Castillo Leslie, Nicolaou Stavroula, Gates Colin
Department of Chemistry and Biochemistry, Loyola University Chicago, Chicago, IL, United States.
Department of Biology, Loyola University Chicago, Chicago, IL, United States.
Front Plant Sci. 2025 Jun 16;16:1584909. doi: 10.3389/fpls.2025.1584909. eCollection 2025.
Efficient photosynthetic light reactions require tight balancing of electrons and protons. In photosystem II, bicarbonate is coordinated to a non-heme iron positioned between the acceptor-side plastoquinones Q and Q, modulating electron transfer. The hypercarbonate-requiring filamentous cyanobacterium has had multiple bicarbonate functions on both acceptor and donor side of PSII determined by depletion. 77K spectrofluorometric investigation of phycobilin and chlorophyll excitation suggests the mild depletion method for bicarbonate results in loss of chlorophyll connectivity to the reaction center in PSII and dissociation of the phycobilisome due to proportional increase of fluorescence emission from allophycocyanin. Using chlorophyll fast repetition rate fluorometry, it was observed under bicarbonate depletion that oscillations were still present in a fraction of PSIIs, confirming the functionality of the water oxidizing complex remains in this fraction of centers. In these fractions of centers only one to two electrons are being released. Q reoxidation kinetics indicate that loss of bicarbonate permits successful electron transfer to Q, forming semiquinone Q . DCMU inhibition of the second electron transfer implies that, in bicarbonate-depleted , electron transfer to Q affects proton delivery needed for plastoquinol formation, which suggests that this depletion targets the non-heme iron. Based on cytochrome bf redox kinetics, depleted cells experience less intensity of oxidation and upon illumination cytochrome b and f are proportionally rapidly and intensely oxidized. P700 redox kinetics exhibit a delay feature in PSI as well as the expected delay of electron delivery to PSII, suggesting a further bicarbonate effect on PSI.
高效的光合光反应需要电子和质子的紧密平衡。在光系统II中，碳酸氢根与位于受体侧质体醌Q和Q之间的非血红素铁配位，调节电子转移。通过缺失实验确定，需要高浓度碳酸氢根的丝状蓝细菌在光系统II的受体侧和供体侧都具有多种碳酸氢根功能。对藻胆素和叶绿素激发的77K光谱荧光研究表明，轻度缺失碳酸氢根的方法会导致光系统II中叶绿素与反应中心的连接丧失，以及藻胆体的解离，这是由于别藻蓝蛋白荧光发射的比例增加所致。使用叶绿素快速重复率荧光测定法，在缺失碳酸氢根的情况下观察到，一部分光系统II中仍存在振荡，这证实了水氧化复合物在这部分中心中仍保持功能。在这些中心部分中，仅释放一到两个电子。Q再氧化动力学表明，碳酸氢根的缺失允许电子成功转移至Q，形成半醌Q。二氯苯基二甲基脲（DCMU）对第二次电子转移的抑制作用表明，在缺失碳酸氢根的情况下，电子向Q的转移会影响质体醌醇形成所需的质子传递，这表明这种缺失作用于非血红素铁。基于细胞色素bf的氧化还原动力学，缺失碳酸氢根的细胞氧化强度较低，光照后细胞色素b和f会按比例迅速且强烈地被氧化。P700氧化还原动力学在光系统I中表现出延迟特征，同时也出现了预期的向光系统II传递电子的延迟，这表明碳酸氢根对光系统I还有进一步的影响。
