Abstract
Interquinone QA− → QB electron-transfer (ET) in isolated photosystem II reaction centers (PSII-RC) is protein-gated. The temperature-dependent gating frequency “k” is described by the Eyring equation till levelling off at T ≥ 240 °K. Although central to photosynthesis, the gating mechanism has not been resolved and due to experimental limitations, could not be explored in vivo. Here we mimic the temperature dependency of “k” by enlarging VD1-208, the volume of a single residue at the crossing point of the D1 and D2 PSII-RC subunits in Synechocystis 6803 whole cells. By controlling the interactions of the D1/D2 subunits, VD1-208 (or 1/T) determines the frequency of attaining an ET-active conformation. Decelerated ET, impaired photosynthesis, D1 repair rate and overall cell physiology upon increasing VD1-208 to above 130 Å3, rationalize the >99% conservation of small residues at D1-208 and its homologous motif in non-oxygenic bacteria. The experimental means and resolved mechanism are relevant for numerous transmembrane protein-gated reactions.
Highlights
PSII-RCs and RCs of green nonsulfur bacteria (Chloroflexi), purple bacteria and the newly discovered photosynthetic Gemmatimonadetes[1], are Type-II RCs responsible for light-induced charge separation across the photosynthetic membrane[2]
In vitro RCs that were cooled to cryogenic temperatures under light, continued to perform light-induced ET while those cooled in the dark did not, an observation made first in non-oxygenic Type-II RCs and termed the Kleinfeld effect[20]
To resolve the mechanism by which D1-208 Vres affects the thermodynamics and kinetics of Q−A →QB ET, we explored the freedom of motion and binding interactions between the d1 and d2 helices mutated at D1-208 using molecular dynamics (MD) simulations
Summary
PSII-RCs and RCs of green nonsulfur bacteria (Chloroflexi), purple bacteria (phototrophic Proteobacteria) and the newly discovered photosynthetic Gemmatimonadetes[1], are Type-II RCs responsible for light-induced charge separation across the photosynthetic membrane[2]. The temperature-dependent Q−A → QB ET rate has been considered evidence for a protein-gated mechanism in both oxygenic[8,11,16] and non-oxygenic[12,17,18,19] Type-II RCs. The first reduction of QB by QA− involves a protein conformational change that brings the reactants from a non-active (dark) to the transient and favorable active (light) ET conformation, followed by rapid electron tunneling from QA to QB, accompanied by protonation[7,10,12,13]. We first set out to find a means of mimicking the temperature effect on protein conformations and the resulting QA− →QB ET rate in vivo, without cooling the cells Once establishing such a model, the impact of “temperature-like” perturbations on light-induced ET, photosynthetic machinery and whole cell physiology could be quantitatively monitored
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