The Catalytic Cycle of Water Oxidation in Crystallized Photosystem II Complexes: Performance and Requirements for Formation of Intermediates

Crystals of photosystem II (PSII) contain the most homogeneous copies of the water-oxidizing reaction center where O₂ is evolved (WOC). However, few functional studies of PSII operation in crystals have been carried out, despite their widespread use in structural studies. Here we apply oximetric methods to determine the quantum efficiency and lifetimes of intermediates of the WOC cycle as a function of added electron acceptors (quinones and ferricyanide), both aerobically and anaerobically. PSII crystals exhibit the highest quantum yield of O₂ production yet observed of any native or isolated PSII (61.6%, theoretically 59 000 μmol O₂/mg Chl/h). WOC cycling can be sustained for thousands of turnovers only using an electron acceptor (quinones, ferricyanide, etc.). Simulations of the catalytic cycle identify four distinct photochemical inefficiencies in both PSII crystals and dissolved PSII cores that are nearly the same magnitude. The exogenous acceptors equilibrate with the native plastoquinone acceptor at the Q_B (or Q_C) site(s), for which two distinct redox couples are observable that regulate flux through PSII. Flux through the catalytic cycle of water oxidation is shown to be kinetically restricted by the Q_AQ_B two-electron gate. The lifetimes of the S2 and S3 states are greatly extended (especially S2) by electron acceptors and depend on their redox reversibility. PSII performance can be pushed in vitro far beyond what it is capable of in vivo. With careful use of precautions and monitoring of populations, PSII microcrystals enable the exploration of WOC intermediates and the mechanism of catalysis.