BBA - Bioenergetics (v.1817, #1)

Photosystem II by Eva-Mari Aro (1).

Supramolecular organization of photosystem II in green plants by Roman Kouřil; Jan P. Dekker; Egbert J. Boekema (2-12).
Green plant photosystem II (PSII) is involved in the light reactions of photosynthesis, which take place in the thylakoid membrane of the chloroplast. PSII is organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. These supercomplexes are dimeric and contain usually 2–4 copies of trimeric LHCII complexes and have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. This review focuses on the overall composition and structure of the PSII supercomplex of green plants and its organization and interactions within the photosynthetic membrane. Further, we present the current knowledge how the thylakoid membrane is three-dimensionally organized within the chloroplast. We also discuss how the supramolecular organization in the thylakoid membrane and the PSII flexibility may play roles in various short-term regulatory mechanisms of green plant photosynthesis. This article is part of a Special Issue entitled: Photosystem II.► We discuss the composition of supercomplexes of major and minor LHCII and PSII. ► We discuss techniques like freeze-fracture EM, AFM and cryo electron tomography. ► We discuss models describing the overall topology of the thylakoid membranes. ► We correlate protein–protein interactions and the shape of the membranes. ► We correlate supramolecular organization and short-term regulatory mechanisms.
Keywords: Photosystem II; Supercomplex; Thylakoid membrane; Electron microscopy; Tomography;

Photosystem II, a growing complex: Updates on newly discovered components and low molecular mass proteins by Lan-Xin Shi; Michael Hall; Christiane Funk; Wolfgang P. Schröder (13-25).
Photosystem II is a unique complex capable of absorbing light and splitting water. The complex has been thoroughly studied and to date there are more than 40 proteins identified, which bind to the complex either stably or transiently. Another special feature of this complex is the unusually high content of low molecular mass proteins that represent more than half of the proteins. In this review we summarize the recent findings on the low molecular mass proteins (<15 kDa) and present an overview of the newly identified components as well. We have also performed co-expression analysis of the genes encoding PSII proteins to see if the low molecular mass proteins form a specific sub-group within the Photosystem II complex. Interestingly we found that the chloroplast-localized genes encoding PSII proteins display a different response to environmental and stress conditions compared to the nuclear localized genes. This article is part of a Special Issue entitled: Photosystem II.► More than 40 proteins have been found associated with Photosystem II. ► Approximately 50% of the PSII proteins have low molecular mass (<15 kDa). ► The low molecular mass proteins do not form a separate cluster. ► Ten new proteins associated with PSII have recently been reported.
Keywords: Psb; Photosynthesis; Arabidopsis; Oxygen evolution;

Charge separation in Photosystem II: A comparative and evolutionary overview by Tanai Cardona; Arezki Sedoud; Nicholas Cox; A. William Rutherford (26-43).
Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.► Charge separation in Photosystem II is reviewed from a historical perspective. ► The electron acceptor reactions of Type II reaction centres is described and compared. ► Differences among Type II reaction centres are associated with photoprotection. ► We discuss some evolutionary aspects of Photosystem II photochemistry.
Keywords: Photosystem II; Charge separation; Quinone; Type II reaction center; Evolution; Photoprotection;

Light-induced quinone reduction in photosystem II by Frank Müh; Carina Glöckner; Julia Hellmich; Athina Zouni (44-65).
The photosystem II core complex is the water:plastoquinone oxidoreductase of oxygenic photosynthesis situated in the thylakoid membrane of cyanobacteria, algae and plants. It catalyzes the light-induced transfer of electrons from water to plastoquinone accompanied by the net transport of protons from the cytoplasm (stroma) to the lumen, the production of molecular oxygen and the release of plastoquinol into the membrane phase. In this review, we outline our present knowledge about the “acceptor side” of the photosystem II core complex covering the reaction center with focus on the primary (QA) and secondary (QB) quinones situated around the non-heme iron with bound (bi)carbonate and a comparison with the reaction center of purple bacteria. Related topics addressed are quinone diffusion channels for plastoquinone/plastoquinol exchange, the newly discovered third quinone QC, the relevance of lipids, the interactions of quinones with the still enigmatic cytochrome b559 and the role of QA in photoinhibition and photoprotection mechanisms. This article is part of a Special Issue entitled: Photosystem II.► Excitation energy transfer and charge separation at the acceptor side of PSII. ► Proton-coupled electron transfer between QA and QB. ► Role of lipids in the environment of the quinone/quinol transport channels. ► Possible function of cyt b559 in quinone reduction. ► Effects of QA redox potential changes on photoinhibition.
Keywords: Bicarbonate; Cytochrome b559; Lipids; Non-heme iron; Photoinhibition; Reaction center;

Cytochrome b 559 and cyclic electron transfer within photosystem II by Katherine E. Shinopoulos; Gary W. Brudvig (66-75).
Cytochrome b 559 (Cyt b 559), β-carotene (Car), and chlorophyll (Chl) cofactors participate in the secondary electron-transfer pathways in photosystem II (PSII), which are believed to protect PSII from photodamage under conditions in which the primary electron-donation pathway leading to water oxidation is inhibited. Among these cofactors, Cyt b 559 is preferentially photooxidized under conditions in which the primary electron-donation pathway is blocked. When Cyt b 559 is preoxidized, the photooxidation of several of the 11 Car and 35 Chl molecules present per PSII is observed. In this review, the discovery of the secondary electron donors, their structures and electron-transfer properties, and progress in the characterization of the secondary electron-transfer pathways are discussed. This article is part of a Special Issue entitled: Photosystem II.► Cytochrome b 559, b-carotenes, and chlorophylls are secondary donors in photosystem II. ► Cytochrome b 559 is also photoreduced in photosystem II, forming a cyclic electron-transfer pathway. ► Cyclic electron transfer is believed to protect photosystem II from photodamage. ► The discovery and progress in characterization of the secondary donors in photosystem II are discussed.
Keywords: β-Carotene; Chlorophyll; Cytochrome b 559; Electron transfer pathway; Photosystem II;

Photosystem II (PSII), the thylakoid membrane enzyme which uses sunlight to oxidize water to molecular oxygen, holds many organic and inorganic redox cofactors participating in the electron transfer reactions. Among them, two tyrosine residues, Tyr-Z and Tyr-D are found on the oxidizing side of PSII. Both tyrosines demonstrate similar spectroscopic features while their kinetic characteristics are quite different. Tyr-Z, which is bound to the D1 core protein, acts as an intermediate in electron transfer between the primary donor, P680 and the CaMn4 cluster. In contrast, Tyr-D, which is bound to the D2 core protein, does not participate in linear electron transfer in PSII and stays fully oxidized during PSII function. The phenolic oxygens on both tyrosines form well-defined hydrogen bonds to nearby histidine residues, HisZ and HisD respectively. These hydrogen bonds allow swift and almost activation less movement of the proton between respective tyrosine and histidine. This proton movement is critical and the phenolic proton from the tyrosine is thought to toggle between the tyrosine and the histidine in the hydrogen bond. It is found towards the tyrosine when this is reduced and towards the histidine when the tyrosine is oxidized. The proton movement occurs at both room temperature and ultra low temperature and is sensitive to the pH. Essentially it has been found that when the pH is below the pK a for respective histidine the function of the tyrosine is slowed down or, at ultra low temperature, halted. This has important consequences for the function also of the CaMn4 complex and the protonation reactions as the critical Tyr–His hydrogen bond also steer a multitude of reactions at the CaMn4 cluster. This review deals with the discovery and functional assignments of the two tyrosines. The pH dependent phenomena involved in oxidation and reduction of respective tyrosine is covered in detail. This article is part of a Special Issue entitled: Photosystem II.► Tyr-Z and Tyr-D form well-defined hydrogen bonds to nearby His residues. ► The hydrogen bonds allow swift and almost activation less movement of the proton. ► The hydrogen bond is affected by pH. ► When the pH is below the pK a for the His the function of the tyrosine is inhibited. ► The Tyr–His hydrogen bond steer a multitude of reactions at the CaMn4 cluster.
Keywords: Photosystem II; Water oxidation; Tyrosine Z; Tyrosine D;

Photosynthetic water oxidation and O2 formation are catalyzed by a Mn4Ca complex bound to the proteins of photosystem II (PSII). The catalytic site, including the inorganic Mn4CaOnHx core and its protein environment, is denoted as oxygen-evolving complex (OEC). Earlier and recent progress in the endeavor to elucidate the structure of the OEC is reviewed, with focus on recent results obtained by (i) X-ray spectroscopy (specifically by EXAFS analyses), and (ii) X-ray diffraction (XRD, protein crystallography). Very recently, an impressive resolution of 1.9 Å has been achieved by XRD. Most likely however, all XRD data on the Mn4CaOnHx core of the OEC are affected by X-ray induced modifications (radiation damage). Therefore and to address (important) details of the geometric and electronic structure of the OEC, a combined analysis of XRD and XAS data has been approached by several research groups. These efforts are reviewed and extended using an especially comprehensive approach. Taking into account XRD results on the protein environment of the inorganic core of the Mn complex, 12 alternative OEC models are considered and evaluated by quantitative comparison to (i) extended-range EXAFS data, (ii) polarized EXAFS of partially oriented PSII membrane particles, and (iii) polarized EXAFS of PSII crystals. We conclude that there is a class of OEC models that is in good agreement with both the recent crystallographic models and the XAS data. On these grounds, mechanistic implications for the O―O bond formation chemistry are discussed. This article is part of a Special Issue entitled: Photosystem II.► Twelve structural models of the oxygen-evolving Mn complex (OEC) are evaluated. ► We combine crystallographic results, molecular mechanics, and EXAFS simulations. ► The O―O bond formation chemistry is discussed for the most favored OEC models.
Keywords: Manganese complex; Oxygen evolution; Photosynthesis; Crystal structure; Water oxidation; X-ray absorption spectroscopy;

The advent of oxygenic photosynthesis through water oxidation by photosystem II (PSII) transformed the planet, ultimately allowing the evolution of aerobic respiration and an explosion of ecological diversity. The importance of this enzyme to life on Earth has ironically been paralleled by the elusiveness of a detailed understanding of its precise catalytic mechanism. Computational investigations have in recent years provided more and more insights into the structural and mechanistic details that underlie the workings of PSII. This review will present an overview of some of these studies, focusing on those that have aimed at elucidating the mechanism of water oxidation at the CaMn4 cluster in PSII, and those exploring the features of the structure and dynamics of this enzyme that enable it to catalyse this energetically demanding reaction. This article is part of a Special Issue entitled: Photosystem II.► The recent studies of PSII using computational methods are reviewed. ► Two main areas of of focus: investigations of the water oxidation mechanism and channels in PSII. ► Brief overviews of three other areas: structural effects on electron and energy transfer, diffusion of PSII in the thylakoid membrane, and structural studies of extrinsic subunits and the QB binding site.
Keywords: Photosystem II; Water oxidation; Channels; Simulation; Computation;

The extrinsic proteins of Photosystem II by Terry M. Bricker; Johnna L. Roose; Robert D. Fagerlund; Laurie K. Frankel; Julian J. Eaton-Rye (121-142).
In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn4CaO5 cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.► We have reviewed the structure and function of the extrinsic proteins of PS II. ► The differences between the higher plant and cyanobacterial systems are examined. ► Putative docking sites of the CyanoQ and CyanoP proteins onto PS II are proposed.
Keywords: PsbO; PsbP; PsbQ; PsbU; PsbV;

Evolution and functional properties of Photosystem II light harvesting complexes in eukaryotes by Matteo Ballottari; Julien Girardon; Luca Dall'Osto; Roberto Bassi (143-157).
Photoautotrophic organisms, the major agent of inorganic carbon fixation into biomass, convert light energy into chemical energy. The first step of photosynthesis consists of the absorption of solar energy by pigments binding protein complexes named photosystems. Within photosystems, a family of proteins called Light Harvesting Complexes (LHC), responsible for light harvesting and energy transfer to reaction centers, has evolved along with eukaryotic organisms. Besides light absorption, these proteins catalyze photoprotective reactions which allowed functioning of oxygenic photosynthetic machinery in the increasingly oxidant environment. In this work we review current knowledge of LHC proteins serving Photosystem II. Balance between light harvesting and photoprotection is critical in Photosystem II, due to the lower quantum efficiency as compared to Photosystem I. In particular, we focus on the role of each antenna complex in light harvesting, energy transfer, scavenging of reactive oxygen species, chlorophyll triplet quenching and thermal dissipation of excess energy. This article is part of a Special Issue entitled: Photosystem II.► Light harvesting complexes are unique proteins evolved in eukaryotic organisms. ► We reviewed the current knowledge on Lhcb proteins, the Photosystem II antennas. ► Lhcb proteins' conformation shift from a light harvesting state to a dissipative state. ► Different Lhcb proteins were reported to have peculiar functions in vitro and in vivo.
Keywords: Photosynthesis; Light harvesting complexes; Chlorophylls; Carotenoids; Photoprotection; Non-photochemical quenching;

Photoprotective mechanisms have evolved in photosynthetic organisms to cope with fluctuating light conditions. Under high irradiance, the production of dangerous oxygen species is stimulated and causes photo-oxidative stress. One of these photoprotective mechanisms, non photochemical quenching (qE), decreases the excess absorbed energy arriving at the reaction centers by increasing thermal dissipation at the level of the antenna. In this review we describe results leading to the discovery of this process in cyanobacteria (qEcya), which is mechanistically distinct from its counterpart in plants, and recent progress in the elucidation of this mechanism. The cyanobacterial photoactive soluble orange carotenoid protein is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein leading to the formation of a red active form. The activated red form interacts with the phycobilisome, the cyanobacterial light-harvesting antenna, and induces a decrease of the phycobilisome fluorescence emission and of the energy arriving to the reaction centers. The orange carotenoid protein is the first photoactive protein to be identified that contains a carotenoid as the chromophore. Moreover, its photocycle is completely different from those of other photoactive proteins. A second protein, called the Fluorescence Recovery Protein encoded by the slr1964 gene in Synechocystis PCC 6803, plays a key role in dislodging the red orange carotenoid protein from the phycobilisome and in the conversion of the free red orange carotenoid protein to the orange, inactive, form. This protein is essential to recover the full antenna capacity under low light conditions after exposure to high irradiance. This article is part of a Special Issue entitled: Photosystem II.► We describe a photoprotective energy dissipating mechanism in cyanobacteria. ► This mechanism decreases the energy arriving at the reaction centers. ► The orange carotenoid protein is the inducer of energy dissipation. ► Energy dissipation occurs at the level of phycobilisomes. ► The Fluorescence Recovery Protein is essential to decrease energy dissipation.
Keywords: Cyanobacteria; Non-photochemical quenching; Orange carotenoid protein; Photoprotection; Photosystem II; Synechocystis;

The photoprotective molecular switch in the photosystem II antenna by Alexander V. Ruban; Matthew P. Johnson; Christopher D.P. Duffy (167-181).
We have reviewed the current state of multidisciplinary knowledge of the photoprotective mechanism in the photosystem II antenna underlying non-photochemical chlorophyll fluorescence quenching (NPQ). The physiological need for photoprotection of photosystem II and the concept of feed-back control of excess light energy are described. The outline of the major component of nonphotochemical quenching, qE, is suggested to comprise four key elements: trigger (ΔpH), site (antenna), mechanics (antenna dynamics) and quencher(s). The current understanding of the identity and role of these qE components is presented. Existing opinions on the involvement of protons, different LHCII antenna complexes, the PsbS protein and different xanthophylls are reviewed. The evidence for LHCII aggregation and macrostructural reorganization of photosystem II and their role in qE are also discussed. The models describing the qE locus in LHCII complexes, the pigments involved and the evidence for structural dynamics within single monomeric antenna complexes are reviewed. We suggest how PsbS and xanthophylls may exert control over qE by controlling the affinity of LHCII complexes for protons with reference to the concepts of hydrophobicity, allostery and hysteresis. Finally, the physics of the proposed chlorophyll–chlorophyll and chlorophyll–xanthophyll mechanisms of energy quenching is explained and discussed. This article is part of a Special Issue entitled: Photosystem II.► The photoprotective mechanism in the photosystem II antenna, qE, is reviewed. ► qE comprises four elements: trigger (ΔpH), site (antenna), mechanics and quencher. ► The role of protons, LHCII complexes, the PsbS protein and xanthophylls is reviewed. ► PsbS and xanthophylls are proposed to control qE. ► Alteration of the affinity of LHCII for protons is key for this regulation.
Keywords: NPQ; Thylakoid membrane; Photosystem II; LHCII; Xanthophyll; PsbS;

Photoprotection of photosystem II (PSII) is essential to avoid the light-induced damage of the photosynthetic apparatus due to the formation of reactive oxygen species (= photo-oxidative stress) under excess light. Carotenoids are known to play a crucial role in these processes based on their property to deactivate triplet chlorophyll (3Chl) and singlet oxygen (1O2 ). Xanthophylls are further assumed to be involved either directly or indirectly in the non-photochemical quenching (NPQ) of excess light energy in the antenna of PSII. This review gives an overview on recent progress in the understanding of the photoprotective role of the xanthophylls zeaxanthin (which is formed in the light in the so-called xanthophyll cycle) and lutein with emphasis on the NPQ processes associated with PSII of higher plants. The current knowledge supports the view that the photoprotective role of Lut is predominantly restricted to its function in the deactivation of 3Chl, while zeaxanthin is the major player in the deactivation of excited singlet Chl (1Chl) and thus in NPQ (non-photochemical quenching). Additionally, zeaxanthin serves important functions as an antioxidant in the lipid phase of the membrane and is likely to act as a key component in the memory of the chloroplast with respect to preceding photo-oxidative stress. This article is part of a Special Issue entitled: Photosystem II.► We review the role of the xanthophylls in photoprotection of photosystem II. ► The photoprotective role of lutein is due to the deactivation of triplet chlorophyll. ► Zeaxanthin has multiple photoprotective functions. ► Zeaxanthin plays a central role in NPQ and as antioxidant in the lipid phase. ► Zeaxanthin acts as a key component in the plant`s memory for photo-oxidative stress.
Keywords: Energy dissipation; Lutein; Photoprotection; Photosynthesis; Xanthophyll cycle; Zeaxanthin;

The role of lipids in photosystem II by Naoki Mizusawa; Hajime Wada (194-208).
The thylakoid membranes of photosynthetic organisms, which are the sites of oxygenic photosynthesis, are composed of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (PG). The identification of many genes involved in the biosynthesis of each lipid class over the past decade has allowed the generation and isolation of mutants of various photosynthetic organisms incapable of synthesizing specific lipids. Numerous studies using such mutants have revealed that deficiency of these lipids primarily affects the structure and function of photosystem II (PSII) but not of photosystem I (PSI). Recent X-ray crystallographic analyses of PSII and PSI complexes from Thermosynechococcus elongatus revealed the presence of 25 and 4 lipid molecules per PSII and PSI monomer, respectively, indicating the enrichment of lipids in PSII. Therefore, lipid molecules bound to PSII may play special roles in the assembly and functional regulation of the PSII complex. This review summarizes our present understanding of the biochemical and physiological roles of lipids in photosynthesis, with a special focus on PSII. This article is part of a Special Issue entitled: Photosystem II.► Lipids play special roles in the assembly and function of PSII complex. ► PG and SQDG are required for structural integrity of the QB binding site. ► DGDG and PG are involved in the binding of extrinsic proteins to PSII core complex. ► Biochemical and physiological roles of lipids in PSII are summarized in this review.
Keywords: Digalactosyldiacylglycerol; Monogalactosyldiacylglycerol; Phosphatidylglycerol; Photosystem II; Sulfoquinovosyldiacylglycerol;

Light induced damage of the photosynthetic apparatus is an important and highly complex phenomenon, which affects primarily the Photosystem II complex. Here the author summarizes the current state of understanding of the molecular mechanisms, which are involved in the light induced inactivation of Photosystem II electron transport together with the relevant mechanisms of photoprotection. Short wavelength ultraviolet radiation impairs primarily the Mn4Ca catalytic site of the water oxidizing complex with additional effects on the quinone electron acceptors and tyrosine donors of PSII. The main mechanism of photodamage by visible light appears to be mediated by acceptor side modifications, which develop under conditions of excess excitation in which the capacity of light-independent photosynthetic processes limits the utilization of electrons produced in the initial photoreactions. This situation of excess excitation facilitates the reduction of intersystem electron carriers and Photosystem II acceptors, and thereby induces the formation of reactive oxygen species, especially singlet oxygen whose production is sensitized by triplet chlorophyll formation in the reaction center of Photosystem II. The highly reactive singlet oxygen and other reactive oxygen species, such as H2O2 and O2 , which can also be formed in Photosystem II initiate damage of electron transport components and protein structure. In parallel with the excess excitation dependent mechanism of photodamage inactivation of the Mn4Ca cluster by visible light may also occur, which impairs electron transfer through the Photosystem II complex and initiates further functional and structural damage of the reaction center via formation of highly oxidizing radicals, such as P680+ • and Tyr-Z+ •. However, the available data do not support the hypothesis that the Mn-dependent mechanism would be the exclusive or dominating pathway of photodamage in the visible spectral range. This article is part of a Special Issue entitled: Photosystem II.► Recent studies on mechanisms of Photosystem II photodamage are reviewed. ► The main areas of focus: photodamage by 1O2 and other ROS, and by inactivation of the Mn4Ca cluster. ► Ultraviolet light inactivates primarily the Mn4Ca cluster. ► Visible light induces multiple photodamage mechanisms.
Keywords: Photoinhibition; Photoprotection; Photosystem II; Charge recombination;

Photosystem II (PSII) is a multisubunit protein complex in cyanobacteria, algae and plants that use light energy for oxidation of water and reduction of plastoquinone. The conversion of excitation energy absorbed by chlorophylls into the energy of separated charges and subsequent water–plastoquinone oxidoreductase activity are inadvertently coupled with the formation of reactive oxygen species (ROS). Singlet oxygen is generated by the excitation energy transfer from triplet chlorophyll formed by the intersystem crossing from singlet chlorophyll and the charge recombination of separated charges in the PSII antenna complex and reaction center of PSII, respectively. Apart to the energy transfer, the electron transport associated with the reduction of plastoquinone and the oxidation of water is linked to the formation of superoxide anion radical, hydrogen peroxide and hydroxyl radical. To protect PSII pigments, proteins and lipids against the oxidative damage, PSII evolved a highly efficient antioxidant defense system comprising either a non-enzymatic (prenyllipids such as carotenoids and prenylquinols) or an enzymatic (superoxide dismutase and catalase) scavengers. It is pointed out here that both the formation and the scavenging of ROS are controlled by the energy level and the redox potential of the excitation energy transfer and the electron transport carries, respectively. The review is focused on the mechanistic aspects of ROS production and scavenging by PSII. This article is part of a Special Issue entitled: Photosystem II.► Triplet chlorophyll is formed either by the intersystem crossing from the singlet chlorophyll in the antenna complex or by the charge recombination of primary radical pair [P680+Pheo] in the reaction center of PSII. ► Production of superoxide anion radical on the electron acceptor side of PSII initiates a cascade of reactions leading to the formation of hydrogen peroxide and hydroxyl radical. ► Incomplete oxidation of water on the electron donor side of PSII brings about the formation of hydrogen peroxide which is either oxidized to superoxide anion radical or reduced to hydroxyl radical.
Keywords: Hydrogen peroxide; Hydroxyl radical; Photosystem II; Redox potential; Singlet oxygen; Superoxide anion radical;

In higher plants, the photosystem (PS) II core and its several light harvesting antenna (LHCII) proteins undergo reversible phosphorylation cycles according to the light intensity. High light intensity induces strong phosphorylation of the PSII core proteins and suppresses the phosphorylation level of the LHCII proteins. Decrease in light intensity, in turn, suppresses the phosphorylation of PSII core, but strongly induces the phosphorylation of LHCII. Reversible and differential phosphorylation of the PSII-LHCII proteins is dependent on the interplay between the STN7 and STN8 kinases, and the respective phosphatases. The STN7 kinase phosphorylates the LHCII proteins and to a lesser extent also the PSII core proteins D1, D2 and CP43. The STN8 kinase, on the contrary, is rather specific for the PSII core proteins. Mechanistically, the PSII-LHCII protein phosphorylation is required for optimal mobility of the PSII-LHCII protein complexes along the thylakoid membrane. Physiologically, the phosphorylation of LHCII is a prerequisite for sufficient excitation of PSI, enabling the excitation and redox balance between PSII and PSI under low irradiance, when excitation energy transfer from the LHCII antenna to the two photosystems is efficient and thermal dissipation of excitation energy (NPQ) is minimised. The importance of PSII core protein phosphorylation is manifested under highlight when the photodamage of PSII is rapid and phosphorylation is required to facilitate the migration of damaged PSII from grana stacks to stroma lamellae for repair. The importance of thylakoid protein phosphorylation is highlighted under fluctuating intensity of light where the STN7 kinase dependent balancing of electron transfer is a prerequisite for optimal growth and development of the plant. This article is part of a Special Issue entitled: Photosystem II.► LHCII protein phosphorylation allows the maintenance of redox balance upon regulation of NPQ. ► Balanced excitation of PSII and PSI is a prerequisite of plant growth under fluctuating intensity of light. ► PSII core protein phosphorylation allows efficient unpacking of PSII-LHCII complexes upon PSII turnover.
Keywords: Photosystem II; Photoinhibition; Protein phosphorylation; State transition; NPQ; stn mutant;

Photosystem II (PSII) catalyzes one of the key reactions of photosynthesis, the light-driven conversion of water into oxygen. Although the structure and function of PSII have been well documented, our understanding of the biogenesis and maintenance of PSII protein complexes is still limited. A considerable number of auxiliary and regulatory proteins have been identified to be involved in the regulation of this process. The carboxy-terminal processing protease CtpA, the serine-type protease DegP and the ATP-dependent thylakoid-bound metalloprotease FtsH are critical for the biogenesis and maintenance of PSII. Here, we summarize and discuss the structural and functional aspects of these chloroplast proteases in these processes. This article is part of a Special Issue entitled: SI: Photosystem II.► The biogenesis and maintenance of PSII protein complexes are largely unknown. ► A considerable number of proteases are involved in this process. ► We summarize the functional aspects of these chloroplast proteases.
Keywords: Photosystem II; CtpA protease; FtsH protease; Deg protease; Protease; Chloroplast;

The Photosystem (PS) II of cyanobacteria, green algae and higher plants is prone to light-induced inactivation, the D1 protein being the primary target of such damage. As a consequence, the D1 protein, encoded by the psbA gene, is degraded and re-synthesized in a multistep process called PSII repair cycle. In cyanobacteria, a small gene family codes for the various, functionally distinct D1 isoforms. In these organisms, the regulation of the psbA gene expression occurs mainly at the level of transcription, but the expression is fine-tuned by regulation of translation elongation. In plants and green algae, the D1 protein is encoded by a single psbA gene located in the chloroplast genome. In chloroplasts of Chlamydomonas reinhardtii the psbA gene expression is strongly regulated by mRNA processing, and particularly at the level of translation initiation. In chloroplasts of higher plants, translation elongation is the prevalent mechanism for regulation of the psbA gene expression. The pre-existing pool of psbA transcripts forms translation initiation complexes in plant chloroplasts even in darkness, while the D1 synthesis can be completed only in the light. Replacement of damaged D1 protein requires also the assistance by a number of auxiliary proteins, which are encoded by the nuclear genome in green algae and higher plants. Nevertheless, many of these chaperones are conserved between prokaryotes and eukaryotes. Here, we describe the specific features and fundamental differences of the psbA gene expression and the regeneration of the PSII reaction center protein D1 in cyanobacteria, green algae and higher plants. This article is part of a Special Issue entitled Photosystem II.Display Omitted► Light damages the D1 protein, which is replaced by a new copy in PSII repair cycle. ► D1 is encoded by a small multigene psbA gene family in cyanobacteria. ► A single chloroplast gene codes for the D1 protein in algae and higher plants. ► In cyanobacteria, psbA gene regulation occurs mainly at the level of transcription. ► In plant chloroplasts, psbA gene expression is mainly regulated at level of translation.
Keywords: Chloroplast; Cyanobacteria; D1 protein; psbA gene; Transcription; Translation;

Parameterization of photosystem II photoinactivation and repair by Douglas A. Campbell; Esa Tyystjärvi (258-265).
The photoinactivation (also termed photoinhibition or photodamage) of Photosystem II (PSII) and the counteracting repair reactions are fundamental elements of the metabolism and ecophysiology of oxygenic photoautotrophs. Differences in the quantification, parameterization and terminology of Photosystem II photoinactivation and repair can erect barriers to understanding, and particular parameterizations are sometimes incorrectly associated with particular mechanistic models. These issues lead to problems for ecophysiologists seeking robust methods to include photoinhibition in ecological models. We present a comparative analysis of terms and parameterizations applied to photoinactivation and repair of Photosystem II. In particular, we show that the target size and quantum yield approaches are interconvertible generalizations of the rate constant of photoinactivation across a range of incident light levels. Our particular emphasis is on phytoplankton, although we draw upon the literature from vascular plants. This article is part of a Special Issue entitled: Photosystem II.► The quantum yield and target size concepts of photoinhibition are interconvertible. ► Both quantum yield and target size can be derivedfrom the concept of rate constant. ► Target size is particularly well suited for phytoplankton.
Keywords: Modeling; Photoinhibition; Rate constant; Quantum yield; Target size; Phytoplankton;