Photosystem II (PSII) is the water-splitting enzyme of oxygenic photosynthesis. Using light energy, PSII catalytically oxidizes two water molecules to fuel downstream metabolism, forming an O-O bond and releasing O 2 as a byproduct. The reaction mechanism requires the strategic removal of four protons via conserved hydrogen-bonding networks, but these pathways remain poorly understood. Site-directed mutagenesis has been used to study these pathways and the role of specific side chains, such as Lys317 of the D2 subunit. Previous studies showed that the D2-Lys317Ala substitution, which abolishes the flexible hydrogen-bonding -NH 3 + group, resulted in delayed O 2 release kinetics and diminished catalytic turnover, suggesting Lys317 has a crucial role in facilitating proton egress. Here, we investigated this proton egress pathway by determining the cryo-EM structure of PSII containing the D2-Lys317Ala substitution at a resolution of 1.97 Å. We observed that four new water molecules fill the space previously occupied by Lys317, but these waters lack specific water-protein interactions, leading to heterogeneity and suboptimal hydrogen bonding. We hypothesize that these waters negatively contribute to the existing hydrogen-bonding network and increase the entropic barrier for proton transfer. Additionally, we observed that a conserved chloride ion (Cl1), which is associated with Lys317, is unexpectedly maintained in D2-Lys317Ala PSII. However, unlike in wild-type, Cl1 has no measured effect on oxygen-evolution rates in D2-Lys317Ala PSII. This suggests that the role of Cl1 is dependent on the Lys317 amino group. These findings provide new insight into proton egress through the Cl1 hydrogen-bonding channel.
Organizational Affiliation:
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA.
Department of Chemistry, Yale University, New Haven, CT 06520, USA.
Department of Biochemistry, University of California, Riverside, CA 92521, USA. Electronic address: richard.debus@ucr.edu.
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA; Department of Chemistry, Yale University, New Haven, CT 06520, USA. Electronic address: gary.brudvig@yale.edu.
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