Spectroelectrochemical determination of the redox potential of pheophytin<i>a</i>, the primary electron acceptor in photosystem II

Oxygenic photosynthesis is the basis for aerobic life on earth. The catalytic Mn 4 O x CaY Z center of photosystem II (PSII), after fourfold oxidation, extracts four electrons from two water molecules to yield dioxygen. This reaction cascade has appeared as a single four-electron transfer that occurs in typically 1 ms. Inevitable redox intermediates have so far escaped detection, probably because of very short lifetime. Previous attempts to stabilize intermediates by high O 2 -back pressure have revealed controversial results. Here we monitored by membrane-inlet mass spectrometry (MIMS) the production of 18 O 2 from 18 O-labeled water against a high background of 16 O 2 in a suspension of PSII-core complexes. We found neither an inhibition nor an altered pattern of O 2 production by up to 50-fold increased concentration of dissolved O 2 . Lack of inhibition is in line with results from previous X-ray absorption and visible-fluorescence experiments, but contradictory to the interpretation of previous UV-absorption data. Because we used essentially identical experimental conditions in MIMS as had been used in the UV work, the contradiction was serious, and we found it was not to be resolved by assuming a significant slowdown of the O 2 release kinetics or a subsequent slow conformational relaxation. This calls for reevaluation of the less direct UV experiments. The direct detection of O 2 release by MIMS shows unequivocally that O 2 release in PSII is highly exothermic. Under the likely assumption that one H þ is released in the S 4 → S 0 transition, the driving force at pH 6.5 and atmospheric O 2 pressure is at least 220 meV, otherwise 160 meV.water oxidation | oxygen evolution | isotope-ratio mass spectrometry | bioenergetics O xygenic photosynthesis is the metabolic basis for the most successful forms of life on earth. In cyanobacteria, algae and plants photosystem II (PSII) uses sunlight to split water into dioxygen and reducing equivalents, and it generates proton motive force. The catalytic centre of O 2 production in PSII, coined the OEC (oxygen evolving complex), comprises the manganese-oxygen-calcium (Mn 4 O x Ca) complex and its ligands, which include two substrate "water" molecules of undefined protonation state. It also includes a redox-active tyrosine residue, coined tyrosine ZðY Z Þ, which is the essential electron transfer link to the photoactive reaction center of PSII. Over 40 years ago Kok et al. (1) proposed-on the basis of flash-induced O 2 -evolution patterns (2)-that PSII, clocked and driven by four quanta of light, cycles through five different redox states, named S i (i ¼ 0;…;4), where i is the number of stored oxidizing equivalents. Once four oxidizing equivalents are accumulated O 2 is produced in the spontaneous reaction from S 4 to yield S 0 .In this S i state transition four electrons are transferred from two bound substrate water molecules that were partially or fully deprotonated (m ¼ 0-2) during the S i state cycle. In addition, one or two new substrate water molecules (n ¼ 1;2) bin...

Section: Discussionmentioning

confidence: 99%

Membrane-inlet mass spectrometry reveals a high driving force for oxygen production by photosystem II

Shevela

Beckmann

Clausen

et al. 2011

“…We examined the effect of betaine (1.0 M) on the redox potential of Phe a, because this reagent was frequently used for recent measurements (13,25) but not for the original report (6)(7)(8). In the case of Synechocystis, the difference spectra in the red region (683 ± 0.3 nm) under the absence of betaine were essentially identical to those observed in the presence of betaine ( Fig.…”

Section: Resultsmentioning

confidence: 99%

“…PCC 6803 (hereafter referred to as Synechocystis) to be about −500 mV. Kato et al (13), using a mutant of Thermosynechococcus elongatus, also supported a high value (−505 ± 6 mV at pH 6.5) under the presence of a stabilizer (1.0 M betaine); this study used a sophisticated spectroelectrochemical method. The controversy over E m (Phe a/ Phe a − ) values indicates that the redox potential of Phe a in PS II is still under debate.…”

mentioning

confidence: 99%

Redox potential of pheophytin a in photosystem II of two cyanobacteria having the different special pair chlorophylls

Allakhverdiev

Tomo

Shimada

et al. 2010

Water oxidation by photosystem (PS) II in oxygenic photosynthetic organisms is a major source of energy on the earth, leading to the production of a stable reductant. Mechanisms generating a high oxidation potential for water oxidation have been a major focus of photosynthesis research. This potential has not been estimated directly but has been measured by the redox potential of the primary electron acceptor, pheophytin (Phe) a. However, the reported values for Phe a are still controversial. Here, we measured the redox potential of Phe a under physiological conditions (pH 7.0; 25°C) in two cyanobacteria with different special pair chlorophylls (Chls): Synechocystis sp. PCC 6803, whose special pair for PS II consists of Chl a, and Acaryochloris marina MBIC 11017, whose special pair for PS II consists of Chl d. We obtained redox potentials of −536 ± 8 mV for Synechocystis sp. PCC 6803 and −478 ± 24 mV for A. marina on PS II complexes in the presence of 1.0 M betaine. The difference in the redox potential of Phe a between the two species closely corresponded with the difference in the light energy absorbed by Chl a versus Chl d. We estimated the potentials of the special pair of PS II to be 1.20 V and 1.18 V for Synechocystis sp. PCC 6803 (P680) and A. marina (P713), respectively. This clearly indicates conservation in the properties of water-oxidation systems in oxygenic photosynthetic organisms, irrespective of the special-pair chlorophylls.hotosynthesis mediates the conversion of solar light energy to chemical-bond energy through multistep reactions. Two photosystems (PSs) are present in oxygenic photosynthetic organisms, and these two PSs function cooperatively to capture light energy and drive electron flow. PS II supplies an energy source (i.e., an electron) by water oxidation, and PS I supplies a highly reduced compound, NADPH, to reduce CO 2 to carbohydrates.Reaction processes in the electron transfer system in photosynthesis are governed by two major factors: the relative geometry and the redox potentials of the electron transfer components. The molecular environment supplied by the amino acid matrix of the components will give a supplemental effect(s). Crystal structures of PS II complexes at atomic resolution have been reported from several laboratories (1-4). Thus, with the exception of some inconsistencies in the water-oxidation reaction system, an essential part of the primary charge separation machinery has been characterized (5). The electron transfer mechanisms, in contrast,have not yet been clarified in most cases.Pheophytin (Phe) a is the primary electron acceptor in PS II (6-8), although the primary electron donor of PS II is still controversial [P680 or accessory chlorophyll (Chl) a] (9, 10). These two are not in disagreement with respect to the nature of the primary charge separation, but they differ in the value of rate constants and the question of "transfer to the trap limited" or "trap-limited reaction" (5). In this report, we used the term the special pair instead of the primary electron...

“…Chl d gains light energy at longer wavelengths and with a lowered redox potential, by ∼80 mV, compared with that of Chl a in Synechocystis sp. PCC 6803 (hereafter referred to as Synechocystis) and spinach (7)(8)(9)(10)(11). However, the overall conditions for the water-cleavage reaction are satisfied even with the difference in the redox potentials (12).…”

mentioning

confidence: 99%

Redox potentials of primary electron acceptor quinone molecule (Q _A ) ⁻ and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

Allakhverdiev

Watabe

Kojima

et al. 2011

In a previous study, we measured the redox potential of the primary electron acceptor pheophytin (Phe) a of photosystem (PS) II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina and a chlorophyll a-containing cyanobacterium, Synechocystis. We obtained the midpoint redox potential (E m ) values of −478 mV for A. marina and −536 mV for Synechocystis. In this study, we measured the redox potentials of the primary electron acceptor quinone molecule (Q A ), i. ) of A. marina was determined to be +64 mV without the Mn cluster and was estimated to be −66 to −86 mV with a Mn-depletion shift (130-150 mV), as observed with other organisms. The E m (Phe a/Phe a − ) in Synechocystis was measured to be −525 mV with the Mn cluster, which is consistent with our previous report. The Mn-depleted downshift of the potential was measured to be approximately −77 mV in Synechocystis, and this value was applied to A. marina (−478 mV); the E m (Phe a/Phe a − ) was estimated to be approximately −401 mV. These values gave rise to a ΔG PhQ of −325 mV for A. marina and −383 mV for Synechocystis. In the two cyanobacteria, the energetics in PS II were conserved, even though the potentials of Q A − and Phe a − were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.photosynthesis | photochemical reaction C hlorophylls (Chls) are key pigments for photosynthesis, and oxygenic photosynthetic organisms containing Chls are found all over the world-they sustain all terrestrial life through the primary production of organic molecules and the production of oxygen. Chl a and its derivatives, pheophytin (Phe) a and Chl a epimer (Chl a′), are used as the electron transfer components in photosystem (PS) II and PS I, respectively, except for in two taxonomic groups, marine cyanobacteria Prochlorococcus spp. containing divinyl-Chl a (1) and marine cyanobacteria Acaryochloris spp. containing Chl d (2). Consequently, experimental and theoretical analyses of the electron transfer system in oxygenic photosynthesis have been exclusively performed in Chl a-containing organisms, and the basic concept of the photosynthetic electron flow has been constructed on the basis of Chl a. However, our understanding of photosynthetic electron transfer is not yet complete because the reaction systems with divinyl-Chl a and Chl d have not been fully analyzed as yet.Since the discovery of Chl d in Acaryochloris marina in 1996 (3, 4), questions have arisen on the whether the electron transfer system in this organism is significantly different from those with Chl a. Interestingly, the functional "special pair" are Chl d molecules in the reaction centers of PS I and II in A. marina (5, 6); however, in PS II, Phe a is the primary electron acceptor. Chl d gains light energy at longer wavelengths and with a lowered redox potential, by ∼80 mV, compared with that of Chl a in Synechocystis sp. PCC 6803 (hereafter referred to as Synechocystis) and spinach (7-11). However, the overall ...