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 Mn 4 O x CaY Z 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 18 O 2 from 18 O-labeled water against a high background of 16 O 2 in a suspension of PSII-core complexes. We found neither an inhibition nor an altered pattern of O 2 production by up to 50-fold increased concentration of dissolved O 2 . 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 O 2 release kinetics or a subsequent slow conformational relaxation. This calls for reevaluation of the less direct UV experiments. The direct detection of O 2 release by MIMS shows unequivocally that O 2 release in PSII is highly exothermic. Under the likely assumption that one H þ is released in the S 4 → S 0 transition, the driving force at pH 6.5 and atmospheric O 2 pressure is at least 220 meV, otherwise 160 meV.water oxidation | oxygen evolution | isotope-ratio mass spectrometry | bioenergetics O xygenic photosynthesis is the metabolic basis for the most successful forms of life on earth. In cyanobacteria, algae and plants photosystem II (PSII) uses sunlight to split water into dioxygen and reducing equivalents, and it generates proton motive force. The catalytic centre of O 2 production in PSII, coined the OEC (oxygen evolving complex), comprises the manganese-oxygen-calcium (Mn 4 O x Ca) complex and its ligands, which include two substrate "water" molecules of undefined protonation state. It also includes a redox-active tyrosine residue, coined tyrosine ZðY Z Þ, which is the essential electron transfer link to the photoactive reaction center of PSII. Over 40 years ago Kok et al. (1) proposed-on the basis of flash-induced O 2 -evolution patterns (2)-that PSII, clocked and driven by four quanta of light, cycles through five different redox states, named S i (i ¼ 0;…;4), where i is the number of stored oxidizing equivalents. Once four oxidizing equivalents are accumulated O 2 is produced in the spontaneous reaction from S 4 to yield S 0 .In this S i state transition four electrons are transferred from two bound substrate water molecules that were partially or fully deprotonated (m ¼ 0-2) during the S i state cycle. In addition, one or two new substrate water molecules (n ¼ 1;2) bin...

Misses during Water Oxidation in Photosystem II Are S State-dependent

“…They were always more efficient than the S 2 3 S 3 transition. This is not entirely surprising because recent studies have shown that O 2 release during the S 3 3 S 0 transition is not limited thermodynamically and highly exothermic, indicating that possibly this is not the most difficult transition in the S cycle (48,49).…”

Thermodynamic Limitations of Photosynthetic Water Oxidation at High Proton Concentrations

“…The influence of elevated O 2 concentrations has been investigated previously (15,16). In time-resolved x-ray experiments, no inhibition of the O 2 evolution transition (S 3 3 S 0 ϩ O 2 ) was detected, up to pO 2 of 16 bar, suggesting that the Gibbs free energy of any reaction intermediate is Ͼ130 meV higher than for the product state (S 0 ϩ O 2 ) (16), as recently confirmed by mass spectroscopy (17) and for intact cells (18). (Here and in the following, we use, for simplicity, the term free energy or ⌬G instead of standard free energy or ⌬G 0 .)…”

Algal evolution in relation to atmospheric CO 2 : carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles

“…The presence of O 2 does not, however, inhibit O 2 production by oxygenic photosynthetic organisms [81,82]. The accumulation of O 2 in the habitats of oxygenic photosynthetic organisms using Rubisco as their autotrophic carboxylase permits Rubisco oxygenase activity to occur, provided the CO 2 concentration is below saturation for Rubisco.…”