Hypothesis: the peroxydicarbonic acid cycle in photosynthetic oxygen evolution

Castelfranco, Paul A.; Lu, Yih-Kuang; Stemler, Alan

doi:10.1007/s11120-007-9134-8

Cited by 11 publications

(4 citation statements)

References 36 publications

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“…For mechanistic considerations, the following characteristics must be taken into account: (i) water is the direct substrate for dioxygen formation (Clausen et al 2005;Hillier et al 2006) without any catalytic function of bicarbonate (for a proposal, see Castelfranco et al 2007) that is not a constituent of the WOC (Ulas et al 2008;Shevela et al 2008); (ii) the S i -state transitions are coupled with proton release in a MS-EPT manner (for a discussion, see Meyer et al 2007); (iii) the oxidation steps of the WOC by Y OX Z are not limited by the rate of nonadiabatic ET but triggered by proton transfer (PT) and/or conformational changes and become blocked below threshold temperatures (for compilation of data, see Renger G 2001) and hydration levels (Noguchi and Sugiura 2002) that depend on the redox state S i ,; and (iv) the redox steps Y OX Z S 0 ! Y z S 1 and Y OX Z S 1 !…”

Section: Mechanismmentioning

confidence: 99%

Photosystem II: The machinery of photosynthetic water splitting

2008

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This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

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Section: Mechanismmentioning

confidence: 99%

Photosystem II: The machinery of photosynthetic water splitting

2008

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“…Since 1995, various data have been accumulated showing that bicarbonate is indeed functional in oxygen evolution ,,,– . In addition to the earlier proposal of a catalytic intermediate ,, , it has been suggested that bicarbonate functions as a ligand to the Mn cluster or an integral cofactor of OEC ,,, , a cofactor crucial in the photoassembly process of the Mn cluster as a transient ligand to Mn ions ,,,– , , a proton transfer mediator bound to a residue in the vicinity of the Mn cluster , and an element to indirectly stabilize OEC by binding to PSII proteins or extrinsic proteins ,, .…”

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confidence: 99%

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Flash-Induced FTIR Difference Spectroscopy Shows No Evidence for the Structural Coupling of Bicarbonate to the Oxygen-Evolving Mn Cluster in Photosystem II

et al. 2008

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Bicarbonate is known to be required for the maximum activity of photosystem II. Although it is well established that bicarbonate is bound to the nonheme iron to regulate the quinone reactions, the effect of bicarbonate on oxygen evolution is still controversial, and its binding site and exact physiological roles remain to be clarified. In this study, the structural coupling of bicarbonate to the oxygen-evolving center (OEC) was studied using Fourier transform infrared (FTIR) difference spectroscopy. Flash-induced FTIR difference spectra during the S-state cycle of OEC were recorded using the PSII core complexes from Thermosynechococcus elongatus in the presence of either unlabeled bicarbonate or (13)C-bicarbonate. The H (12)CO 3 (-)-minus-H (13)CO 3 (-) double difference spectra showed prominent bicarbonate bands at the first flash, whereas no appreciable bands were detected at the second to fourth flashes. The bicarbonate bands at the first flash were virtually identical to those from the nonheme iron, which was preoxidized by ferricyanide and photoreduced by a single flash, recorded using Mn-depleted PSII complexes. Using the bicarbonate bands of the nonheme iron as an internal standard, it was concluded that no bicarbonate band arising from OEC exists in the S-state FTIR spectra. This conclusion indicates that bicarbonate is not affected by the structural changes in OEC upon the four S-state transitions. It is thus strongly suggested that bicarbonate is neither a ligand to the Mn cluster nor a cofactor closely coupled to OEC, although the possibility cannot be fully excluded that nonexchangeable bicarbonate exists in OEC as a constituent of the Mn-cluster core. The data also provide strong evidence that bicarbonate does not function as a substrate or a catalytic intermediate. Bicarbonate may play major roles in the photoassembly process of the Mn cluster and in the stabilization of OEC by a rather indirect interaction.

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“…Currently, the way bicarbonate is possibly involved in the events on the donor side of PSII has had many speculations. The two viewpoints considering bicarbonate as an intermediate substrate for photosynthetic water oxidation or as a direct ligand of Mn4CaO5 cluster are thought to be unlikely since no strong experimental evidence exists or supports it [10,11]. There are suggestions that bicarbonate may play a role in stabilizing the OEC indirectly by binding to extrinsic proteins or some other nearby protein components [12,13], which, however, needs to be investigated.…”

Section: Introductionmentioning

confidence: 99%

Light-driven CO2 assimilation by photosystem II and its relation to photosynthesis

Wang

et al. 2023

Chinese Journal of Catalysis

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Hypothesis: the peroxydicarbonic acid cycle in photosynthetic oxygen evolution

Cited by 11 publications

References 36 publications

Photosystem II: The machinery of photosynthetic water splitting

Photosystem II: The machinery of photosynthetic water splitting

Flash-Induced FTIR Difference Spectroscopy Shows No Evidence for the Structural Coupling of Bicarbonate to the Oxygen-Evolving Mn Cluster in Photosystem II

Light-driven CO2 assimilation by photosystem II and its relation to photosynthesis

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