Probing the structure of photosystem II with amines and phenylhydrazine

Light-induced oxidative stress, N-formylkynurenine, and oxygenic photosynthesis

Light stress in plants results in damage to the water oxidizing reaction center, photosystem II (PSII). Redox signaling, through oxidative modification of amino acid side chains, has been proposed to participate in this process, but the oxidative signals have not yet been identified. Previously, we described an oxidative modification, N-formylkynurenine (NFK), of W365 in the CP43 subunit. The yield of this modification increases under light stress conditions, in parallel with the decrease in oxygen evolving activity. In this work, we show that this modification, NFK365-CP43, is present in thylakoid membranes and may be formed by reactive oxygen species produced at the Mn(4)CaO(5) cluster in the oxygen-evolving complex. NFK accumulation correlates with the extent of photoinhibition in PSII and thylakoid membranes. A modest increase in ionic strength inhibits NFK365-CP43 formation, and leads to accumulation of a new, light-induced NFK modification (NFK317) in the D1 polypeptide. Western analysis shows that D1 degradation and oligomerization occur under both sets of conditions. The NFK modifications in CP43 and D1 are found 17 and 14 Angstrom from the Mn(4)CaO(5) cluster, respectively. Based on these results, we propose that NFK is an oxidative modification that signals for damage and repair in PSII. The data suggest a two pathway model for light stress responses. These pathways involve differential, specific, oxidative modification of the CP43 or D1 polypeptides.

N-formylkynurenine as a marker of high light stress in photosynthesis

Photosystem II (PSII) is the membrane protein complex that catalyzes the photo-induced oxidation of water at a manganese-calcium active site. Light-dependent damage and repair occur in PSII under conditions of high light stress. The core reaction center complex is composed of the D1, D2, CP43, and CP47 intrinsic polypeptides. In this study, a new chromophore formed from the oxidative post-translational modification of tryptophan is identified in the CP43 subunit. Tandem mass spectrometry peptide sequencing is consistent with the oxidation of the CP43 tryptophan side chain, Trp-365, to produce N-formylkynurenine (NFK). Characterization with ultraviolet visible absorption and ultraviolet resonance Raman spectroscopy supports this assignment. An optical assay suggests that the yield of NFK increases 2-fold (2.2 ± 0.5) under high light illumination. A concomitant 2.4 ± 0.5-fold decrease is observed in the steady-state rate of oxygen evolution under the high light conditions. NFK is the product formed from reaction of tryptophan with singlet oxygen, which can be produced under high light stress in PSII. Reactive oxygen species reactions lead to oxidative damage of the reaction center, D1 protein turnover, and inhibition of electron transfer. Our results are consistent with a role for the CP43 NFK modification in photoinhibition.

Proton-coupled electron transfer in photosystem II: proton inventory of a redox active tyrosine

Photosystem II (PSII) catalyzes the light driven oxidation of water and the reduction of plastoquinone. PSII is a multisubunit membrane protein; the D1 and D2 polypeptides form the heterodimeric core of the PSII complex. Water oxidation occurs at a manganese-containing oxygen evolving complex (OEC). PSII contains two redox active tyrosines, Y(Z) and Y(D), which form the neutral tyrosyl radicals, Y(z)(*) and Y(D)(*). Y(D) has been assigned as tyrosine 160 in the D2 polypeptide through isotopic labeling and site-directed mutagenesis. Whereas Y(D) is not directly involved in the oxidation of water, it has been implicated in the formation and stabilization of the OEC. PSII structures have shown Y(D) to be within hydrogen-bonding distance of histidine 189 in the D2 polypeptide. Spectroscopic studies have suggested that a proton is transferred between Y(D) and histidine 189 when Y(D) is oxidized and reduced. In our previous work, we used (2)H(2)O solvent exchange to demonstrate that the mechanism of Y(D) proton-coupled electron transfer (PCET) differs at high and low pH. In this article, we utilize the proton inventory technique to obtain more information concerning PCET mechanism at high pH. The hypercurvature of the proton inventory data provides evidence for the existence of multiple, proton-donation pathways to Y(D)(*). In addition, at least one of these pathways must involve the transfer of more than one proton.

Redox proteomics: basic principles and future perspectives for the detection of protein oxidation in plants

The production and scavenging of chemically reactive species, such as ROS/RNS, are central to a broad range of biotic and abiotic stress and physiological responses in plants. Among the techniques developed for the identification of oxidative stress-induced modifications on proteins, the so-called 'redox proteome', proteomics appears to be the best-suited approach. Oxidative or nitrosative stress leaves different footprints in the cell in the form of different oxidatively modified components and, using the redox proteome, it will be possible to decipher the potential roles played by ROS/RNS-induced modifications in stressed cells. The purpose of this review is to present an overview of the latest research endeavours in the field of plant redox proteomics to identify the role of post-translational modifications of proteins in developmental cell stress. All the strategies set up to analyse the different oxidized/nitrosated amino acids, as well as the different reactivities of ROS and RNS for different amino acids are revised and discussed. A growing body of evidence indicates that ROS/RNS-induced protein modifications may be of physiological significance, and that in some cellular stresses they may act causatively and not arise as a secondary consequence of cell damage. Thus, although previously the oxidative modification of proteins was thought to represent a detrimental process in which the modified proteins were irreversibly inactivated, it is now clear that, in plants, oxidatively/nitrosatively modified proteins can be specific and reversible, playing a key role in normal cell physiology. In this sense, redox proteomics will have a central role in the definition of redox molecular mechanisms associated with cellular stresses.

An Activated Glutamate Residue Identified in Photosystem II at the Interface between the Manganese-stabilizing Subunit and the D2 Polypeptide

Photosystem II (PSII) catalyzes the oxidation of water during oxygenic photosynthesis. PSII is composed both of intrinsic subunits, such as D1, D2, and CP47, and extrinsic subunits, such as the manganese-stabilizing subunit (MSP). Previous work has shown that amines covalently bind to amino acid residues in the CP47, D1, and D2 subunits of plant and cyanobacterial PSII, and that these covalent reactions are prevented by the addition of chloride in plant preparations depleted of the 18- and 24-kDa extrinsic subunits. It has been proposed that these reactive groups are carbonyl-containing, post-translationally modified amino acid side chains (Ouellette, A. J. A., Anderson, L. B., and Barry, B. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 2204-2209 and Anderson, L. B., Ouellette, A. J. A., and Barry, B. A. (2000) J. Biol. Chem. 275, 4920-4927). To identify the amino acid binding site in the spinach D2 subunit, we have employed a biotin-amine labeling reagent, which can be used in conjunction with avidin affinity chromatography to purify biotinylated peptides from the PSII complex. Multidimensional chromato-graphic separation and multistage mass spectrometry localizes a novel post-translational modification in the D2 subunit to glutamate 303. We propose that this glutamate is activated for amine reaction by post-translational modification. Because the modified glutamate is located at a contact site between the D2 and manganese-stabilizing subunits, we suggest that the modification is important in vivo in stabilizing the interaction between these two PSII subunits. Consistent with this conclusion, mutations at the modified glutamate alter the steady-state rate of photosynthetic oxygen evolution.

Calcium exchange and structural changes during the photosynthetic oxygen evolving cycle

PSII catalyzes the oxidation of water and reduction of plastoquinone in oxygenic photosynthesis. PSII contains an oxygen-evolving complex, which is located on the lumenal side of the PSII reaction center and which contains manganese, calcium, and chloride. Four sequential photooxidation reactions are required to generate oxygen. This process produces five Sn-states, where n refers to the number of oxidizing equivalents stored. Calcium is required for oxygen production. Strontium is the only divalent cation that replaces calcium and maintains activity. In our previous FT-IR work, we assessed the effect of strontium substitution on substrate-limited PSII preparations, which were inhibited at the S3 to S0 transition. In this work, we report reaction-induced FT-IR studies of hydrated PSII preparations, which undergo the full S-state cycle. The observed difference FT-IR spectra reflect long-lived photoinduced conformational changes in the oxygen-evolving complex; strontium exchange identifies vibrational bands sensitive to substitutions at the calcium site. During the S1' to S2' transition, the data are consistent with an electrostatic or structural perturbation of the calcium site. During the S3' to S0' and S0' to S1' transitions, the data are consistent with a perturbation of a hydrogen bonding network, which contains calcium, water, and peptide carbonyl groups. To explain our data, persistent shifts in divalent cation coordination must occur when strontium is substituted for calcium. A modified S-state model is proposed to explain these results and results in the literature.

Structural studies of the manganese stabilizing subunit in photosystem II

Photosystem II (PSII) is the plant photosynthetic reaction center that carries out the light driven oxidation of water. The water splitting reactions are catalyzed at a tetranuclear manganese cluster. The manganese stabilizing protein (MSP) of PSII stabilizes the manganese cluster and accelerates the rate of oxygen evolution. MSP can be removed from PSII, with an accompanying decrease in activity. Either an Escherichia coli expressed version of MSP or native, plant MSP can be rebound to the PSII reaction center; MSP reconstitution reverses the deleterious effects associated with MSP removal. We have employed Fourier transform infrared (FTIR) spectroscopy and solution small angle x-ray scattering (SAXS) techniques to investigate the structure of MSP in solution and to define the structural changes that occur before and after reconstitution to PSII. FTIR and SAXS are complementary, because FTIR spectroscopy detects changes in MSP secondary structure and SAXS detects changes in MSP size/shape. From the SAXS data, we conclude that the size/shape and domain structure of MSP do not change when MSP binds to PSII. From FTIR data acquired before and after reconstitution, we conclude that the reconstitution-induced increase in beta-sheet content, which was previously reported, persists after MSP is removed from the PSII reaction center. However, the secondary structural change in MSP is metastable after removal from PSII, which indicates that this form of MSP is not the lowest energy conformation in solution.

Specific isotopic labeling and photooxidation-linked structural changes in the manganese-stabilizing subunit of photosystem II

Photosystem II (PSII) oxidizes water to molecular oxygen; the catalytic site is a cluster of four manganese ions. The catalytic site undergoes four sequential light-driven oxidation steps to form oxygen; these sequentially oxidized states are referred to as the Sn states, where n refers to the number of oxidizing equivalents stored. The extrinsic manganese stabilizing protein (MSP) of PSII influences the efficiency and stability of the manganese cluster, as well as the rates of the S state transitions. To understand how MSP influences photosynthetic water oxidation, we have employed isotope editing and difference Fourier transform infrared spectroscopy. MSP was expressed in Escherichia coli under conditions in which MSP aspartic and glutamic acid residues label at yields of 65 and 41%, respectively. Asparagine and glutamine were also labeled by this approach. GC/MS analysis was consistent with minimal scrambling of label into other amino acid residues and with no significant scrambling into the peptide bond. Selectively labeled MSP was then reconstituted to PSII, which had been stripped of native MSP. Difference Fourier transform infrared spectroscopy was used to probe the S1QA to S2QA- transition at 200 K, as well as the S1QB to S2QB- transition at 277 K. These experiments show that aspargine, glutamine, and glutamate residues in MSP are perturbed by photooxidation of manganese during the S1 to S2 transition.

Evidence for spontaneous structural changes in a dark-adapted state of photosystem II

Photosystem II catalyzes photosynthetic water oxidation in plants, green algae, and cyanobacteria. The manganese-containing active site cycles through a series of five oxidation states, S(n), where n refers to the number of oxidizing equivalents stored. In this report, reaction-induced Fourier transform infrared and electron paramagnetic resonance spectra of the S(1)-to-S(2) transition are presented. These data suggest that changes in carboxylate ligation to manganese, changes in secondary structure, and/or changes in polarity occur during dark adaptation in the S(1) state. These spontaneous structural changes are attributed to a S(1)' intermediate, at the same oxidation level as S(1), in the process of photosynthetic water oxidation.

Posttranslational modifications in the CP43 subunit of photosystem II

Photosystem II (PSII) catalyzes the light-driven oxidation of water and the reduction of plastoquinone; the oxidation of water occurs at a cluster of four manganese. The PSII CP43 subunit functions in light harvesting, and mutations in the fifth luminal loop (E) of CP43 have established its importance in PSII structure and/or assembly [Kuhn, M. G. & Vermaas, V. F. J. (1993) Plant Mol. Biol. 23, 123-133]. The sequence A(350)PWLEPLR(357) in luminal loop E is conserved in CP43 genes from 50 organisms. To map important posttranslational modifications in this sequence, tandem mass spectrometry (MS/MS) was used. These data show that the indole side chain of Trp-352 is posttranslationally modified to give mass shifts of +4, +16, and +18 daltons. The masses of the modifications suggest that the tryptophan is modified to kynurenine (+4), a keto-/amino-/hydroxy- (+16) derivative, and a dihydro-hydroxy- (+18) derivative of the indole side chain. Peptide synthesis and MS/MS confirmed the kynurenine assignment. The +16 and +18 tryptophan modifications may be intermediates formed during the oxidative cleavage of the indole ring to give kynurenine. The site-directed mutations, W352C, W352L, and W352A, exhibit an increased rate of photoinhibition relative to wild type. We hypothesize that Trp-352 oxidative modifications are a byproduct of PSII water-splitting or electron transfer reactions and that these modifications target PSII for turnover. As a step toward understanding the tertiary structure of this CP43 peptide, structural modeling was performed by using molecular dynamics.