Oxygenic Photosynthesis

Nugent, Jonathan H. A.

doi:10.1111/j.1432-1033.1996.00519.x

Cited by 125 publications

(108 citation statements)

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“…cyanobacteria | energy conversion | proinhibition | photosynthesis | protein engineering P hotosynthesis is the major source of useful chemical energy in the biosphere. All photosynthetic processes require efficient electron transfer (ET) pathways that are utilized for proton gradient formation (to be used for the production of ATP) and/or accumulation of reducing equivalents (1,2). Light energy, absorbed by light-harvesting antenna complexes, is transferred to photochemical reaction centers (RC), initiating charge separation in specific chlorophyll molecules bound to the RC proteins.…”

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confidence: 99%

Engineering of an alternative electron transfer path in photosystem II

Larom

Salama

Schuster

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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The initial steps of oxygenic photosynthetic electron transfer occur within photosystem II, an intricate pigment/protein transmembrane complex. Light-driven electron transfer occurs within a multistep pathway that is efficiently insulated from competing electron transfer pathways. The heart of the electron transfer system, composed of six linearly coupled redox active cofactors that enable electron transfer from water to the secondary quinone acceptor Q B , is mainly embedded within two proteins called D1 and D2. We have identified a site in silico, poised in the vicinity of the Q A intermediate quinone acceptor, which could serve as a potential binding site for redox active proteins. Here we show that modification of Lysine 238 of the D1 protein to glutamic acid (Glu) in the cyanobacterium Synechocystis sp. PCC 6803, results in a strain that grows photautotrophically. The Glu thylakoid membranes are able to perform light-dependent reduction of exogenous cytochrome c with water as the electron donor. Cytochrome c photoreduction by the Glu mutant was also shown to significantly protect the D1 protein from photodamage when isolated thylakoid membranes were illuminated. We have therefore engineered a novel electron transfer pathway from water to a soluble protein electron carrier without harming the normal function of photosystem II. cyanobacteria | energy conversion | proinhibition | photosynthesis | protein engineering P hotosynthesis is the major source of useful chemical energy in the biosphere. All photosynthetic processes require efficient electron transfer (ET) pathways that are utilized for proton gradient formation (to be used for the production of ATP) and/or accumulation of reducing equivalents (1, 2). Light energy, absorbed by light-harvesting antenna complexes, is transferred to photochemical reaction centers (RC), initiating charge separation in specific chlorophyll molecules bound to the RC proteins. Following charge separation, electrons are transferred sequentially to a series of acceptor molecules, each with a redox potential determined by its immediate environment (3, 4). The source of electron replenishment differs according to the reaction center type. For instance in purple non-sulfur bacteria, electrons are cycled back to the oxidized reaction center by a soluble cytochrome c (cyt c) type protein (5, 6). Oxygenic photosynthetic organisms (cyanobacteria, red and green algae and plants) contain two photosystems: photosystem I (PSI) and photosystem II (PSII) that work linearly (1, 7), and the source of electrons is water. One common facet of all ET pathways is the requirement for insulation of the redox active cofactors from potentially reducing/oxidizing molecules within the RC or in the surrounding media. Insulation provides the system with maximal ET rates and efficiencies and also prevents damage to the RC.PSII has a redox potential of up to 1.2 V (8, 9), required to abstract electrons from water (Fig. 1A). The photoexcited P 680 reaction center chlorophyll a primary donor transfers electron...

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confidence: 99%

Engineering of an alternative electron transfer path in photosystem II

Larom

Salama

Schuster

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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“…It has been proposed that Y Z not only acts as an electron transmitter between the OEC (Mn 4 X) and P 680 + , but participates more directly in water oxidation. The neutral radical Y Z ox , formed by electron abstraction, was suggested to pick up a hydrogen atom from bound water after its deprotonation into the bulk (18)(19)(20)(21). This concept has been mainly based on three sets of data and interpretations: (1) Y Z ox is a neutral radical (22).…”

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Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

1998

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In oxygen-evolving photosystem II (PSII), a tyrosine residue, D1Tyr161 (Y Z ), serves as the intermediate electron carrier between the catalytic Mn cluster and the photochemically active chlorophyll moiety P 680 . A more direct catalytic role of Y Z , as a hydrogen abstractor from bound water, has been postulated. That Y Z ox appears as a neutral (i.e. deprotonated) radical, Y Z • , in EPR studies is compatible with this notion. Data based on electrochromic absorption transients, however, are conflicting because they indicate that the phenolic proton remains on or near to Y Z ox . In Mn-depleted PSII the electron transfer between Y Z and P 680 + can be almost as fast as in oxygen-evolving material, however, only at alkaline pH. With an apparent pK of about 7 the fast reaction is suppressed and converted into an about 100-fold slower one which dominates at acid pH. In the present work we investigated the optical difference spectra attributable to the transition Y Z f Y Z ox as function of the pH. We scanned the UV and VIS range and used Mn-depleted PSII core particles and also oxygen-evolving ones. Comparing these spectra with published in vitro and in vivo spectra of phenolic compounds, we arrived at the following conclusions: In oxygen-evolving PSII Y Z resembles a hydrogen-bonded tyrosinate, Y Z (-) ‚‚‚H (+) ‚B. The phenolic proton is shifted toward a base B already in the reduced state and even more so in the oxidized state. The retention of the phenolic proton in a hydrogen-bonded network gives rise to a positive net charge in the immediate vicinity of the neutral radical Y Z • . It may be favorable both for the very rapid reduction by Y Z of P 680 + and for electron (not hydrogen) abstraction by Y Z • from the Mn-water cluster.Photosystem II (PSII) 1 produces molecular oxygen from water. It is located in photosynthetic membranes of plants and cyanobacteria. The inner core of PSII is a heterodimer of the D1D2 subunits which show homology with the L and M subunits of the purple bacterial RC (1). The primary electron donor of PSII, P 680 , is located close to the lumenal side of the membrane. The absorption of a quantum of light drives the charge separation between P 680 and the quinone acceptor at the stromal side (2). The very high redox potential of P 680 + drives water oxidation. It is a four-stepped process with at least four increasingly oxidized, metastable states S 0 to S 3 , plus a transient state S 4 that decays in milliseconds into S 0 under release of O 2 . P 680 + is reduced (in nanoseconds) by a redox-active tyrosine residue, Y Z (D1Tyr161 (3, 4)). Y Z ox is, in turn, reduced in microseconds by the oxygenevolving complex (OEC). The structure and exact location of the OEC is still ill-defined. It is formed by four manganese atoms (5, 6), possibly another redox cofactor, X (7-9), plus at least one Cl -ion (10) and one Ca 2+ (11, 12) ion. These cofactors serve various functions during water oxidation: The Mn cluster stores at least two of the oxidizing equivalents (namely on transitions S ...

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“…Structurally, P700 consists of a chlorophyll a dimer (eC 1 /eC 1 ), whose mutually parallel dihydroporphyrin ring planes are aligned with the membrane normal. Upon excitation, P700* passes an electron to the primary electron acceptor A (probably eC 2 or eC 2 ; see below).…”

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“…Photosystem II catalyzes the light-driven luminal oxidation of water and the reduction of plastoquinone near the stromal side of the photosynthetic membrane. Photosystem I (PSI) 1 luminally oxidizes the soluble electron donor plastocyanin (alternatively cytochrome c 6 ) and stromally reduces the extrinsic electron acceptor ferredoxin or flavodoxin. The reduced ferredoxin induces the reduction of NADP ϩ , a reaction catalyzed by ferredoxin: NADP ϩ reductase.…”

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“…Photosystem I receives electrons from photosystem II via an intermediate plastoquinone pool, the cytochrome b 6 /f complex, and water soluble electron carriers. The difference in proton concentration across the thylakoid membrane, which results from the proton pumping of the plastoquinone pool and the cytochrome b 6 /f complex, the stromal consumption of protons by NADP ϩ reduction, and the luminal release of protons following water oxidation, is used by the ATP-synthase for phosphorylation of ADP to ATP (1,2).…”

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Localization of Two Phylloquinones, QK and QK′, in an Improved Electron Density Map of Photosystem I at 4-Å Resolution

Klukas¹,

Schubert²,

Jordan³

et al. 1999

Journal of Biological Chemistry

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An improved electron density map of photosystem I from Synechococcus elongatus calculated at 4-Å resolution for the first time reveals a second phylloquinone molecule and thereby completes the set of cofactors constituting the electron transfer system of this iron-sulfur type photosynthetic reaction center: six chlorophyll a, two phylloquinones, and three Fe 4 S 4 clusters. The location of the newly identified phylloquinone pair, the individual plane orientations of these molecules, and the resulting distances to other cofactors of the electron transfer system are discussed and compared with those determined by magnetic resonance techniques.The electron transfer processes of oxygenic photosynthesis, as observed in cyanobacteria, eukaryotic algae, and higher plants, involve two distinct types of photosynthetic reaction centers located in the thylakoid membrane. Photosystem II catalyzes the light-driven luminal oxidation of water and the reduction of plastoquinone near the stromal side of the photosynthetic membrane. Photosystem I (PSI) 1 luminally oxidizes the soluble electron donor plastocyanin (alternatively cytochrome c 6 ) and stromally reduces the extrinsic electron acceptor ferredoxin or flavodoxin. The reduced ferredoxin induces the reduction of NADP ϩ , a reaction catalyzed by ferredoxin: NADP ϩ reductase. Photosystem I receives electrons from photosystem II via an intermediate plastoquinone pool, the cytochrome b 6 /f complex, and water soluble electron carriers. The difference in proton concentration across the thylakoid membrane, which results from the proton pumping of the plastoquinone pool and the cytochrome b 6 /f complex, the stromal consumption of protons by NADP ϩ reduction, and the luminal release of protons following water oxidation, is used by the ATP-synthase for phosphorylation of ADP to ATP (1, 2).Cyanobacterial PSI consists of 11 subunits referred to as PsaA to PsaF and PsaI to PsaM. An x-ray structural model of a cyanobacterial PSI complex from the thermophile Synechococcus elongatus has been postulated on the basis of an electron density map calculated at 4-Å resolution (3, 4). Despite the comparatively low resolution, it was possible to suggest an assignment of 43 ␣-helices to the individual subunits of PSI by correlating the information provided by the electron density map with available biochemical and biophysical data. Furthermore, the electron density map allowed the positions of 89 Chl a molecules, constituents of both the core antenna system and electron transfer system, one phylloquinone, and three ironsulfur clusters to be modeled.The electron transfer reactions of PSI are initiated through excitation of the primary electron donor P700 positioned near the luminal side of the membrane-integral complex. Structurally, P700 consists of a chlorophyll a dimer (eC 1 /eC 1 ), whose mutually parallel dihydroporphyrin ring planes are aligned with the membrane normal. Upon excitation, P700* passes an electron to the primary electron acceptor A (probably eC 2 or eC 2 ; see below). Spectros...

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Oxygenic Photosynthesis

Cited by 125 publications

References 118 publications

Engineering of an alternative electron transfer path in photosystem II

Engineering of an alternative electron transfer path in photosystem II

Tyrosine-Z in Oxygen-Evolving Photosystem II: A Hydrogen-Bonded Tyrosinate

Localization of Two Phylloquinones, QK and QK′, in an Improved Electron Density Map of Photosystem I at 4-Å Resolution

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