期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2019
卷号：116
期号：39
页码：19458-19463
DOI：10.1073/pnas.1910675116
出版社：The National Academy of Sciences of the United States of America
摘要：Photosystem II (PSII), the light-driven water/plastoquinone photooxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH 2 ), is released into the membrane from the Q B site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the Q B site. Despite their functional importance, the thermodynamic properties of the PQ in the Q B site, Q B , in its different redox forms have received relatively little attention. Here we report the midpoint potentials ( E m ) of Q B in PSII from Thermosynechococcus elongatus using electron paramagnetic resonance (EPR) spectroscopy: E m Q B /Q B •− ≈ 90 mV, and E m Q B •− /Q B H 2 ≈ 40 mV. These data allow the following conclusions: 1) The semiquinone, Q B •− , is stabilized thermodynamically; 2) the resulting E m Q B /Q B H 2 (∼65 mV) is lower than the E m PQ/PQH 2 (∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for Q B H 2 release into the pool; 3) PQ is ∼50× more tightly bound than PQH 2 ; and 4) the difference between the E m Q B /Q B •− measured here and the E m Q A /Q A •− from the literature is ∼234 meV, in principle corresponding to the driving force for electron transfer from Q A •− to Q B . The pH dependence of the thermoluminescence associated with Q B •− provided a functional estimate for this energy gap and gave a similar value (≥180 meV). These estimates are larger than the generally accepted value (∼70 meV), and this is discussed. The energetics of Q B in PSII are comparable to those in the homologous purple bacterial reaction center..
关键词：photosynthesis ; redox potential ; proton;coupled electron transfer ; photoinhibition
