The D1-V185N mutation alters substrate water exchange by stabilizing alternative structures of the Mn4Ca-cluster in photosystem II

Casper De Lichtenberg,Anton P Avramov,Minquan Zhang,Fikret Mamedov,Robert L Burnap,Johannes Messinger

doi:10.1016/j.bbabio.2020.148319

Casper De Lichtenberg, Anton P Avramov + Show 4 more

Open Access

https://doi.org/10.1016/j.bbabio.2020.148319

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Abstract

In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O2-release and strongly perturb the water network surrounding the Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H218O/H216O-exchange kinetics of the fast (Wf) and slow (Ws) exchanging substrate waters bound in the S1, S2 and S3 states to the Mn4Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for Ws was increased in the S1 and S2 states, while both Wf and Ws exchange rates were decreased in the S3 state. Additionally, we used EPR spectroscopy to characterize the Mn4Ca cluster and its interaction with the redox active D1-Tyr161 (YZ). In the S2 state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S1 state was not affected, while the split signal in the S3 state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn4Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.

Highlights

Photosynthesis is arguably one of the most important chemical processes on Earth
We investigated the exchange rates of substrate waters bound at the Mn4Ca-cluster with bulk water in photosystem II (PSII) core complexes isolated from Synechocystis sp
This indicates that all centers participate in substrate water exchange

Summary

Introduction

Photosynthesis is arguably one of the most important chemical processes on Earth. Driven by sunlight it supports aerobic life by sup plying oxygen for breathing and by storing the energy of the photons in chemical bonds of, for example, lipids and carbohydrates. In a process known as water-splitting, the energy from light is used to extract electrons and protons from water in order to reduce the plastoquinone pool and support the proton gradient across the thylakoid membrane. In this process, oxygen is released as a side product. The water-splitting chemistry occurs at a Mn4Ca-cluster that constitutes the active site of the oxygen evolving complex (OEC), while the photochemistry and electron transfer reactions are performed by chlorophyll (Chl), pheophytin (Pheo) and quinone cofactors as well as by a tyrosine re sidue [1,2,3,4,5]

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