Electron transfer from Cyt b(559) and tyrosine-D to the S2 and S3 states of the water oxidizing complex in photosystem II at cryogenic temperatures

The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.

Light-adapted charge-separated state of photosystem II: structural and functional dynamics of the closed reaction center

Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons for life on Earth. The photochemical reaction center of PSII is known to possess two stationary states. In the open state (PSIIO), the absorption of a single photon triggers electron-transfer steps, which convert PSII into the charge-separated closed state (PSIIC). Here, by using steady-state and time-resolved spectroscopic techniques on Spinacia oleracea and Thermosynechococcus vulcanus preparations, we show that additional illumination gradually transforms PSIIC into a light-adapted charge-separated state (PSIIL). The PSIIC-to-PSIIL transition, observed at all temperatures between 80 and 308 K, is responsible for a large part of the variable chlorophyll-a fluorescence (Fv) and is associated with subtle, dark-reversible reorganizations in the core complexes, protein conformational changes at noncryogenic temperatures, and marked variations in the rates of photochemical and photophysical reactions. The build-up of PSIIL requires a series of light-induced events generating rapidly recombining primary radical pairs, spaced by sufficient waiting times between these events-pointing to the roles of local electric-field transients and dielectric relaxation processes. We show that the maximum fluorescence level, Fm, is associated with PSIIL rather than with PSIIC, and thus the Fv/Fm parameter cannot be equated with the quantum efficiency of PSII photochemistry. Our findings resolve the controversies and explain the peculiar features of chlorophyll-a fluorescence kinetics, a tool to monitor the functional activity and the structural-functional plasticity of PSII in different wild-types and mutant organisms and under stress conditions.

Microsolvation of the Redox-Active Tyrosine-D in Photosystem II: Correlation of Energetics with EPR Spectroscopy and Oxidation-Induced Proton Transfer

Photosystem II (PSII) of oxygenic photosynthesis captures sunlight to drive the catalytic oxidation of water and the reduction of plastoquinone. Among the several redox-active cofactors that participate in intricate electron transfer pathways there are two tyrosine residues, YZ and YD. They are situated in symmetry-related electron transfer branches but have different environments and play distinct roles. YZ is the immediate oxidant of the oxygen-evolving Mn4CaO5 cluster, whereas YD serves regulatory and protective functions. The protonation states and hydrogen-bond network in the environment of YD remain debated, while the role of microsolvation in stabilizing different redox states of YD and facilitating oxidation or mediating deprotonation, as well the fate of the phenolic proton, is unclear. Here we present detailed structural models of YD and its environment using large-scale quantum mechanical models and all-atom molecular dynamics of a complete PSII monomer. The energetics of water distribution within a hydrophobic cavity adjacent to YD are shown to correlate directly with electron paramagnetic resonance (EPR) parameters such as the tyrosyl g-tensor, allowing us to map the correspondence between specific structural models and available experimental observations. EPR spectra obtained under different conditions are explained with respect to the mode of interaction of the proximal water with the tyrosyl radical and the position of the phenolic proton within the cavity. Our results revise previous models of the energetics and build a detailed view of the role of confined water in the oxidation and deprotonation of YD. Finally, the model of microsolvation developed in the present work rationalizes in a straightforward way the biphasic oxidation kinetics of YD, offering new structural insights regarding the function of the radical in biological photosynthesis.

Unpaired electrons as probes of catalytic systems

An overview is provided of the importance of molecular species containing unpaired electrons in catalytic systems, as revealed using ESR spectroscopy. The review aims to demonstrate the considerable extent of scientific progress that has been made in this broad topic during the past few decades. Studies of catalytically active surfaces, including zeolites, are surveyed, and the detection of radical species, formed as intermediates in their reactions, using matrix isolation and spin-trapping techniques. Radical cation formation in zeolites is discussed, and the employment of muon spin rotation and relaxation techniques to study the mobility of labelled radicals in various porous and catalytic media. Among the specific types of catalytic media considered are those for photocatalysis, water splitting, degradation of environmental pollutants, hydrocarbon conversions, fuel cells and sensor devices employing graphene. The review concludes with recent developments in the study of enzymes and their reactions, using ESR-based methods.

Electron transfer from Cyt b(559) and tyrosine-D to the S2 and S3 states of the water oxidizing complex in photosystem II at cryogenic temperatures

The Mn(4)CaO(5) cluster of photosystem II (PSII) catalyzes the oxidation of water to molecular oxygen through the light-driven redox S-cycle. The water oxidizing complex (WOC) forms a triad with Tyrosine(Z) and P(680), which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, Tyrosine(D) (Y(D)) and Cytochrome b(559) (Cyt b(559)) can also interact with the S-states. In the present work we investigate the electron transfer from Cyt b(559) and Y(D) to the S(2) and S(3) states at 195 K. First, Y(D)(•) and Cyt b(559) were chemically reduced. The S(2) and S(3) states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S(1) state. EPR signals of the WOC (the S(2)-state multiline signal, ML-S(2)), Y(D)(•) and oxidized Cyt b(559) were simultaneously detected during a prolonged dark incubation at 195 K. During 163 days of incubation a large fraction of the S(2) population decayed to S(1) in the S(2) samples by following a single exponential decay. Differently, S(3) samples showed an initial increase in the ML-S(2) intensity (due to S(3) to S(2) conversion) and a subsequent slow decay due to S(2) to S(1) conversion. In both cases, only a minor oxidation of Y(D) was observed. In contrast, the signal intensity of the oxidized Cyt b(559) showed a two-fold increase in both the S(2) and S(3) samples. The electron donation from Cyt b(559) was much more efficient to the S(2) state than to the S(3) state.

Two-electron reactions S2QB -->S0QB and S3QB -->S1QB are involved in deactivation of higher S states of the oxygen-evolving complex of Photosystem II

The oxygen-evolving complex of Photosystem II cycles through five oxidation states (S(0)-S(4)), and dark incubation leads to 25% S(0) and 75% S(1). This distribution cannot be reached with charge recombination reactions between the higher S states and the electron acceptor Q(B)(-). We measured flash-induced oxygen evolution to understand how S(3) and S(2) are converted to lower S states when the electron required to reduce the manganese cluster does not come from Q(B)(-). Thylakoid samples preconditioned to make the concentration of the S(1) state 100% and to oxidize tyrosine Y(D) were illuminated by one or two laser preflashes, and flash-induced oxygen evolution sequences were recorded at various time intervals after the preflashes. The distribution of the S states was calculated from the flash-induced oxygen evolution pattern using an extended Kok model. The results suggest that S(2) and S(3) are converted to lower S states via recombination from S(2)Q(B)(-) and S(3)Q(B)(-) and by a slow change of the state of oxygen-evolving complex from S(3) and S(2) to S(1) and S(0) in reactions with unspecified electron donors. The slow pathway appears to contain two-electron routes, S(2)Q(B) -->S(0)Q(B), and S(3)Q(B) -->S(1)Q(B). The two-electron reactions dominate in intact thylakoid preparations in the absence of chemical additives. The two-electron reaction was replaced by a one-electron-per-step pathway, S(3)Q(B) -->S(2)Q(B) -->S(1)Q(B) in PS II-enriched membrane fragments and in thylakoids measured in the presence of artificial electron acceptors. A catalase effect suggested that H(2)O(2) acts as an electron donor for the reaction S(2)Q(B) -->S(0)Q(B) but added H(2)O(2) did not enhance this reaction.

Multiple redox-active chlorophylls in the secondary electron-transfer pathways of oxygen-evolving photosystem II

Photosystem II (PS II) is unique among photosynthetic reaction centers in having secondary electron donors that compete with the primary electron donors for reduction of P680(+). We have characterized the photooxidation and dark decay of the redox-active accessory chlorophylls (Chl) and beta-carotenes (Car) in oxygen-evolving PS II core complexes by near-IR absorbance and EPR spectroscopies at cryogenic temperatures. In contrast to previous results for Mn-depleted PS II, multiple near-IR absorption bands are resolved in the light-minus-dark difference spectra of oxygen-evolving PS II core complexes including two fast-decaying bands at 793 and 814 nm and three slow-decaying bands at 810, 825, and 840 nm. We assign these bands to chlorophyll cation radicals (Chl(+)). The fast-decaying bands observed after illumination at 20 K could be generated again by reilluminating the sample. Quantization by EPR gives a yield of 0.85 radicals per PS II, and the yield of oxidized cytochrome b 559 by optical difference spectroscopy is 0.15 per PS II. Potential locations of Chl(+) and Car(+) species, and the pathways of secondary electron transfer based on the rates of their formation and decay, are discussed. This is the first evidence that Chls in the light-harvesting proteins CP43 and CP47 are oxidized by P680(+) and may have a role in Chl fluorescence quenching. We also suggest that a possible role for negatively charged lipids (phosphatidyldiacylglycerol and sulfoquinovosyldiacylglycerol identified in the PS II structure) could be to decrease the redox potential of specific Chl and Car cofactors. These results provide new insight into the alternate electron-donation pathways to P680(+).