The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme

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Title: The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme
Authors: Boussac, A
Ugur, I
Marion, A
Sugiura, M
Kaila, VRI
Rutherford, AW
Item Type: Journal Article
Abstract: In Photosystem II (PSII), the Mn4CaO5-cluster of the active site advances through five sequential oxidation states (S0to S4) before water is oxidized and O2is generated. Here, we have studied the transition between the low spin (LS) and high spin (HS) configurations of S2using EPR spectroscopy, quantum chemical calculations using Density Functional Theory (DFT), and time-resolved UV-visible absorption spectroscopy. The EPR experiments show that the equilibrium between S2LSand S2HSis pH dependent, with a pKa ≈ 8.3 (n ≈ 4) for the native Mn4CaO5and pKa ≈ 7.5 (n ≈ 1) for Mn4SrO5. The DFT results suggest that exchanging Ca with Sr modifies the electronic structure of several titratable groups within the active site, including groups that are not direct ligands to Ca/Sr, e.g., W1/W2, Asp61, His332 and His337. This is consistent with the complex modification of the pKaupon the Ca/Sr exchange. EPR also showed that NH3addition reversed the effect of high pH, NH3-S2LSbeing present at all pH values studied. Absorption spectroscopy indicates that NH3is no longer bound in the S3TyrZstate, consistent with EPR data showing minor or no NH3-induced modification of S3and S0. In both Ca-PSII and Sr-PSII, S2HSwas capable of advancing to S3at low temperature (198 K). This is an experimental demonstration that the S2LSis formed first and advances to S3via the S2HSstate without detectable intermediates. We discuss the nature of the changes occurring in the S2LSto S2HStransition which allow the S2HSto S3transition to occur below 200 K. This work also provides a protocol for generating S3in concentrated samples without the need for saturating flashes.
Issue Date: 28-Feb-2018
Date of Acceptance: 24-Feb-2018
ISSN: 0006-3002
Publisher: Elsevier
Start Page: 342
End Page: 356
Journal / Book Title: Biochim Biophys Acta
Volume: 1859
Issue: 5
Copyright Statement: © 2018 Elsevier B.V. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International
Sponsor/Funder: The Royal Society
Biotechnology and Biological Sciences Research Council (BBSRC)
Biotechnology and Biological Sciences Research Cou
Funder's Grant Number: WM110001
ERI031013 - BB/L0115061/1
Keywords: DFT
Mn(4)CaO(5) cluster
Oxygen evolution
Photosystem II
Spin state
06 Biological Sciences
02 Physical Sciences
Publication Status: Published
Conference Place: Netherlands
Appears in Collections:Faculty of Natural Sciences

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