Mechanism of Proton Release during Water Oxidation in Photosystem II

Abstract

Photosystem II (PSII) catalyzes the light-driven water oxidation that releases dioxygen into our atmosphere and provides the electrons needed for the synthesis of biomass. The catalysis occurs in the oxygen-evolving oxo-manganese-calcium (Mn4O5Ca) cluster that drives the stepwise oxidation and deprotonation of substrate water molecules leading to the O2 formation. However, despite recent advances, the mechanism of these reactions remains unclear and much debated. Here we show that the light-driven Tyr161D1 oxidation adjacent to the Mn4O5Ca cluster, significantly decreases the barrier for proton transfer from the putative substrate water molecule (W3/Wx) to Glu310D2, which is accessible to the luminal bulk. By combining hybrid quantum/classical (QM/MM) free energy calculations with atomistic molecular dynamics (MD) simulations, we probe the energetics of the proton transfer along the Cl1 pathway. We demonstrate that the proton transfer occurs via water molecules and a cluster of conserved carboxylates, driven by redox-triggered electric fields directed along the pathway. Glu65D1 establishes a local molecular gate that controls the proton transfer to the luminal bulk, whilst Glu312D2 acts as a local proton storage site. The identified gating region could be important in preventing back-flow of protons to the Mn4O5Ca cluster. The structural changes, derived here based on the dark-state PSII structure, strongly support recent time-resolved XFEL data of the S3[->]S4 transition (Nature 617, 2023), and reveal the mechanistic basis underlying deprotonation of the substrate water molecules. Our combined findings provide insight into the water oxidation mechanism of PSII and show how the interplay between redox-triggered electric fields, ion-pairs, and hydration effects control proton transport reactions. Significance StatementPhotosystem II is natures water splitting enzyme that produces the oxygen in the atmosphere and drives the synthesis of biomass. The water splitting reaction releases protons to the luminal bulk contributing to the protonmotive force that drives the synthesis of ATP. Key mechanistic principles of the light-driven water splitting reaction remain debated, amongst them the catalytically important deprotonation steps. Here we show how the oxygen-evolving oxo-manganese-calcium cluster transports protons via conserved carboxylates and water molecules in proton arrays that lead to the luminal bulk. We identify a local proton storage site and molecular gates that prevent wasteful back reactions by undergoing conformational changes, and we show how electric field effects control the protonation dynamics in Photosystem II.