In photosystem I (PSI) complexes at room temperature electron transfer from A1- to FX is an order of magnitude faster on the B-branch compared to the A-branch. One factor that might contribute to this branch asymmetry in time constants is TrpB673 (Thermosynechococcus elongatus numbering), which is located between A1B and FX. The corresponding residue on the A-branch, between A1A and FX, is GlyA693. Here, microsecond time-resolved step-scan FTIR difference spectroscopy at 77 K has been used to study isolated PSI complexes from wild type and TrpB673Phe mutant (WB673F mutant) cells from Synechocystis sp. PCC 6803. WB673F mutant cells require glucose for growth and are light sensitive. Photoaccumulated FTIR difference spectra indicate changes in amide I and II protein vibrations upon mutation of TrpB673 to Phe, indicating the protein environment near FX is altered upon mutation. In the WB673F mutant PSI samples, but not in WT PSI samples, the phylloquinone molecule that occupies the A1 binding site is likely doubly protonated following long periods of repetitive flash illumination at room temperature. PSI with (doubly) protonated quinone in the A1 binding site are not functional in electron transfer. However, electron transfer functionality can be restored by incubating the light-treated mutant PSI samples in the presence of added phylloquinone.

中文翻译：

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光系统I中电子转移的可逆抑制和激活。

在室温下，在光系统I（PSI）中，电子从A1-转移到FX的速度比A分支快一个数量级。可能导致时间常数分支不对称的一个因素是TrpB673（伸长热菌球菌编号），它位于A1B和FX之间。A分支与FX之间的A分支上的相应残基为GlyA693。在这里，微秒时间分辨的阶跃扫描FTIR差异光谱在77 K下已用于研究来自野生型的分离的PSI复合物和来自集胞藻属的TrpB673Phe突变体（WB673F突变体）细胞。PCC6803。WB673F突变细胞需要葡萄糖才能生长，并且对光敏感。光积累的FTIR差异光谱表明，在TrpB673突变为Phe后，酰胺I和II蛋白振动会发生变化，表明FX附近的蛋白质环境在突变后发生了变化。在WB673F突变PSI样品中，但在WT PSI样品中则没有，占据A1结合位点的叶醌分子在室温下长时间重复闪光后可能会双重质子化。在A1结合位点带有（双）质子化醌的PSI在电子转移中不起作用。但是，通过在添加的叶醌存在下孵育经过光处理的突变PSI样品，可以恢复电子转移功能。在A1结合位点带有（双）质子化醌的PSI在电子转移中不起作用。但是，通过在添加的叶醌存在下孵育经过光处理的突变PSI样品，可以恢复电子转移功能。在A1结合位点带有（双）质子化醌的PSI在电子转移中不起作用。但是，通过在添加的叶醌存在下孵育经过光处理的突变PSI样品，可以恢复电子转移功能。