The strontium inorganic mutant of the water oxidizing center (CaMn₄O₅) of PSII improves WOC efficiency but slows electron flux through the terminal acceptors

Herein we extend prior studies of biosynthetic strontium replacement of calcium in PSII-WOC core particles to characterize whole cells. Previous studies of Thermosynechococcus elongatus found a lower rate of light-saturated O₂ from isolated PSII-WOC(Sr) cores and 5-8 × slower rate of oxygen release. We find similar properties in whole cells, and show it is due to a 20% larger Arrhenius activation barrier for O₂ evolution. Cellular adaptation to the sluggish PSII-WOC(Sr) cycle occurs in which flux through the Q_AQ_B acceptor gate becomes limiting for turnover rate in vivo. Benzoquinone derivatives that bind to Q_B site remove this kinetic chokepoint yielding 31% greater O₂ quantum yield (QY) of PSII-WOC(Sr) vs. PSII-WOC(Ca). QY and efficiency of the WOC(Sr) catalytic cycle are greatly improved at low light flux, due to fewer misses and backward transitions and 3-fold longer lifetime of the unstable S3 state, attributed to greater thermodynamic stabilization of the WOC(Sr) relative to the photoactive tyrosine Y_Z. More linear and less cyclic electron flow through PSII occurs per PSII-WOC(Sr). The organismal response to the more active PSII centers in Sr-grown cells at 45 °C is to lower the number of active PSII-WOC per Chl, producing comparable oxygen and energy per cell. We conclude that redox and protonic energy fluxes created by PSII are primary determinants for optimal growth rate of T. elongatus. We further conclude that the (Sr-favored) intermediate-spin S = 5/2 form of the S2 state is the active form in the catalytic cycle relative to the low-spin S = 1/2 form.