The strontium inorganic mutant of the water oxidizing center (CaMn4O5) of PSII improves WOC efficiency but slows electron flux through the terminal acceptors

Exceptional Quantum Efficiency Powers Biomass Production in Halotolerant Algae Picochlorum sp.. PHOTOSYNTHESIS RESEARCH 2024;162:439-457. [PMID: 38329705 DOI: 10.1007/s11120-024-01075-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The green algal genus Picochlorum is of biotechnological interest because of its robust response to multiple environmental stresses. We compared the metabolic performance of P. SE3 and P. oklahomense to diverse microbial phototrophs and observed exceptional performance of photosystem II (PSII) in light energy conversion in both Picochlorum species. The quantum yield (QY) for O2 evolution is the highest of any phototroph yet observed, 32% (20%) by P. SE3 (P. okl) when normalized to total PSII subunit PsbA (D1) protein, and 80% (75%) normalized per active PSII, respectively. Three factors contribute: (1) an efficient water oxidizing complex (WOC) with the fewest photochemical misses of any organism; (2) faster reoxidation of reduced (PQH2)B in P. SE3 than in P. okl. (period-2 Fourier amplitude); and (3) rapid reoxidation of the plastoquinol pool by downstream electron carriers (Cyt b6f/PETC) that regenerates PQ faster in P. SE3. This performance gain is achieved without significant residue changes around the QB site and thus points to a pull mechanism involving faster PQH2 reoxidation by Cyt b6f/PETC that offsets charge recombination. This high flux in P. SE3 may be explained by genomically encoded plastoquinol terminal oxidases 1 and 2, whereas P. oklahomense has neither. Our results suggest two distinct types of PSII centers exist, one specializing in linear electron flow and the other in PSII-cyclic electron flow. Several amino acids within D1 differ from those in the low-light-descended D1 sequences conserved in Viridiplantae, and more closely match those in cyanobacterial high-light D1 isoforms, including changes near tyrosine Yz and a water/proton channel near the WOC. These residue changes may contribute to the exceptional performance of Picochlorum at high-light intensities by increasing the water oxidation efficiency and the electron/proton flux through the PSII acceptors (QAQB).

Insights from Ca2+→Sr2+ substitution on the mechanism of O-O bond formation in photosystem II

In recent years, there has been a steady interest in unraveling the intricate mechanistic details of water oxidation mechanism in photosynthesis. Despite the substantial progress made over several decades, a comprehensive understanding of the precise kinetics underlying O-O bond formation and subsequent evolution remains elusive. However, it is well-established that the oxygen evolving complex (OEC), specifically the CaMn4O5 cluster, plays a crucial role in O-O bond formation, undergoing a series of four oxidative events as it progresses through the S-states of the Kok cycle. To gain further insights into the OEC, researchers have explored the substitution of the Ca2+ cofactor with strontium (Sr), the sole atomic replacement capable of retaining oxygen-evolving activity. Empirical investigations utilizing spectroscopic techniques such as XAS, XRD, EPR, FTIR, and XANES have been conducted to probe the structural consequences of Ca2+→Sr2+ substitution. In parallel, the development of DFT and QM/MM computational models has explored different oxidation and protonation states, as well as variations in ligand coordination at the catalytic center involving amino acid residues. In this review, we critically evaluate and integrate these computational and spectroscopic approaches, focusing on the structural and mechanistic implications of Ca2+→Sr2+ substitution in PS II. We contribute DFT modelling and simulate EXAFS Fourier transforms of Sr-substituted OEC, analyzing promising structures of the S3 state. Through the combination of computational modeling and spectroscopic investigations, valuable insights have been gained, developing a deeper understanding of the photosynthetic process.

How chloride functions to enable proton conduction in photosynthetic water oxidation: Time-resolved kinetics of intermediates (S-states) in vivo and bromide substitution

Chloride (Cl-) is essential for O2 evolution during photosynthetic water oxidation. Two chlorides near the water-oxidizing complex (WOC) in Photosystem II (PSII) structures from Thermosynechococcus elongatus (and T. vulcanus) have been postulated to transfer protons generated from water oxidation. We monitored four criteria: primary charge separation flash yield (P* → P+QA-), rates of water oxidation steps (S-states), rate of proton evolution, and flash O2 yield oscillations by measuring chlorophyll variable fluorescence (P* quenching), pH-sensitive dye changes, and oximetry. Br-substitution slows and destabilizes cellular growth, resulting from lower light-saturated O2 evolution rate (-20 %) and proton release (-36 % ΔpH gradient). The latter implies less ATP production. In Br- cultures, protonogenic S-state transitions (S2 → S3 → S0') slow with increasing light intensity and during O2/water exchange (S0' → S0 → S1), while the non-protonogenic S1 → S2 transition is kinetically unaffected. As flash rate increases in Cl- cultures, both rate and extent of acidification of the lumen increase, while charge recombination is suppressed relative to Br-. The Cl- advantage in rapid proton escape from the WOC to lumen is attributed to correlated ion-pair movement of H3O+Cl- in dry water channels vs. separated Br- and H+ ion movement through different regions (>200-fold difference in Bronsted acidities). By contrast, at low flash rates a previously unreported reversal occurs that favors Br- cultures for both proton evolution and less PSII charge recombination. In Br- cultures, slower proton transfer rate is attributed to stronger ion-pairing of Br- with AA residues lining the water channels. Both anions charge-neutralize protons and shepherd them to the lumen using dry aqueous channels.

Regulation of light energy conversion between linear and cyclic electron flow within photosystem II controlled by the plastoquinone/quinol redox poise

Ultrapurified Photosystem II complexes crystalize as uniform microcrystals (PSIIX) of unprecedented homogeneity that allow observation of details previously unachievable, including the longest sustained oscillations of flash-induced O₂ yield over > 200 flashes and a novel period-4.7 water oxidation cycle. We provide new evidence for a molecular-based mechanism for PSII-cyclic electron flow that accounts for switching from linear to cyclic electron flow within PSII as the downstream PQ/PQH₂ pool reduces in response to metabolic needs and environmental input. The model is supported by flash oximetry of PSIIX as the LEF/CEF switch occurs, Fourier analysis of O₂ flash yields, and Joliot-Kok modeling. The LEF/CEF switch rebalances the ratio of reductant energy (PQH₂) to proton gradient energy (H⁺_o/H⁺_i) created by PSII photochemistry. Central to this model is the requirement for a regulatory site (Q_C) with two redox states in equilibrium with the dissociable secondary electron carrier site Q_B. Both sites are controlled by electrons and protons. Our evidence fits historical LEF models wherein light-driven water oxidation delivers electrons (from Q_A^-) and stromal protons through Q_B to generate plastoquinol, the terminal product of PSII-LEF in vivo. The new insight is the essential regulatory role of Q_C. This site senses both the proton gradient (H⁺_o/H⁺_i) and the PQ pool redox poise via e^-/H⁺ equilibration with Q_B. This information directs switching to CEF upon population of the protonated semiquinone in the Qc site (Q^-H⁺)_C, while the WOC is in the reducible S2 or S3 states. Subsequent photochemical primary charge separation (P⁺Q_A^-) forms no (QH₂)_B, but instead undergoes two-electron backward transition in which the Q_C protons are pumped into the lumen, while the electrons return to the WOC forming (S1/S2). PSII-CEF enables production of additional ATP needed to power cellular processes including the terminal carboxylation reaction and in some cases PSI-dependent CEF.

Cyclic electron flow around photosystem II in silico: How it works and functions in vivo

To date, cyclic electron flow around PSI (PSI-CEF) has been considered the primary (if not the only) mechanism accepted to adjust the ratio of linear vs cyclic electron flow that is essential to adjust the ratio of ATP/NADPH production needed for CO₂ carboxylation. Here we provide a kinetic model showing that cyclic electron flow within PSII (PSII-CEF) is essential to account for the accelerating rate of decay in flash-induced oscillations of O₂ yield as the PQ pool progressively reduces to PQH₂. Previously, PSII-CEF was modeled by backward transitions using empirical Markov models like Joliot-Kok (J-K) type. Here, we adapted an ordinary differential equation methodology denoted RODE1 to identify which microstates within PSII are responsible for branching between PSII-CEF and Linear Electron Flow (LEF). We applied it to simulate the oscillations of O₂ yield from both Chlorella ohadii, an alga that shows strong PSII-CEF attributed to high backward transitions, and Synechococcus elongatus sp. 7002, a widely studied model cyanobacterium. RODE2 simulations reveal that backward transitions occur in microstates that possess a Q_B^- semiquinone prior to the flash. Following a flash that forms microstates populating (Q_AQ_B)^2-, PSII-CEF redirects these two electrons to the donor side of PSII only when in the oxidized S₂ and S₃ states. We show that this backward transition pathway is the origin of the observed period-2 oscillations of flash O₂ yield and contributes to the accelerated decay of period-4 oscillations. This newly added pathway improved RODE1 fits for cells of both S. elongatus and C. ohadii. RODE2 simulations show that cellular adaptation to high light intensity growth is due to a decrease in Q_B availability (empty or blocked by Q^2-_B), or equivalently due to a decrease in the difference in reduction potential relative to Q_A/Q_A^-. PSII-CEF provides an alternative mechanism for rebalancing the NADPH:ATP ratio that occurs rapidly by adjusting the redox level of the PQ:PQH₂ pool and is a necessary process for energy metabolism in aquatic phototrophs.

Why Did Nature Choose Manganese over Cobalt to Make Oxygen Photosynthetically on the Earth?

All contemporary oxygenic phototrophs─from primitive cyanobacteria to complex multicellular plants─split water using a single invariant cluster comprising Mn₄CaO₅ (the water oxidation catalyst) as the catalyst within photosystem II, the universal oxygenic reaction center of natural photosynthesis. This cluster is unstable outside of PSII and can be reconstituted, both in vivo and in vitro, using elemental aqueous ions and light, via photoassembly. Here, we demonstrate the first functional substitution of manganese in any oxygenic reaction center by in vitro photoassembly. Following complete removal of inorganic cofactors from cyanobacterial photosystem II microcrystal (PSIIX), photoassembly with free cobalt (Co²⁺), calcium (Ca²⁺), and water (OH^-) restores O₂ evolution activity. Photoassembly occurs at least threefold faster using Co²⁺ versus Mn²⁺ due to a higher quantum yield for PSIIX-mediated charge separation (P*): Co²⁺ → P* → Co³⁺Q_A^-. However, this kinetic preference for Co²⁺ over native Mn²⁺ during photoassembly is offset by significantly poorer catalytic activity (∼25% of the activity with Mn²⁺) and ∼3- to 30-fold faster photoinactivation rate. The resulting reconstituted Co-PSIIX oxidizes water by the standard four-flash photocycle, although they produce 4-fold less O₂ per PSII, suggested to arise from faster charge recombination (Co³⁺Q_A ← Co⁴⁺Q_A^-) in the catalytic cycle. The faster photoinactivation of reconstituted Co-PSIIX occurs under anaerobic conditions during the catalytic cycle, suggesting direct photodamage without the involvement of O₂. Manganese offers two advantages for oxygenic phototrophs, which may explain its exclusive retention throughout Darwinian evolution: significantly slower charge recombination (Mn³⁺Q_A ← Mn⁴⁺Q_A^-) permits more water oxidation at low and fluctuating solar irradiation (greater net energy conversion) and much greater tolerance to photodamage at high light intensities (Mn⁴⁺ is less oxidizing than Co⁴⁺). Future work to identify the chemical nature of the intermediates will be needed for further interpretation.

Realtime kinetics of the light driven steps of photosynthetic water oxidation in living organisms by "stroboscopic" fluorometry

We develop a rapid "stroboscopic" fluorescence induction method, using the fast repetition rate fluorometry (FRRF) technique, to measure changes in the quantum yield of light emission from chlorophyll in oxygenic photosynthesis arising from competition with primary photochemical charge separation (P₆₈₀* ➔ P₆₈₀⁺Q_A^-). This method determines the transit times of electrons that pass through PSII during the successive steps in the catalytic cycle of water oxidation/O₂ formation (S states) and plastoquinone reduction in any oxygenic phototroph (in vivo or in vitro). We report the first measurements from intact living cells, illustrated by a eukaryotic alga (Nannochloropsis oceanica). We demonstrate that S state transition times depend strongly on the redox state of the PSII acceptor side, at both Q_B and the plastoquinone pool which serve as the major locus of regulation of PSII electron flux. We provide evidence for a kinetic intermediate S3' state (lifetime 220 μs) following formation of S3 and prior to the release of O₂. We compare the FRRF-detected kinetics to other previous spectroscopic methods (optical absorbance, EPR, and XES) that are applicable only to in vitro samples.

'Birth defects' of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water-oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane-inlet mass spectrometry and O₂ -polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these 'PSII birth defects' in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de-etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O₂ -polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, Q_B -inhibitor binding, and thermoluminescence studies indicate that the decline of the high-light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability Q_A ^-  → Q_B during de-etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer-range energy transfer.

Energetics and Kinetics of S-State Transitions Monitored by Delayed Chlorophyll Fluorescence

Understanding energetic and kinetic parameters of intermediates formed in the course of the reaction cycle (S-state cycle) of photosynthetic water oxidation is of high interest and could support the rationale designs of artificial systems for solar fuels. We use time-resolved measurements of the delayed chlorophyll fluorescence to estimate rate constants, activation energies, free energy differences, and to discriminate between the enthalpic and the entropic contributions to the decrease of the Gibbs free energy of the individual transitions. Using a joint-fit simulation approach, kinetic parameters are determined for the reaction intermediates in the S-state transitions in buffers with different pH in H₂O and in D₂O.

Photosystem II-cyclic electron flow powers exceptional photoprotection and record growth in the microalga Chlorella ohadii

The desert microalga Chlorella ohadii was reported to grow at extreme light intensities with minimal photoinhibition, tolerate frequent de/re-hydrations, yet minimally employs antenna-based non-photochemical quenching for photoprotection. Here we investigate the molecular mechanisms by measuring Photosystem II charge separation yield (chlorophyll variable fluorescence, Fv/Fm) and flash-induced O₂ yield to measure the contributions from both linear (PSII-LEF) and cyclic (PSII-CEF) electron flow within PSII. Cells grow increasingly faster at higher light intensities (μE/m²/s) from low (20) to high (200) to extreme (2000) by escalating photoprotection via shifting from PSII-LEF to PSII-CEF. This shifts PSII charge separation from plastoquinone reduction (PSII-LEF) to plastoquinol oxidation (PSII-CEF), here postulated to enable proton gradient and ATP generation that powers photoprotection. Low light-grown cells have unusually small antennae (332 Chl/PSII), use mainly PSII-LEF (95%) and convert 40% of PSII charge separations into O₂ (a high O₂ quantum yield of 0.06mol/mol PSII/flash). High light-grown cells have smaller antenna and lower PSII-LEF (63%). Extreme light-grown cells have only 42 Chl/PSII (no LHCII antenna), minimal PSII-LEF (10%), and grow faster than any known phototroph (doubling time 1.3h). Adding a synthetic quinone in excess to supplement the PQ pool fully uncouples PSII-CEF from its natural regulation and produces maximum PSII-LEF. Upon dark adaptation PSII-LEF rapidly reverts to PSII-CEF, a transient protection mechanism to conserve water and minimize the cost of antenna biosynthesis. The capacity of the electron acceptor pool (plastoquinone pool), and the characteristic times for exchange of (PQH₂)_B with PQ_pool and reoxidation of (PQH₂)_pool were determined.