The exchange of the fast substrate water in the S2 state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism.

Casper De Lichtenberg,Christopher J Kim,Petko Chernev,Johannes Messinger,Richard J Debus

doi:10.1039/d1sc02265b

Casper De Lichtenberg, Christopher J Kim + Show 3 more

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https://doi.org/10.1039/d1sc02265b

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Abstract

The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the ‘slow exchanging substrate water (Ws)’, while, in the S2 state, the terminal water ligands W2 and W3 are both discussed as the ‘fast exchanging substrate water (Wf)’. A critical point for the assignment of Wf is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of H216O/H218O substrate water exchange in the S2 and S3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of Wf and Ws were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that Wf exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S2 state, namely the indifference of the rate of Wf exchange towards Ca/Sr substitution.

Highlights

Photosynthesis performed by plants, algae and cyanobacteria is critical for life on Earth as it releases molecular oxygen into the atmosphere and stores solar energy as biomass
To probe if the fast water exchange (Wf) in the S2 state is limited by diffusion through channels or by the chemical exchange process, we study here the effects of the D1-D61A and D1-E189Q mutations on the rates of substrate water exchange with bulk water in the S2 and S3 states
For the 18,18O2 signal (m/z 36), which requires that both substrate waters exchange against H218O added to the bulk water, a mono-exponential rise with the rate ks was detected (Fig. 2B and 3B)

Summary

Introduction

Photosynthesis performed by plants, algae and cyanobacteria is critical for life on Earth as it releases molecular oxygen into the atmosphere and stores solar energy as biomass. The protein complex photosystem II (PSII) generates and stabilizes charge pairs that are employed for the extraction of 4 electrons and 4 protons from 2 water molecules, and to reduce plastoquinone to plastohydroquinone.[1,2] The solar-tochemical energy conversion efficiency of PSII has been estimated to reach values of up to 15%.3. The water oxidation reaction is catalyzed by a metal–oxygen cluster comprising the metals manganese and calcium in a 4 : 1 stoichiometry as well as ve oxo bridges (O1–O5).[4,5,6] During the reaction cycle, the Mn4CaO5 cluster is stepwise oxidized by light-induced charge separations in the chlorophyll containing reaction center of PSII. Proton release is facilitated by an intricate H-bonding network that is pivotal to the function of PSII and its earth-abundant water oxidation catalyst.[17,21,22,23,24]

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