Transmembrane charge transfer in photosynthetic reaction centers: some similarities and distinctions

Comparison of photosynthetic responses between haptophyte Phaeocystis globosa and diatom Skeletonema costatum under phosphorus limitation

The diatom Skeletonema costatum and the haptophyte Phaeocystis globosa often form blooms in the coastal waters of the South China Sea. Skeletonema costatum commonly dominates in nutrient enrichment coastal waters, whereas P. globosa starts flourishing after the diatom blooms when phosphorus (P) is limited. Therefore, P limitation was proposed to be a critical factor affecting diatom-haptophyte transition. To elucidate the tolerance to P limitation in P. globosa compared with S. costatum, the effect of P limitation on their photosystem II (PSII) performance was investigated and their photosynthesis acclimation strategies in response to P limitation were evaluated. P limitation did not affect the growth of P. globosa over 7 days but decreased it for S. costatum. Correspondingly, the PSII activity of S. costatum was significantly inhibited by P limitation. The decline in PSII activity in S. costatum under P limitation was associated with the impairment of the oxygen-evolving complex (the donor side of PSII), the hindrance of electron transport from Q_A ^- to Q_B (the acceptor side of PSII), and the inhibition of electron transport to photosystem I (PSI). The 100% decrease in D1 protein level of S. costatum after P limitation for 6 days and PsbO protein level after 2 days of P limitation were attributed to its enhanced photoinhibition. In contrast, P. globosa maintained its photosynthetic activity with minor impairment of the function of PSII. With accelerated PSII repair and highly increased non-photochemical quenching (NPQ), P. globosa can avoid serious PSII damage under P limitation. On the contrary, S. costatum decreased its D1 restoration under P limitation, and the maximum NPQ value in S. costatum was only one-sixth of that in P. globosa. The present work provides extensive evidence that a close interaction exists between the tolerance to P limitation and photosynthetic responses of S. costatum and P. globosa.

Evolutionary Loss of the Ability of the Photosystem I Primary Electron Donor for the Redox Interaction with Mn-Bicarbonate Complexes

The structure and functional organization of the photosystem I (PSI) reaction center (RC) donor side has a significant similarity to the reaction centers of purple bacteria (bRCs), despite the fact that they belong to different types of RCs. Moreover, the redox potential values of their primary electron donors are identical (~0.5 V). In our earlier reports [Khorobrykh et al. (2008) Phylos. Trans. R. Soc. B., 363, 1245-1251; Terentyev et al. (2011) Biochemistry (Moscow), 76, 1360-1366; Khorobrykh et al. (2018) ChemBioChem, 14, 1725-1731], we have demonstrated redox interaction of low-potential Mn2+-bicarbonate complexes with bRCs, which might have been one of the first steps in the evolutionary origin of Mn-cluster of the photosystem II water-oxidizing complex that occurred in the Archean (over 3 billion years ago). In this study, we investigated redox interactions between Mn2+-bicarbonate complexes and PSI. Such interactions were almost absent in the original PSI preparations and emerged only in preoxidized PSI preparations containing ~50% oxidized RCs. The interaction between Mn2+-bicarbonate complexes and PSI required increased Mn2+ concentrations, while its dependence on the HCO₃^- concentration indicated involvement of electroneutral low-potential [Mn(HCO₃)₂] complex in the process. Analysis of the PSI crystal structure revealed steric hindrances on the RC donor side, which could block the redox interaction between Mn2+-bicarbonate complexes and oxidized primary electron donor. Comparison of structures of RCs from the PSI and ancient RCs from heliobacteria belonging to the same type of RCs suggested that such hindrances should be absent in the primitive PSI in the Archean and allowed to explain their evolutionary origin as a consequence of PSI RCs into the united electron transport chain (ETC) of the photosynthetic membrane that was accompanied by the evolutionary loss of PSI capacity for the redox interaction with Mn2+-bicarbonate complexes.

Comparison of tryptophan fluorescence lifetimes in cyanobacterial photosystem I frozen in the light and in the dark

The dependence on temperature of tryptophan fluorescence lifetime in trimeric photosystem I (PSI) complexes from cyanobacteria Synechocystis sp. PCC 6803 during the heating of pre-frozen to - 180 °C in the dark or in the light-activated preparations has been studied. Fluorescence lifetime in samples frozen in the light was longer than in samples frozen in the dark. For samples in 65% glycerol at λ_reg = 335 nm and at 20 °C, the lifetime of components were as follows: τ₁ ≈ 1.2 ns, τ₂ ≈ 4.9 ns, and τ₃ ≈ 20 ns. The contribution of the first component was negligible. To analyze the contribution of components 2 and 3 derived from frozen-thawed samples, two temperature ranges from - 180 to - 90 °C and above - 90 °C are considered. In doing so, the contributions of these components appear antiphase course to each other. The dependence on temperature of these contributions is explained by the influence of the microconformational protein dynamics on the tryptophan fluorescence lifetime. In the present work, a comparative analysis of temperature-dependent conformational dynamics and electron transfer in cyanobacterial PSI (Schlodder et al., in Biochemistry 37:9466-9476, 1998) and Rhodobacter sphaeroides reaction center complexes (Knox et al., in J Photochem Photobiol B 180:140-148, 2018) was also carried out.

Non-photochemical Fluorescence Quenching in Photosystem II Antenna Complexes by the Reaction Center Cation Radical

In direct experiments, rate constants of photochemical (kP) and non-photochemical (kP(+)) fluorescence quenching were determined in membrane fragments of photosystem II (PSII), in oxygen-evolving PSII core particles, as well as in core particles deprived of the oxygen-evolving complex. For this purpose, a new approach to the pulse fluorometry method was implemented. In the "dark" reaction center (RC) state, antenna fluorescence decay kinetics were measured under low-intensity excitation (532 nm, pulse repetition rate 1 Hz), and the emission was registered by a streak camera. To create a "closed" [P680(+)QA(-)] RC state, a high-intensity pre-excitation pulse (pump pulse, 532 nm) of the sample was used. The time advance of the pump pulse against the measuring pulse was 8 ns. In this experimental configuration, under the pump pulse, the [P680(+)QA(-)] state was formed in RC, whereupon antenna fluorescence kinetics was measured using a weak testing picosecond pulsed excitation light applied to the sample 8 ns after the pump pulse. The data were fitted by a two-exponential approximation. Efficiency of antenna fluorescence quenching by the photoactive RC pigment in its oxidized (P680(+)) state was found to be ~1.5 times higher than that of the neutral (P680) RC state. To verify the data obtained with a streak camera, control measurements of PSII complex fluorescence decay kinetics by the single-photon counting technique were carried out. The results support the conclusions drawn from the measurements registered with the streak camera. In this case, the fitting of fluorescence kinetics was performed in three-exponential approximation, using the value of τ1 obtained by analyzing data registered by the streak camera. An additional third component obtained by modeling the data of single photon counting describes the P680(+)Pheo(-) charge recombination. Thus, for the first time the ratio of kP(+)/kP = 1.5 was determined in a direct experiment. The mechanisms of higher efficiency for non-photochemical antenna fluorescence quenching by RC cation radical in comparison to that of photochemical quenching are discussed.

The efficiency of non-photochemical fluorescence quenching by cation radicals in photosystem II reaction centers

In a direct experiment, the rate constants of photochemical k _p and non-photochemical k _p⁺ quenching of the chlorophyll fluorescence have been determined in spinach photosystem II (PS II) membrane fragments, oxygen-evolving PS II core, as well as manganese-depleted PS II particles using pulse fluorimetry. In the dark-adapted reaction center(s) (RC), the fluorescence decay kinetics of the antenna were measured at low-intensity picosecond pulsed excitation. To create a "closed" P680⁺Q _A^- state, RCs were illuminated by high-intensity actinic flash 8 ns prior to the measuring flash. The obtained data were approximated by the sum of two decaying exponents. It was found that the antennae fluorescence quenching efficiency by the oxidized photoactive pigment of RC P680⁺ was about 1.5 times higher than that of the neutral P680 state. These results were confirmed by a single-photon counting technique, which allowed to resolve the additional slow component of the fluorescence decay. Slow component was assigned to the charge recombination of P680⁺Pheo^- in PS II RC. Thus, for the first time, the ratio k _p⁺ /k _p ≅ 1.5 was found directly. The mechanism of the higher efficiency of non-photochemical quenching comparing to photochemical quenching is discussed.

Primary electron transfer processes in photosynthetic reaction centers from oxygenic organisms

This minireview is written in honor of Vladimir A. Shuvalov, a pioneer in the area of primary photochemistry of both oxygenic and anoxygenic photosyntheses (See a News Report: Allakhverdiev et al. 2014). In the present paper, we describe the current state of the formation of the primary and secondary ion-radical pairs within photosystems (PS) II and I in oxygenic organisms. Spectral-kinetic studies of primary events in PS II and PS I, upon excitation by ~20 fs laser pulses, are now available and reviewed here; for PS II, excitation was centered at 710 nm, and for PS I, it was at 720 nm. In PS I, conditions were chosen to maximally increase the relative contribution of the direct excitation of the reaction center (RC) in order to separate the kinetics of the primary steps of charge separation in the RC from that of the excitation energy transfer in the antenna. Our results suggest that the sequence of the primary electron transfer reactions is P680 → ChlD1 → PheD1 → QA (PS II) and P700 → A 0A/A 0B → A 1A/A 1B (PS I). However, alternate routes of charge separation in PS II, under different excitation conditions, are not ruled out.

The nonheme iron in photosystem II

Photosystem II (PSII), the light-driven water:plastoquinone (PQ) oxidoreductase of oxygenic photosynthesis, contains a nonheme iron (NHI) at its electron acceptor side. The NHI is situated between the two PQs QA and QB that serve as one-electron transmitter and substrate of the reductase part of PSII, respectively. Among the ligands of the NHI is a (bi)carbonate originating from CO2, the substrate of the dark reactions of oxygenic photosynthesis. Based on recent advances in the crystallography of PSII, we review the structure of the NHI in PSII and discuss ideas concerning its function and the role of bicarbonate along with a comparison to the reaction center of purple bacteria and other enzymes containing a mononuclear NHI site.

pH-dependent regulation of electron transport and ATP synthesis in chloroplasts

This review is focused on pH-dependent mechanisms of regulation of photosynthetic electron transport and ATP synthesis in chloroplasts. The light-induced acidification of the thylakoid lumen is known to decelerate the plastoquinol oxidation by the cytochrome b 6 f complex, thus impeding the electron flow between photosystem II and photosystem I. Acidification of the lumen also triggers the dissipation of excess energy in the light-harvesting antenna of photosystem II, thereby protecting the photosynthetic apparatus against a solar stress. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the nature of the rate-limiting step of electron transfer between photosystem II and photosystem I. In the context of pH-dependent mechanism of photosynthetic control in chloroplasts, the mechanisms of plastoquinol oxidation by the cytochrome b 6 f complex have been considered. The light-induced alkalization of stroma is another factor of pH-dependent regulation of electron transport in chloroplasts. Alkalization of stroma induces activation of the Bassham-Benson-Calvin cycle reactions, thereby promoting efflux of electrons from photosystem I to NADP(+). The mechanisms of the light-induced activation of ATP synthase are briefly considered.

Chlorophyll fluorescence in the leaves of Tradescantia species of different ecological groups: induction events at different intensities of actinic light

Chlorophyll fluorescence analysis is one of the most convenient and widespread techniques used to monitor photosynthesis performance in plants. In this work, after a brief overview of the mechanisms of regulation of photosynthetic electron transport and protection of photosynthetic apparatus against photodamage, we describe results of our study of the effects of actinic light intensity on photosynthetic performance in Tradescantia species of different ecological groups. Using the chlorophyll fluorescence as a probe of photosynthetic activity, we have found that the shade-tolerant species Tradescantia fluminensis shows a higher sensitivity to short-term illumination (≤20min) with low and moderate light (≤200μEm(-2)s(-1)) as compared with the light-resistant species Tradescantia sillamontana. In T. fluminensis, non-photochemical quenching of chlorophyll fluorescence (NPQ) and photosystem II operational efficiency (parameter ΦPSII) saturate as soon as actinic light reaches ≈200μEm(-2)s(-1). Otherwise, T. sillamontana revealed a higher capacity for NPQ at strong light (≥800μEm(-2)s(-1)). The post-illumination adaptation of shade-tolerant plants occurs slower than in the light-resistant species. The data obtained are discussed in terms of reactivity of photosynthetic apparatus to short-term variations of the environment light.