The terminal reaction cascade of water oxidation: Proton and oxygen release

In cyanobacteria, algae and plants Photosystem II produces the oxygen we breathe. Driven and clocked by light quanta, the catalytic Mn4Ca-tyrosine centre accumulates four oxidising equivalents before it abstracts four electrons from water, liberating dioxygen and protons. Aiming at intermediates of the terminal four-electron cascade, we previously have suppressed this reaction by elevating the oxygen pressure, thereby stabilising one redox intermediate. Here, we established a similar suppression by increasing the proton concentration. Data were analysed in terms of only one (peroxy) redox intermediate between the fourfold oxidised Mn4Ca-tyrosine centre and oxygen release. The surprising result was that the release into the bulk of one proton per dioxygen is linked to the first and rate-limiting electron transfer in the cascade rather than to the second which produces free oxygen. The penultimate intermediate might thus be conceived as a fully deprotonated peroxy-moiety.