A hydrogen-bonding network plays a catalytic role in photosynthetic oxygen evolution

Abstract

In photosystem II, oxygen evolution occurs by the accumulation of photo-induced oxidizing equivalents at the oxygen-evolving complex (OEC). The sequentially oxidized states are called the S 0 -S 4 states, and the dark stable state is S 1 . Hydrogen bonds to water form a network around the OEC; this network is predicted to involve multiple peptide carbonyl groups. In this work, we tested the idea that a network of hydrogen bonded water molecules plays a catalytic role in water oxidation. As probes, we used OEC peptide carbonyl frequencies, the substrate-based inhibitor, ammonia, and the sugar, trehalose. Reaction-induced FT-IR spectroscopy was used to describe the protein dynamics associated with the S 1 to S 2 transition. A shift in an amide CO vibrational frequency (1664 (S 1 ) to 1653 (S 2 ) cm −1 ) was observed, consistent with an increase in hydrogen bond strength when the OEC is oxidized. Treatment with ammonia/ammonium altered these CO vibrational frequencies. The ammonia-induced spectral changes are attributed to alterations in hydrogen bonding, when ammonia/ammonium is incorporated into the OEC hydrogen bond network. The ammonia-induced changes in CO frequency were reversed or blocked when trehalose was substituted for sucrose. This trehalose effect is attributed to a displacement of ammonia molecules from the hydrogen bond network. These results imply that ammonia, and by extension water, participate in a catalytically essential hydrogen bond network, which involves OEC peptide CO groups. Comparison to the ammonia transporter, AmtB, reveals structural similarities with the bound water network in the OEC.amide carbonyl frequency | Amt B transporter | photosystem II | vibrational spectroscopy | water oxidation I n oxygenic photosynthesis, photosystem II (PSII) catalyzes the oxidation of water and reduction of plastoquinone (1). Each reaction center includes the transmembrane subunits D1, D2, CP43, and CP47, which bind the redox-active cofactors. After photoexcitation, a charge separation is generated between the dimeric chl donor, P 680 , and a bound plastoquinone, Q A . Q A − reduces a second quinone, Q B , and P 680 þ oxidizes a tyrosine residue, YZ, Y161 in the D1 polypeptide. The radical, YZ•, is a powerful oxidant (2). Under physiological conditions, YZ• oxidizes the Mn 4 CaO 5 cluster, where water oxidation occurs (3). Four sequential photooxidations lead to the release of molecular oxygen (4). The oxygen yield fluctuates with period four. The sequentially oxidized states of the Mn cluster are called the Sn states, where n refers to the number of oxidizing equivalents stored at the OEC. S 1 is the dark stable state of the OEC. A single flash given to a dark-adapted sample of PSII generates the S 2 state, which corresponds to the oxidation of Mn(III) to Mn (IV) (5). The water oxidation chemistry in the S state cycle occurs on the microsecond to millisecond time scale, and oxygen is released during the S 3 to S 0 transition (4, 6).The structure of PSII has been reported at a resolution of 1.9 Å (3). T...

An Intrinsically Disordered Photosystem II Subunit, PsbO, Provides a Structural Template and a Sensor of the Hydrogen-bonding Network in Photosynthetic Water Oxidation

“…Second, spectra were then normalized to the intensity of the ferricyanide and ferrocyanide bands (2116 and 2038 cm Ϫ1 , data not shown), which reflect any small changes in the amount of charge separation in the preparations. These methods have been used previously for PSII spectra (70,71,84) and give control double difference spectra ( Fig. 5F) with flat baselines and no vibrational bands, as expected.…”

“…How are these conformational changes in PsbO linked with oxidation of the metal cluster? PSII contains a catalytically important hydrogen-bonded water network surrounding the OEC (70). This network (Fig.…”

“…This reaction-induced FT-IR technique has been used recently (70,71) to study the S state cycle in a plant PSII preparation, isolated with octylthioglucoside (72) and Triton X-100 (73). This preparation provides an advantage for spectroscopic studies in that it is highly resolved and stable.…”

“…FT-IR spectroscopy was performed at 263 and 277 K, as previously described (70,71). Samples (PSII, NaCl-PSII, urea-PSII, CaCl 2 -PSII, native-PSII, 12 C-PSII, or 13 C-PSII) for FT-IR experiments were suspended in 400 mM sucrose, 50 mM MES-NaOH (pH 6.0), 60 mM NaCl, and 20 mM CaCl 2 by three rounds of centrifugation (50,000 ϫ g, 4°C, 15 min) and resuspension and frozen (Ϫ70°C) until use.…”

Detection of an intermediary, protonated water cluster in photosynthetic oxygen evolution

“…In another approach, the recombination kinetics of YZ• were used as a probe of electrostatic changes in the network in the S 2 and S 0 states (21, 22). In both studies, ammonia, a substrate-based inhibitor (23-25), was used to perturb hydrogen bonding in the OEC and was shown to have significant effects on the spectroscopic signals (18,20,22).Proton-coupled electron-transfer reactions occur during the S-state cycle (11,12). Although the S 1 -to-S 2 transition is not accompanied by a net proton release to sucrose-containing buffers, proton release accompanies the other S-state transitions (26).…”