Water in Photosystem II: Structural, functional and mechanistic considerations

Abstract

Water is clearly important for the functioning of Photosystem II (PSII). Apart from being the very substrate that needs to be transported in this water oxidation enzyme, water is also vital for the transport of protons to and from the catalytic center as well as other important co-factors and key residues in the enzyme. The latest crystal structural data of PSII have enabled detailed analyses of the location and possible function of water molecules in the enzyme. Significant progress has also been made recently in the investigation of channels and pathways through the protein complex. Through these studies, the mechanistic significance of water for PSII is becoming increasingly clear. An overview and discussion of key aspects of the current research on water in PSII is presented here. The role of water in three other systems (aquaporin, bacteriorhodopsin and cytochrome P450) is also outlined to illustrate further points concerning the central significance that water can have, and potential applications of these ideas for continued research on PSII. It is advocated that water be seen as an integral part of the protein and far from a mere solvent.

Oxygen-evolving complex of Photosystem II: an analysis of second-shell residues and hydrogen-bonding networks

“…Subunits with residues that line the channels are highlighted in color: D1 and D2 in blues, antenna proteins in greens, and PsbO in purple. Water molecules shown in orange form the 'narrow' [9,13,14] channel (also known as E/F [10], or channel 2 [12 ]). The yellow surface shows the water network corresponding to the channels termed 'broad' [9,13,14], 'iii' [8], D/C/G [10], or 1/3 [12 ].…”

“…Water molecules shown in orange form the 'narrow' [9,13,14] channel (also known as E/F [10], or channel 2 [12 ]). The yellow surface shows the water network corresponding to the channels termed 'broad' [9,13,14], 'iii' [8], D/C/G [10], or 1/3 [12 ]. In red is the branching network described as the 'large channel system' [9,13,14], 'ii' [8], B1/B2 [10], or 4a/4b [12 ].…”

“…The yellow surface shows the water network corresponding to the channels termed 'broad' [9,13,14], 'iii' [8], D/C/G [10], or 1/3 [12 ]. In red is the branching network described as the 'large channel system' [9,13,14], 'ii' [8], B1/B2 [10], or 4a/4b [12 ]. (b) The water molecules in the 1.9 Å crystal structure are shown as spheres within each colored-mesh surface for the channels shown in panel (a) [6].…”

“…This channel has been shown via molecular dynamics to be impermeable to water [11], with a barrier of $22 kcal/mol at the bottleneck composed of D1-L91, CP43-F292, and CP43-F358 [12 ]. This leaves the 'narrow,' 'broad,' and 'large' channel systems as options for water entry from the lumen to the OEC ( Figure 2, with nomenclature from [9,13,14]). The narrow channel connects to D1-D61 and the O4 m-oxo ligand of the OEC and has a calculated 9 kcal/mol barrier for water entry at the interface of D1-N338, D2-N350, and CP43-P334 [12 ].…”

Localization and functional characterization of the extrinsic subunits of photosystem II: an update

“…It is of note that crystal structures and theoretical calculations suggest another role for the extrinsic proteins-namely, the maintenance of access channels for substrate water to the Mn 4 CaO 5 cluster and exit channels for the products (molecular oxygen and protons). 18,19) A number of amino acid residues in the extrinsic proteins have been predicted to be associated with these channels, but their actual roles still need to be investigated experimentally.…”