Water Network Dynamics Next to the Oxygen-Evolving Complex of Photosystem II

Krystle Reiss,Victor Batista,Alex Grigas,Uriel Morzan

doi:10.3390/inorganics7030039

Abstract

The influence of the environment on the functionality of the oxygen-evolving complex (OEC) of photosystem II has long been a subject of great interest. In particular, various water channels, which could serve as pathways for substrate water diffusion, or proton translocation, are thought to be critical to catalytic performance of the OEC. Here, we address the dynamical nature of hydrogen bonding along the water channels by performing molecular dynamics (MD) simulations of the OEC and its surrounding protein environment in the S1 and S2 states. Through the eigenvector centrality (EC) analysis, we are able to determine the characteristics of the water network and assign potential functions to the major channels, namely that the narrow and broad channels are likely candidates for proton/water transport, while the large channel may serve as a path for larger ions such as chloride and manganese thought to be essential during PSII assembly.

Highlights

Photosystem II (PSII) is a 650 kDa complex composed of 20 proteins embedded in the thylakoid membrane of green plant chloroplast and internal membranes of cyanobacteria
We computed the Root Mean Square Fluctuation (RMSF) per water molecule to quantify the mobility of waters in the system
The narrow channel shows a straight pathway of hydrogen-bonding water molecules with moderate mobility

Summary

Introduction

Photosystem II (PSII) is a 650 kDa complex composed of 20 proteins embedded in the thylakoid membrane of green plant chloroplast and internal membranes of cyanobacteria. PSII sustains aerobic life on Earth by harvesting solar light, initiating the process of photosynthesis, and producing oxygen from water. The water oxidation reaction is catalyzed by the oxygen-evolving complex (OEC), which is a CaMn4 O5 cuboidal cluster embedded in the D1 protein subunit. In each turn of the cycle, the OEC evolves through five storage states (S-states) of oxidizing equivalents, with S0 and S4 being the most reduced and oxidized states, respectively [1,2]. The stable dark-adapted S1 state (structure and oxidation states shown in Figure 1) is advanced to the S2 state by a single flash of light and generates a mixture of two redox isomers. Isomer A is an “open” cubane structure where Mn1 and Mn4 have formal oxidation states III and IV, respectively [3]

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Discussion

Conclusion