The inhibitory effects of acidification and augmented oxygen pressure on water oxidation.

Cyanobacteria, algae, and plants produce dioxygen from water. Driven and clocked by light quanta, the catalytic Mn(4)Ca Tyrosine centre accumulates four oxidizing equivalents before it abstracts four electrons from water and liberates dioxygen and protons. Intermediates of this reaction cascade are short-lived (<100 micros) and difficult to detect. By application of high oxygen pressure to cyanobacterial PSII-core-complexes, we have previously suppressed the transition from the highest oxidation state of the centre to the lowest by stabilizing a (peroxy) intermediate. Here, we investigated the inhibitory interplay of acidification and augmented oxygen pressure. Starting from pH 6.5, acidification increasingly inhibited the reduction of the highest oxidized state and resulted in a lower oxygen partial pressure for half inhibition. Oxygen and proton interfere with different steps of the reaction cascade.