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Figure 5
Water and proton channels in PS II. The Mn4CaO5 cluster in PS II is connected to the lumen of the thylakoid membrane by three distinct channels: the O1 channel, O4 channel and Cl1 channel. (a) Shown are the various waters and key amino acids that are found in these channels in the room-temperature crystal structures of PS II. Time-resolved XFEL data show the dynamics of the waters and the amino acids in these channels, correlated with changes in the Mn cluster. Two examples for the S2 → S3 transition are highlighted in the insets: (i) schematic of the proton gate in the Cl1 channel likely to be responsible for the transport of a proton to the bulk; and (ii) the electron density changes in the water wheel, adjacent to the Mn4CaO5 cluster, possibly a signature of water shuttling to the catalytic center. For the water wheel, the 2mFo − DFc map is shown at 3 contour levels (green: 0.5σ, light blue: 1σ, blue: 1.5σ) (b) The water molecules in a high-resolution room-temperature structure of PS II (1.89 Å, PDB entry 7rf1; Hussein et al., 2021BB33), colored by a gradient scale representing the B factor of each water (white for a value of 25 Å2 and red for a value of 50 Å2). The mFo − DFc map is overlaid in blue (showing only peaks >3σ). The O1 channel is colored in red, the O4 channel in blue and the Cl1 channel in green. Parts of the figure have been adapted from Ibrahim et al. (2020BB35) and Hussein et al. (2021BB33).

IUCrJ
Volume 10| Part 6| November 2023| Pages 642-655
ISSN: 2052-2525
https://doi.org/10.1107/S2052252523008928