Membrane-inlet mass spectrometry reveals a high driving force for oxygen production by photosystem II

Abstract

Oxygenic photosynthesis is the basis for aerobic life on earth. The catalytic Mn4OxCaYZ center of photosystem II (PSII), after fourfold oxidation, extracts four electrons from two water molecules to yield dioxygen. This reaction cascade has appeared as a single four-electron transfer that occurs in typically 1 ms. Inevitable redox intermediates have so far escaped detection, probably because of very short lifetime. Previous attempts to stabilize intermediates by high O 2-back pressure have revealed controversial results. Here we monitored by membrane-inlet mass spectrometry (MIMS) the production of 18O2 from 18O-labeled water against a high background of 16O2 in a suspension of PSII-core complexes. We found neither an inhibition nor an altered pattern of O2 production by up to 50-fold increased concentration of dissolved O2. Lack of inhibition is in line with results from previous X-ray absorption and visible-fluorescence experiments, but contradictory to the interpretation of previous UV-absorption data. Because we used essentially identical experimental conditions in MIMS as had been used in the UV work, the contradiction was serious, and we found it was not to be resolved by assuming a significant slowdown of the O2 release kinetics or a subsequent slow conformational relaxation. This calls for reevaluation of the less direct UV experiments. The direct detection of O2 release by MIMS shows unequivocally that O2 release in PSII is highly exothermic. Under the likely assumption that one H+ is released in the S4 → S0 transition, the driving force at pH 6.5 and atmospheric O2 pressure is at least 220 meV, otherwise 160 meV.