Shutova Tatiana, Kenneweg Hella, Buchta Joachim, Nikitina Julia, Terentyev Vasily, Chernyshov Sergey, Andersson Bertil, Allakhverdiev Suleyman I, Klimov Vyacheslav V, Dau Holger, Junge Wolfgang, Samuelsson Göran
Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå, Sweden.
EMBO J. 2008 Mar 5;27(5):782-91. doi: 10.1038/emboj.2008.12. Epub 2008 Jan 31.
Water oxidation in photosystem II (PSII) is still insufficiently understood and is assumed to involve HCO(3)(-). A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O(2) evolution in the absence of HCO(3)(-). The O(2) evolution for saturating, continuous illumination (R(O2)) was slower than in the wild type, but was elevated by HCO(3)(-) and increased further by Cah3. The R(O2) limitation in the absence of Cah3/HCO(3)(-) was amplified by H(2)O/D(2)O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO(3)(-) in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO(3)(-) effect at the donor side of PSII. Time-resolved delayed fluorescence and O(2)-release measurements suggest specific effects on proton-release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO(3)(-), which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O(2) formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.
光系统II（PSII）中的水氧化过程仍未得到充分理解，且被认为涉及HCO₃⁻。一种缺乏与PSII供体侧相关的碳酸酐酶的衣藻突变体，在没有HCO₃⁻的情况下，其氧气释放受损。饱和连续光照下的氧气释放速率（R(O₂)）比野生型慢，但在添加HCO₃⁻后升高，添加Cah3后进一步增加。在没有Cah3/HCO₃⁻的情况下，R(O₂)的限制因H₂O/D₂O交换而加剧，但可被两亲性质子载体缓解，这表明Cah3/HCO₃⁻在质子转运中起作用。叶绿素荧光表明Cah3/HCO₃⁻在PSII供体侧有影响。时间分辨延迟荧光和氧气释放测量表明，其对质子释放步骤有特定影响，但对电子转移没有影响。我们提出，Cah3通过局部提供HCO₃⁻促进质子从锰复合物中去除，HCO₃⁻可能起到质子载体的作用。没有Cah3时，质子去除可能在氧气形成过程中成为限速步骤，从而限制高光下的水氧化。我们的结果揭示了水氧化过程中PSII供体侧质子释放的普遍重要性。
