PSI, was also observed and the shape of the (
F
A
F
B
) NIR difference spectrum is similar to that of Fd. The NIR difference spectra of the electron-transfer cofactors were compared between different organisms and to those previously measured in vivo, whereas the relative absorption coefficients of all cofactors were determined by using single PSI turnover conditions. Thus, the (840 nm
minus
965 nm) extinction coefficients of the light-induced species (oxidized
minus
reduced for PC and cyt.
c
6
, reduced
minus
oxidized for (
F
A
F
B
), and Fd) were determined with values of 0.207 ± 0.004, – 0.033 ± 0.006, – 0.036 ± 0.008, and – 0.021 ± 0.005 for PCy, cyt.
c
6
, (
F
A
F
B
) (single reduction), and Fd, respectively, by taking a reference value of + 1 for P700
+
. The fact that the NIR P700 coefficient is larger than that of PCy and much larger than that of other contributing species, combined with the observed variability in the NIR P700 spectral shape, emphasizes that deconvolution of NIR signals into different components requires a very precise determination of the P700 spectrum.
The quinone-iron complex of the electron acceptor complex of Photosystem II was studied by EPR spectroscopy in Thermosynechococcus elongatus. New g ∼ 2 features belonging to the EPR signal of the ...semiquinone forms of the primary and secondary quinone, i.e., Q(A)(•-)Fe(2+) and Q(B)(•-)Fe(2+), respectively, are reported. In previous studies, these signals were missed because they were obscured by the EPR signal arising from the stable tyrosyl radical, TyrD(•). When the TyrD(•) signal was removed, either by chemical reduction or by the use of a mutant lacking TyrD, the new signals dominated the spectrum. For Q(A)(•-)Fe(2+), the signal was formed by illumination at 77 K or by sodium dithionite reduction in the dark. For Q(B)(•-)Fe(2+), the signal showed the characteristic period-of-two variations in its intensity when generated by a series of laser flashes. The new features showed relaxation characteristics comparable to those of the well-known features of the semiquinone-iron complexes and showed a temperature dependence consistent with an assignment to the low-field edge of the ground state doublet of the spin system. Spectral simulations are consistent with this assignment and with the current model of the spin system. The signal was also present in Q(B)(•-)Fe(2+) in plant Photosystem II, but in plants, the signal was not detected in the Q(A)(•-)Fe(2+) state.
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