On the role of cytochrome b559 in oxygen evolution in photosynthesis

Analysis of the transformation effect in cytochrome b559 of photosystem II in terms of the model of the heme-quinone redox interaction

Transformation of three-component redox pattern of cytochrome (Cyt) b559 in PS II membrane fragments upon various treatments is manifested in decrease of the relative content (R) of the high potential (HP) redox form of Cyt b559 and concomitant increase in the fractions of the two lower potential forms. Redox titration of Cyt b559 in different types of PS II membrane preparations was performed and revealed that (1) alteration of redox titration curve of Cyt b559 upon treatment of a sample is not specific to the type of treatment; (2) each value of R_HP defines the individual shape of the redox titration curve; (3) population of Cyt b559 may exist in several stable forms with multicomponent redox pattern: three types of three-component redox pattern and one type of two-component redox pattern as well as in the form with a single E_m; (4) transformation of Cyt b559 proceeds as successive conversion between the stable forms with multicomponent redox pattern; (5) upon harsh treatments, Cyt b559 abruptly converts into the state with a single E_m which value is intermediate between the E_m values of the two lower potential forms. Analysis of the data using the model of Cyt b559-quinone redox interaction revealed that diminution of R_HP in a range from 80 to 10% reflects a shift in redox equilibrium between the heme group of Cyt b559 and the interacting quinone, due to a gradual decrease of 90 mV in E_m of the heme group at the virtually unchanged E_m of the quinone component.

Biphasic reduction of cytochrome b559 by plastoquinol in photosystem II membrane fragments: evidence for two types of cytochrome b559/plastoquinone redox equilibria

In photosystem II membrane fragments with oxidized cytochrome (Cyt) b559 reduction of Cyt b559 by plastoquinol formed in the membrane pool under illumination and by exogenous decylplastoquinol added in the dark was studied. Reduction of oxidized Cyt b559 by plastoquinols proceeds biphasically comprising a fast component with a rate constant higher than (10s)(-1), named phase I, followed by a slower dark reaction with a rate constant of (2.7min)(-1) at pH6.5, termed phase II. The extents of both components of Cyt b559 reduction increased with increasing concentrations of the quinols, with that, maximally a half of oxidized Cyt b559 can be photoreduced or chemically reduced in phase I at pH6.5. The photosystem II herbicide dinoseb but not 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) competed with the quinol reductant in phase I. The results reveal that the two components of the Cyt b559 redox reaction reflect two redox equilibria attaining in different time domains. One-electron redox equilibrium between oxidized Cyt b559 and the photosystem II-bound plastoquinol is established in phase I of Cyt b559 reduction. Phase II is attributed to equilibration of Cyt b559 redox forms with the quinone pool. The quinone site involved in phase I of Cyt b559 reduction is considered to be the site regulating the redox potential of Cyt b559 which can accommodate quinone, semiquinone and quinol forms. The properties of this site designated here as QD clearly suggest that it is distinct from the site QC found in the photosystem II crystal structure.

Light-induced quinone reduction in photosystem II

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 (Q(A)) and secondary (Q(B)) 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 Q(C), the relevance of lipids, the interactions of quinones with the still enigmatic cytochrome b559 and the role of Q(A) in photoinhibition and photoprotection mechanisms. This article is part of a Special Issue entitled: Photosystem II.

Two reaction pathways for transformation of high potential cytochrome b559 of PS II into the intermediate potential form

This study describes an analysis of different treatments that influence the relative content and the midpoint potential of HP Cyt b559 in PS II membrane fragments from higher plants. Two basically different types of irreversible modification effects are distinguished: the HP form of Cyt b559 is either predominantly affected when the heme group is oxidized ("O-type" effects) or when it is reduced ("R-type" effects). Transformation of HP Cyt b559 to lower potential redox forms (IP and LP forms) by the "O-type" mechanism is induced by high pH and detergent treatments. In this case the effects consist of a gradual decrease in the relative content of HP Cyt b559 while its midpoint potential remains unaffected. Transformation of HP Cyt b559 via an "R-type" mechanism is caused by a number of exogenous compounds denoted L: herbicides, ADRY reagents and tetraphenylboron. These compounds are postulated to bind to the PS II complex at a quinone binding site designated as Q(C) which interacts with Cyt b559 and is clearly not the Q(B) site. Binding of compounds L to the Q(C) site when HP Cyt b559 is oxidized gives rise to a gradual decrease in the E(m) of HP Cyt b559 with increasing concentration of L (up to 10 K(ox)(L) values) while the relative content of HP Cyt b559 is unaffected. Higher concentrations of compounds L required for their binding to Q(C) site when HP Cyt b559 is reduced (described by K(red)(L)) induce a conversion of HP Cyt b559 to lower potential redox forms ("R-type" transformation). Two reaction pathways for transitions of Cyt b559 between the different protein conformations that are responsible for the HP and IP/LP redox forms are proposed and new insights into the functional regulation of Cyt b559 via the Q(C) site are discussed.

Evidence for a Novel Quinone-Binding Site in the Photosystem II (PS II) Complex That Regulates the Redox Potential of Cytochrome b559

The present study provides a thorough analysis of effects on the redox properties of cytochrome (Cyt) b559 induced by two photosystem II (PS II) herbicides [3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,4-dinitro-6-sec-butylphenol (dinoseb)], an acceleration of the deactivation reactions of system Y (ADRY) agent carbonylcyanide-m-chlorophenylhydrazone (CCCP), and the lipophilic PS II electron-donor tetraphenylboron (TPB) in PS II membrane fragments from higher plants. The obtained results revealed that (1) all four compounds selectively affected the midpoint potential (E(m)) of the high potential (HP) form of Cyt b559 without any measurable changes of the E(m) values of the intermediate potential (IP) and low potential (LP) forms; (2) the control values from +390 to +400 mV for HP Cyt b559 gradually decreased with increasing concentrations of DCMU, dinoseb, CCCP, and TPB; (3) in the presence of high TPB concentrations, a saturation of the E(m) decrease was obtained at a level of about +240 mV, whereas no saturation was observed for the other compounds at the highest concentrations used in this study; (4) the effect of the phenolic herbicide dinoseb on the E(m) is independent of the occupancy of the Q(B)-binding site by DCMU; (5) at high concentrations of TPB or dinoseb, an additional slow and irreversible transformation of HP Cyt b559 into IP Cyt b559 or a mixture of the IP and LP Cyt b559 is observed; and (6) the compounds stimulate autoxidation of HP Cyt b559 under aerobic conditions. These findings lead to the conclusion that a binding site Q(C) exists for the studied substances that is close to Cyt b559 and different from the Q(B) site. On the basis of the results of the present study and former experiments on the effect of PQ extraction and reconstitution on HP Cyt b559 [Cox, R. P., and Bendall, D. S. (1974) The functions of plastoquinone and beta-carotene in photosystem II of chloroplasts, Biochim. Biophys. Acta 347, 49-59], it is postulated that the binding of a plastoquinone (PQ) molecule to Q(C) is crucial for establishing the HP form of Cyt b559. On the other hand, the binding of plastoquinol (PQH2) to Q(C) is assumed to cause a marked decrease of E(m), thus, giving rise to a PQH2 oxidase function of Cyt b559. The possible physiological role of the Q(C) site as a regulator of the reactivity of Cyt b559 is discussed.

Redox and spectral properties of cytochrome b559 in different preparations of photosystem II

A detailed analysis of the properties of cytochrome b(559) (Cyt b(559)) in photosystem II (PS II) preparations with different degrees of structural complexity is presented. It reveals that (i) D1-D2-Cyt b(559) complexes either in solubilized form or incorporated into liposomes contain only one type of Cyt b(559) with E(m) values of 60 +/- 5 and 100 +/- 10 mV, respectively, at pH 6.8; (ii) in oxygen-evolving solubilized PS II core complexes Cyt b(559) exists predominantly (>85%) as an LP form with an E(m,7) of 125 +/- 10 mV and a minor fraction with an E(m,7) of -150 +/- 15 mV; (iii) in oxygen-evolving PS II membrane fragments three different redox forms are discernible with E(m) values of 390 +/- 15 mV (HP form), 230 +/- 20 mV (IP form), and 105 +/- 25 mV (LP form) and relative amplitudes of 58, 24, and 18%, respectively, at pH 7.3; (iv) the E(m) values are almost pH-independent between pH 6 and 9.5 in all sample types except D1-D2-Cyt b(559) complexes incorporated into liposomes with a slope of -29 mV/pH unit, when the pH increases from 6 to 9.5 (IP and LP form in PS II membrane fragments possibly within a restricted range from pH 6.5 to 8); (v) at pH >8 the HP Cyt b(559) progressively converts to the IP form with increasing pH; (vi) the reduced-minus-oxidized optical difference spectra of Cyt b(559) are very similar in the lambda range of 360-700 nm for all types except for the HP form which exhibits pronounced differences in the Soret band; and (vii) PS II membrane fragments and core complexes are inferred to contain about two Cyt b(559) hemes per PS II. Possible implications of conformational changes near the heme group and spin state transitions of the iron are discussed.

Divergent pathways of photosynthetic electron transfer: The autonomous oxygenic and anoxygenic photosystems

The aim of this article is to assemble and integrate, from a personal perspective of a research participant, seldom examined evidence that is incompatible with some basic tenets of photosynthetic electron transport, the cornerstone of which is the Z scheme. The nonconforming evidence pertaining to the mode of ferredoxin reduction and the role of the copper redox protein, plastocyanin, indicates that contrary to the Z scheme ferredoxin is reduced in two experimentally distinguishable ways: oxygenically by PS II (renamed the oxygenic photosystem), without the participation of PS I, and anoxygenically by PS I (renamed the anoxygenic photosystem). It also indicates that plastocyanin is not only, as the Z scheme asserts, the electron donor to the reaction center chlorophyll of PS I (P700) but also to the reaction center chlorophyll of PS II (P680). Other unconventional findings include evidence that the fully functional oxygenic photosystem, when operating separately from the anoxygenic photosystem, reduces plastoquinone to plastoquinol and subsequently oxidizes plastoquinol by two pathways acting in concert: one being the universally recognized DBMIB-sensitive pathway via the Rieske iron-sulfur center of the cytochrome bf complex and the other, a hitherto unrecognized, DBMIB-insensitive electron transport pathway around P680 that centers on cytochrome b-559. These nonconforming findings form the basis of an alternate hypothesis of photosynthetic electron transport that modifies and complements the Z scheme.

pH sensitivity of the redox state of cytochrome b559 may regulate its function as a protectant against donor and acceptor side photoinhibition

A series of experiments have been conducted with isolated reaction centers of photosystem two (PS II) with the aim to elucidate the functional role of cytochrome (Cyt b 559). At pH 6.5 it was found that Cyt b 559 was reversibly photoreduced by red actinic light when Mn(2+) was present as an electron donor while at pH 8.5 a photo-oxidation was observed under the same lighting conditions, which was dark reversible in the presence of hydroquinone. These pH dependent light induced changes were measured under anaerobic conditions and correlated with changes in the relative levels of high (HP) and low (LP) potential forms of the cytochrome. At pH 6.5 the cytochrome was mainly in its LP form while at pH 8.5 a significant proportion was converted to the HP form as detected by dark titrations with hydroquinone. This pH dependent difference in the levels of HP and LP Cyt b 559 was also detected when bright white light was used to monitor the level of the LP form using a novel reaction involving direct electron donation from the flavin of glucose oxidase (present in the medium and used together with glucose and catalase as an oxygen trap). The results suggest that PS II directly oxidises and reduces the HP and LP forms, respectively and that the extent of these photo-reactions is dependent on the relative levels of the two forms, which are in turn governed by the pH. This conclusion is interpreted in terms of the model presented previously (Barber J and De Las Rivas J (1993) Proc Natl Acad Sci USA 90: 10942-10946) whereby the pH induced effect is considered as a possible mechanism by which interconversion of LP and HP forms of Cyt b 559 is achieved. In agreement with this was the finding that as the extent of photo-oxidisable HPCyt b 559 increases, with increasing pH, the rate of irreversible photo-oxidation of β-carotene decreases, a result expected if the HP form protects against donor side photoinhibition.

O2-dependent electron flow, membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence

Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (qP and qN) by the 'saturation pulse method'. This is part of an introductory talk at the Wageningen Meeting on 'The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology'. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of qN. The role of O2-dependent electron flow for ΔpH- and qN-formation is emphasized. Previous conclusions on the rate of 'pseudocyclic' transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the qN developed when CO2-assimilation is limited. Dithiothreitol is shown to inhibit part of qN-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 μs luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of ΔpH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.

Nucleotide sequence of the genes encoding cytochrome b-559 from the cyanelle genome of Cyanophora paradoxa

Cyanophora paradoxa is a flagellated protozoan which possesses unusual, chloroplast-like organelles referred to as cyanelles. The psbE and psbF genes, which encode the two apoprotein subunits of cytochrome b-559, have been cloned from the cyanelle genome of C. paradoxa. The complete nucleotide sequences of these genes and their flanking sequences were determined by the chain-termination, dideoxy method. The psbE gene is composed of 75 codons and predicts a polypeptide of 8462 Da that is seven to nine residues smaller than most other psbE gene products. The psbF gene consists of 43 codons and predicts a polypeptide of 4761 Da. Two open reading frames, whose sequences are highly conserved among cyanobacteria and numerous higher plants, were located in the nucleotide sequence downstream from the psbF gene. The first open reading frame, denoted psbI, is composed of 39 codons, while the second open reading frame, denoted psbJ, is composed of 41 codons. The predicted amino acid sequences of the psbI and psbJ gene products predict proteins of 5473 and 3973 Da respectively. These proteins are probably integral membrane proteins anchored in the membrane by a single, transmembrane alpha helix. The psbEFIJ genes are probably co-transcribed and constitute an operon as found for other organisms. Each of the four genes is preceded by a polypurine sequence which resembles the consensus ribsosome binding sequences for Escherichia coli.

Organization of the psbE, psbF, orf38, and orf42 gene loci on the Euglena gracilis chloroplast genome

The genes for cytochrome b559, designated psbE and psbF, and two highly conserved open reading frames of 38 and 42 codons have been located and characterized on the chloroplast genome of Euglena gracilis. The organization of the genes is psbE - 8 bp spacer - psbF - 110 bp spacer - orf38 - 87 bp spacer - orf42. All genes are of the same polarity. The psbE gene contains two introns of 350 and 326 bp. The psbF gene contains a single large intron of 1,042 bp. The orf38 and orf42 loci lack introns. The introns are extremely AT rich with a pronounced base composition bias of T greater than A greater than G greater than C in the mRNA-like strand and group II-like boundary sequences at their 3' and 5' ends having the consensus 5'-GTGTG .. INTRON .. TTAATTTNAT-3'. The psbE gene consists of 82 codons and encodes a polypeptide with a predicted molecular weight of 9,212. The psbF gene consists of 42 codons, which specify a polypeptide with a predicted molecular weight of 4,785. The highly conserved open reading frames of 38 and 42 codons code for polypeptides with predicted molecular weights of 4,405 and 4,426, respectively. The gene products of psbE, psbF, orf38 and orf42 are, respectively, 69.5%, 70% and 61.5% identical to those found in higher plants. The predicted secondary structure of the proteins from hydropathy plots is consistent with each containing a single membrane-spanning domain of at least 20 amino acids. Each of the genes is preceded by sequences which may serve as ribosome binding sites. All four genes are transcribed.

[Molecular mechanisms activated during the emission of oxygen in photosynthesis]

Photosynthetic oxygen evolution results from the dehydrogenation of water molecules by the chlorophyll-center cation of photosystem II. The primary charge separation occurs between the chlorophyll center and the primary acceptor. The positive charges are then transferred to the manganese cluster able to accumulate four positive equivalents. Protons are liberated during this process. The oxygen evolution requires the presence of Ca2+ and Cl-. The oxidation of two water molecules, and the transfer of the electrons to the plastoquinone pool, are done by a supra-molecular complex which utilizes photons as substrate, and can therefore transfer electrons against a large redox potential difference.

On the structure and function of cytochrome b-559

A sumary of biochemical, biophysical, and molecular biological data is presented which led to the identification of two different polypeptides (α and β, MW=9.16 and 4.27 kDa) in the cytochrome b-559 protein. The presence of a single His residue on each polypeptide, and the conclusion from spectroscopy that the heme coordination must be bis-histidine led to an obligatory requirement for coordination of a single heme through a heme cross-linked dimer. This structure does not have a precedent among soluble or membrane bound cytochromes. The possible participation of the cytochrome in the pathway of photoactivation is discussed.

Cytochrome b-559 genes from Oenothera hookeri and Nicotiana tabacum show a remarkably high degree of conservation as compared to spinach. The enigma of cytochrome b-559: highly conserved genes and proteins but no known function

Recent data suggest that cytochrome b-559, an intrinsic membrane protein of the oxygen-evolving photosystem II in chloroplasts, is a heme cross-linked heteromeric polypeptide unit (Herrmann et al. 1984, FEBS Lett 176:239-244). The genes for this cytochrome, designated psbE and psbF, have been located on the chloroplast chromosome of Oenothera hookeri and Nicotiana tabacum by hybridization with fragments of the corresponding spinach genes, and characterized. In both cases, the nucleotide sequence discloses 2 uninterrupted reading frames of 83 and 39 codons separated by a few nucleotides, as in spinach. The amber translation stop codon of psbE overlaps the putative ribosome-binding site for psbF in all cases. The predicted molecular weights of the proteins are 9.2 and 4.3 kd, respectively. In each of the three plant species, and the direction of transcription is opposite to that of the gene for cytochrome f which is located distal to the b-559 genes. Comparison of the deduced amino acid sequences with those from the corresponding spinach genes shows 97% homology. The ubiquitous presence and remarkably high degree of structural and functional conservation of this gene cluster supports the hypothesis of a heteromeric assembly and an important functional role for cytochrome b-559.

ESR spectroscopy demonstrates that cytochrome b559 remains low potential in Ca (2+)-reactivated, salt-washed PSII particles

Cytochrome b559 in various Photosystem II preparations was studled by using low temperature ESR spectroscopy. This technique was used because it is able to distinguish high from low potential forms of the cytochrome owing to the g-value differences between these species. Moreover, by using low temperature irradiation to oxidize cyt b559 we have avoided the use of redox mediators. Previous work (Ghanotakis DF., Topper J.N. and Yocum, C.F. (1984) Biochim. Biophys. Acta 767, 524-531) demonstrated that reduction and extraction of manganese of the oxygen evolving complex, which might be expected to alter the redox properties of cyt b559, occurs when certain PSII preparations are exposed to reductants. The ESR data presented here show that a mixture of high potential and lower potential cyt b559 species is observed in the oxygen evolving Photosystem II complex. Treatment of PSII membranes with 0.8 M Tris converts the high potential form(s) to those of lower potential. Exposure of the membranes to 2M NaCl shifts a significant amount of high potential cyt b559 to lower potential form(s); addition of CaCl2 reconstituted oxygen evolution activity but did not restore cyt b559 to its high potential form(s).

Photosynthetic water oxidation. A new chemical model

A sequential four-step chemical model for the water oxidation process in photosystem II is presented, based on the observation that a peroxide-linked biquinone complex can be chemically formed as a result of hydroxide ion addition to quinone. In our model, the hydroxide ion intermediate is generated in photosystem II as a result of proton abstraction from water. In the model, the first two flashes of light raise the oxidation state of the bimanganese center, while the third and fourth flashes of light sequentially generate the peroxide-linked biquinone which is then directly oxidized by the bimanganese center to produce oxygen and regenerate quinone.

Reconstitution of oxygen evolution in high salt washed photosystem II particles

Photosystem II thylakoid particles possessing high rates of oxygen evolution, were shown to have a very simple polypeptide composition. Upon washing of these particles with 250 mM NaCl the oxygen evolution was inhibited up to 80% concomitant with a release of two polypeptides of 23 and 16 kDa. Readdition of the pure 23 kDa protein to the depleted thylakoids under low ionic strength reconstituted more than half of the lost activity. No stimulation was obtained with the 16 kDa protein alone or in combination with glycerol. The results give further strong evidence that the 23 kDa protein is an essential component in the oxygen evolving complex. The possible involvement of other proteins in this complex is discussed in light of the demonstrated simple polypeptide pattern of the photosystem II particles.

The redox potentials of the b-type cytochromes of higher plant chloroplasts

1. In fresh chloroplasts,three b-type cytochromes exist. These are b-559HP (lambda max, 559 nm; Em at pH 7, +370 mV; pH-independent Em), b-559LP (lambda max, 559 nm; Em at pH 7, +20 mV; pH-independent Em) and b-563 (lambda max, 563 nm; Em at pH 7, -110 mV; pH-independent Em), b-559HP may be converted to a lower potential form (lambda max, 559 nm; Em at pH 7, +110 mV; pH-independent Em). 2. In catalytically active b-f particle preparations, three cytochromes exist. These are cytochrome f (lambda max, 554 nm; Em at pH 7, +375 mV, pK on oxidised cytochrome at pH 9), b-563 (lambda max, 563 nm; Em at pH 7, -90 mV, small pH-dependence of Em) and a b-559 species (lambda max, 559 nm, Em at pH 7, +85 mV; pH-independent Em). 3. A positive method of demonstration and estimation of b-559LP in fresh chloroplasts is described which involves the use of menadiol as a selective reductant of b-559LP.

Photoreactions of cytochromes in algal chloroplasts

Light-induced cytochrome redox reactions were investigated with algal chloroplasts capable of high rates of electron transport coupled to phosphorylation. The electron-donor pool preceding photosystem I consists of membrane-bound cytochrome f-554.5 and a soluble cytochrome c-553; the latter replaces the function of plastocyanin of higher-plant chloroplasts. Both cytochromes are reduced by photosystem II and oxidized by photosystem I. A site of energy conservation precedes these c-type cytochromes. The data obtained with respect to the function of b-type cytochromes are comparable to those obtained with higher-plant chloroplasts. Cyclic electron transport is mediated by cytochrome b-563 in a photosystem-I-dependent reaction. In addition, cytochrome b-563 may be reduced by photosystem II, in accordance with recent findings with intact spinach chloroplasts. It therefore appears that cytochrome b-563 is a member of both cyclic and non-cyclic electron transport. In contrast to higher-plant chloroplasts, redox reactions of cytochrome b-559 are observable without any pretreatments. Cytochrome b-559, high-potential, is reduced by photosystem II through plastoquinone. In the presence of carbonyl cyanide m-chlorophenylhydrazone a cytochrome-b-559 oxidation by photosystem II is measured.