Chapter 6 The photosynthetic oxygen-evolving process

The reactivity of hydrazine with photosystem II strongly depends on the redox state of the water oxidizing system

The decay kinetics of the redox states S2 and S3 of the water-oxidizing enzyme have been analyzed in isolated spinach thylakoids in the absence and presence of the exogenous reductant hydrazine. In control samples without NH2NH2 a biphasic decay is observed. The rapid decline of S2 and S3 with YD as reductant exhibits practically the same kinetics with t1/2 = 6-7 s at pH = 7.2 and 7 degrees C. The slow reduction (order of 5-10 min at 7 degrees C) of S2 and S3 with endogenous electron donors other than YD is about twice as fast for S2 as for S3 under these conditions. In contrast, the hydrazine-induced reductive shifts of the formal redox states Si (i = 0...3) are characterized by a totally different kinetic pattern: (a) at 1 mM NH2NH2 and incubation on ice the decay of S2 is estimated to be at least 25 times faster (t1/2 less than or equal to 0.4 min) than the corresponding reaction of S3 (t1/2 approximately 13 min); (b) the NH2NH2-induced decay of S3 is even slower (about twice) than the transformation of S1 into the formal redox state 'S-1' (t1/2 approximately 6 min), which gives rise to the two-digit phase shift of the oxygen-yield pattern induced by a flash train in dark adapted thylakoids. (c) the NH2NH2-induced transformation S0----'S-2' [Renger, Messinger and Hanssum (1990) in: Curr.' Res. Photosynth. (Baltscheffsky, M., ed), Vol. 1, pp. 845-848, Kluwer, Dordrecht] is about three times faster (t1/2 approximately 2 min) than the reaction [see text]. Based on these results, the following dependence on the redox state Si of the reactivity towards NH2NH2 is obtained: S3 less than S1 less than S0 much less than S2. The implications of this surprising order of reactivity are discussed.

An astrophysical view of Earth-based metabolic biosignature gases

Microbial life on Earth uses a wide range of chemical and energetic resources from diverse habitats. An outcome of this microbial diversity is an extensive and varied list of metabolic byproducts. We review key points of Earth-based microbial metabolism that are useful to the astrophysical search for biosignature gases on exoplanets, including a list of primary and secondary metabolism gas byproducts. Beyond the canonical, unique-to-life biosignature gases on Earth (O(2), O(3), and N(2)O), the list of metabolic byproducts includes gases that might be associated with biosignature gases in appropriate exoplanetary environments. This review aims to serve as a starting point for future astrophysical biosignature gas research.

STRUCTURE AND MEMBRANE ORGANIZATION OF PHOTOSYSTEM II IN GREEN PLANTS

Photosystem II (PSII) is the pigment protein complex embedded in the thylakoid membrane of higher plants, algae, and cyanobacteria that uses solar energy to drive the photosynthetic water-splitting reaction. This chapter reviews the primary, secondary, tertiary, and quaternary structures of PSII as well as the function of its constituent subunits. The understanding of in vivo organization of PSII is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These images show a resolution of about 40-50 A and so provide information mainly on the localization, heterogeneity, dimensions, and shapes of membrane-embedded PSII complexes. Higher resolution of about 15-40 A has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. Observations are discussed in terms of the oligomeric state and subunit organization of PSII and its antenna components.

Point-mutations affecting the properties of tyrosineD in photosystem II. Characterization by isotopic labeling and spectral simulation

The reaction center of photosystem II (PSII) contains two redox-active tyrosines, TyrD and TyrZ, which are Tyr160 and Tyr161 of the D2 and D1 proteins, respectively. We have introduced five site-directed mutations in the vicinity of TyrD to analyze the consequences of the mutations on spectral and functional properties of TyrD(ox). Characterization of three mutants, P161A and P161L (Pro161 changed to Ala and Leu, respectively) and Q164L (Gln164 mutated to Leu), is emphasized. Of these three mutants, only P161L is an obligate photoheterotroph; it is capable of oxygen evolution, but is photoinactivated rapidly. The D2 protein of this mutant migrates slower on a SDS-polyacrylamide gel. The EPR spectrum of TyrD(ox) is modified in the three mutants. The EPR spectra of TyrD(ox) in wild type and the mutants were characterized in detail by comparison of EPR spectra of thylakoids from cells grown in the presence and absence of tyrosine that was deuterated in specific positions. The experimentally obtained EPR spectra of wild type, P161A, and Q164L could be simulated satisfactorily using current theoretical models. The angle between one of the hydrogens on the beta-methylene carbon and the 2pz orbital at C1 of the tyrosine ring was found to change slightly but significantly as a function of the mutations (52 degrees in wild type, 50 degrees in P161A, and 48 degrees in Q164L). The overall electronic structure of TyrDox is quite unaffected; only minor redistribution of the unpaired electron spin is observed between the wild type and the mutated systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Where plants make oxygen: a structural model for the photosynthetic oxygen-evolving manganese cluster

In the photosynthetic evolution of oxygen, water oxidation occurs at a catalytic site that includes four manganese atoms together with the essential cofactors, the calcium and chlorine ions. A structural model and a determination of the manganese oxidation states based on x-ray absorption spectroscopy are presented. The salient features, in both higher plants and cyanobacteria, are a pair of di-mu-oxo bridged manganese binuclear clusters linked by a mono-mu-oxo bridge, one proximal calcium atom, and one halide. In dark-adapted samples, manganese occurs in oxidation states (III) and (IV). Data from oriented membranes display distinct dichroism, precluding highly symmetrical structures for the manganese complex.

Proton release during successive oxidation steps of the photosynthetic water oxidation process: stoichiometries and pH dependence

Flash-induced absorption changes of pH-indicating dyes were investigated in photosystem II enriched membrane fragments, in order to retrieve the individual contributions to proton release of the successive transitions of the Kok cycle. These stoichiometric coefficients were found to be, in general, noninteger and to vary as a function of pH. Proton release on the S0----S1 step decreases from 1.75 at pH 5.5 to 1 at pH 8, while, on S1----S2 the stoichiometry increases from 0 to 0.5 in the same pH range and remains close to 1 for S2----S3. These findings are analyzed in terms of pK shifts of neighboring amino acid residues caused by electrostatic interactions with the redox centers involved in the two first transitions. The electrochromic shift of a chlorophyll, associated with the S transitions, responding to local electrostatic effects was investigated under similar conditions. The pH dependence of this signal upon the successive transitions was found correlated with the titration of the proton release stoichiometries, expressing the electrostatic balance between the oxidation and deprotonation processes.

The carboxyl modifier 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) inhibits half of the high-affinity Mn-binding site in photosystem II membrane fragments

The diphenylcarbazide(DPC)/Mn2+ assay [Hsu, B.-D., Lee, J.-Y., & Pan, R.-L. (1987) Biochim. Biophys. Acta 890, 89-96] was used to assess the amount of the high-affinity Mn-binding site in manganese-depleted photosystem II (PS II) membrane fragments from spinach and Scenedesmus obliquus. The assay mechanism at high DPC concentration was shown to involve noncompetitive inhibition of only half of the control level of DPC donation to PS II by micromolar concentrations of Mn at pH 6.5 (i.e., one of two DPC donation sites is inhibited). At low DPC concentration both DPC and Mn2+ donate to PS II additively. Treatment with the carboxyl amino acid modifier 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) inhibited half of the high-affinity Mn-binding site in spinach and Scenedesmus WT PS II membranes and all of the available site in Scenedesmus LF-1 mutant PS II membranes. A similar EDC concentration dependence was observed in all cases. Addition of 2 mM MnCl2 to the 10 mM EDC modification buffer provided complete protection for the Mn-binding site from modification. This protection was specific for Mn2+; six other divalent cations were ineffective. We conclude that EDC modifies that half of the high-affinity Mn-binding site that is insensitive to the histidine modifier diethyl pyrocarbonate (DEPC) [Seibert, M., Tamura, N., & Inoue, Y. (1989) Biochim. Biophys. Acta 974, 185-191] and directly affects ligands that bind Mn. The effects of EDC and DEPC that influence the high-affinity site are mutually exclusive and are specific to the lumenal side of the PS II membrane. Removal of the two more loosely bound of the four functional Mn from PS II membranes uncovers that part of the high-affinity site associated with carboxyl but not histidyl residues. We suggest that carboxyl residues on reaction center proteins are associated with half of the high-affinity Mn-binding site in PS II and are involved along with histidine residues in binding Mn functional in the O2-evolving process.

A guide to electron paramagnetic resonance spectroscopy of Photosystem II membranes

This guide is intended to aid in the detection and identification of paramagnetic species in Photosystem II membranes, by electron paramagnetic resonance spectroscopy. The spectral features and occurrence of each of the electron paramagnetic resonance signals from Photosystem II are discussed, in relation to the nature of the moiety giving rise to the signal and the role of that species in photosynthetic electron transport. Examples of most of the signals discussed are shown. The electron paramagnetic resonance signals produced by the cytochrome b6f and Photosystem I complexes, as well as the signals from other common contaminants, are also reviewed. Furthermore, references to seminal experiments on bacterial reaction centers are included. By reviewing both the spectroscopic and biochemical bases for the electron paramagnetic resonance signals of the cofactors that mediate photosynthetic electron transport, this paper provides an introduction to the use and interpretation of electron paramagnetic resonance spectroscopy in the study of Photosystem II.

Dicyclohexylcarbodiimide-binding proteins related to the short circuit of the proton-pumping activity of photosystem II. Identified as light-harvesting chlorophyll-a/b-binding proteins

In photosynthesis of higher plants, photosystem II drives electron transfer from the water-oxidizing manganese centre at the lumenal side to bound plastoquinone at the stromal side of the thylakoid membrane. Proton release into the lumen and proton uptake from the stroma, i.e. net proton pumping, follows as consequence of vectoral electron transport. The proton pumping activity can be short circuited by covalent modification with N,N'-dicyclohexylcarbodiimide (cHxN)2C of certain proteins in the 20-28-kDa range. After modification, protons from water oxidation are no longer released into the thylakoid lumen, but instead transferred through the photosystem complex to protonate the photoreduced bound quinone at the other side of the membrane [Jahns, P., Polle, A. & Junge, W. (1988) EMBO J. 7, 589-594]. Here we identify the pertinent (cHxN)2C-binding proteins by amino acid sequence analysis and localize (cHxN)2C-binding sites within their primary structure. The proteins that are associated with the proton short circuit are light-harvesting chlorophyll-a/b-binding proteins. Our results imply that in addition to acting as antennae they may serve another function: the funneling into the thylakoid lumen of protons, which are liberated in the water-oxidizing Mn centre.

Functional and structural analysis of photosystem II core complexes from spinach with high oxygen evolution capacity

Oxygen-evolving photosystem II core complexes were prepared from spinach by solubilizing photosystem II membrane fragments with dodecyl-beta-D-maltoside. The core complexes consist of the intrinsic 47-kDa, 43-kDa, D1 and D2 polypeptides, the two subunits of cytochrome b559 and the extrinsic 33-kDa protein. In the presence of 50 mM CaCl2 they exhibit a high oxygen evolution rate of 1.3 +/- 0.2 mmol O2.mg chlorophyll-1.h-1 with either 2,6-dichloro-benzoquinone or K3[Fe(CN)6] as acceptor. Electron micrographs of these complexes reveal an obtuse triangular structure in when viewed from the top measuring 15.3 nm on one side and 10.6 nm on the other two sides. An average height of 7.3 nm was determined from the side view position. These data are in good agreement with previously reported dimensions for photosystem II core complexes [Irrgang, K.-D., Boekema, E. J., Vater, J. and Renger, G. (1988) Eur. J. Biochem. 178, 209-217]. In contrast to previous reports the extrinsic 33-kDa subunit could be resolved for the first time. It appears as a small protrusion when the complex is viewed from the side and seems to cover the lumenal side of the core complex appearing as a disk with a thickness of 1.5-3.3 nm.

Chloride binding proteins: mechanistic implications for the oxygen-evolving complex of Photosystem II

Chloride plays a key role in activating the photosynethetic oxygen-evolving complex (OEC) of Photosystem II, but the OEC is only one of many enzymes affected by this anion. Some of the mechanistic features of Cl(-) involvement in water-splitting resemble those of other proteins whose structure and chemistry are known in detail. An overview of the similarities and differences between these Cl(-)-binding systems is presented.

Lumen-side topography of the alpha-subunit of the chloroplast cytochrome b-559

Removal of the extrinsic 33 kDa polypeptide increased the accessibility to trypsin of a COOH-terminal tridecapeptide epitope of the alpha subunit of cytochrome b-559 (psbE gene product). The sensitivity of the cytochrome epitope to trypsin was not measurably affected by removal of the 16 and 23 kDa extrinsic polypeptides, nor increased by removal of the OEC manganese along with the 33 kDa protein. While protecting alpha-cytochrome b-559 against trypsin, the 33 kDa protein is also proteolyzed, suggesting the possibility of an additional protein component involved in the shielding of the cytochrome. Shielding of the COOH-terminal epitope of alpha-cytochrome b-559 by the OEC 33 kDa protein implies that these COOH-terminal chains of the cytochrome are part of a protein network in the lumen space near the photosystem II reaction center. This network may contain residues that are involved in the binding of essential OEC metal ions.

Studies on the electron transfer from Tyr-161 of polypeptide D-1 to P680(+) in PS II membrane fragments from spinach

The functional connection between redox component Y z identified as Tyr-161 of polypeptide D-1 (Debus et al. 1988) and P680(+) was analyzed by measurements of laser flash induced absorption changes at 830 nm in PS II membrane fragments from spinach. It was found that neither DCMU nor the ADRY agent 2-(3-chloro-4-trifluoromethyl) anilino-3,5-dinitrothiophene (ANT 2p) affects the rate of P680(+) reduction by Y z under conditions where the catalytic site of water oxidation stays in the redox state S1. In contrast to that, a drastic retardation is observed after mild trypsin treatment at pH=6.0. This effect which is stimualted by flash illumination can be largely reversed by Ca(2+). The above mentioned data lead to the following conclusions: (a) the segment of polypeptide D-1 containing Tyr-161 and coordination sites of P680 is not allosterically affected by structural changes due to DCMU binding at the QB-site which is also located in D-1. (b) ANT 2p as a strong protonophoric uncoupler and ADRY agent does not modify the reaction coordinate of P680(+) reduction by Y z , and (c) Ca(2+) could play a functional role for the electronic and vibrational coupling between the redox groups Y z and P680. The electron transport from Y z to P680(+) is discussed within the framework of a nonadiabatic process. Based on thermodynamic considerations the reorganization energy is estimated to be in the order of 0.5 V.

Structural determination of the photosystem II core complex from spinach

A photosystem II core complex was purified with high yield from spinach by solubilization with beta-dodecylmaltoside. The complex consisted of polypeptides with molecular mass 47, 43, 34, 31, 9 and 4 kDa and some minor components, as detected by silver-staining of polyacrylamide gels. There was no indication for the chlorophyll-a/b-binding, light-harvesting complex polypeptides. The core complex revealed electron-transfer activity (1,5-diphenylcarbazide----2,6-dichloroindophenol) of about 30 mumol reduced 2,6-dichloroindophenol/mg chlorophyll/h. The structural integrity was analyzed by electron microscopy. The detergent-solubilized protein complex has the shape of a triangular disk with a maximum diameter of 13 nm and a maximum height of 6.8 nm. The shape of this core complex differs considerably from that of cyanobacterial photosystem II membrane fragments, which are elongated particles. The structural differences between both the complexes of higher plants and cyanobacteria are discussed with special emphasis on their association with the antenna apparatus in the photosynthetic membranes.

The molecular mechanism of the bicarbonate effect at the plastoquinone reductase site of photosynthesis

It has been known for some time that bicarbonate reverses the inhibition, by formate under HCO3 (-)-depletion conditions, of electron transport in thylakoid membranes. It has been shown that the major effect is on the electron acceptor side of photosystem II, at the site of plastoquinone reduction. After presenting a historical introduction, and a minireview of the bicarbonate effect, we present a hypothesis on how HCO3 (-) functions in vivo as (a) a proton donor to the plastoquinone reductase site in the D1-D2 protein; and (b) a ligand to Fe(2+) in the QA-Fe-QB complex that keeps the D1-D2 proteins in their proper functional conformation. They key points of the hypothesis are: (1) HCO3 (-) forms a salt bridge between Fe(2+) and the D2 protein. The carboxyl group of HCO3 (-) is a bidentate ligand to Fe(2+), while the hydroxyl group H-bonds to a protein residue. (2) A second HCO3 (-) is involved in protonating a histidine near the QB site to stabilize the negative charge on QB. HCO3 (-) provides a rapidly available source of H(+) for this purpose. (3) After donation of a H(+), CO3 (2-) is replaced by another HCO3 (-). The high pKa of CO3 (2-) ensures rapid reprotonation from the bulk phase. (4) An intramembrane pool of HCO3 (-) is in equilibrium with a large number of low affinity sites. This pool is a H(+) buffering domain functionally connecting the external bulk phase with the quinones. The low affinity sites buffer the intrathylakoid [HCO3 (-)] against fluctuations in the intracellular CO2. (5) Low pH and high ionic strength are suggested to disrupt the HCO3 (-) salt bridge between Fe(2+) and D2. The resulting conformational change exposes the intramembrane HCO3 (-) pool and low affinity sites to the bulk phase.Two contrasting hypotheses for the action of formate are: (a) it functions to remove bicarbonate, and the low electron transport left in such samples is due to the left-over (or endogenous) bicarbonate in the system; or (b) bicarbonate is less of an inhibitor and so appears to relieve the inhibition by formate. Hypothesis (a) implies that HCO3 (-) is an essential requirement for electron transport through the plastoquinones (bound plastoquinones QA and QB and the plastoquinone pool) of photosystem II. Hypothesis (b) implies that HCO3 (-) does not play any significant role in vivo. Our conclusion is that hypothesis (a) is correct and HCO3 (-) is an essential requirement for electron transport on the electron acceptor side of PS II. This is based on several observations: (i) since HCO3 (-), not CO2, is the active species involved (Blubaugh and Govindjee 1986), the calculated concentration of this species (220 μM at pH 8, pH of the stroma) is much higher than the calculated dissociation constant (Kd) of 35-60 μM; thus, the likelihood of bound HCO3 (-) in ambient air is high; (ii) studies on HCO3 (-) effect in thylakoid samples with different chlorophyll concentrations suggest that the "left-over" (or "endogenous") electron flow in bicarbonate-depleted chloroplasts is due to "left-over" (or endogenous) HCO3 (-) remaining bound to the system (Blubaugh 1987).