Photosynthetic O₂ evolution is catalyzed by the Mn₄CaO₅ cluster of the water oxidation complex of the photosystem II (PSII) complex. The photooxidative self-assembly of the Mn₄CaO₅ cluster, termed photoactivation, utilizes the same highly oxidizing species that drive the water oxidation in order to drive the incorporation of Mn²⁺ into the high-valence Mn₄CaO₅ cluster. This multistep process proceeds with low quantum efficiency, involves a molecular rearrangement between light-activated steps, and is prone to photoinactivation and misassembly. A sensitive polarographic technique was used to track the assembly process under flash illumination as a function of the constituent Mn²⁺ and Ca²⁺ ions in genetically engineered membranes of the cyanobacterium Synechocystis sp. PCC6803 to elucidate the action of Ca²⁺ and peripheral proteins. We show that the protein scaffolding organizing this process is allosterically modulated by the assembly protein Psb27, which together with Ca²⁺ stabilizes the intermediates of photoactivation, a feature especially evident at long intervals between photoactivating flashes. The results indicate three critical metal-binding sites: two Mn and one Ca, with occupation of the Ca site by Ca²⁺ critical for the suppression of photoinactivation. The long-observed competition between Mn²⁺ and Ca²⁺ occurs at the second Mn site, and its occupation by competing Ca²⁺ slows the rearrangement. The relatively low overall quantum efficiency of photoactivation is explained by the requirement of correct occupancy of these metal-binding sites coupled to a slow restructuring of the protein ligation environment, which are jointly necessary for the photooxidative trapping of the first stable assembly intermediate.

光合作用II（PSII）络合物的水氧化络合物的Mn ₄ CaO ₅簇催化光合O ₂的释放。Mn ₄ CaO ₅团簇的光氧化自组装，称为光活化，利用驱动水氧化的相同高氧化性物质来驱动Mn ²⁺掺入高价Mn ₄ CaO _5中簇。该多步骤过程以低量子效率进行，涉及光激活步骤之间的分子重排，并且易于光灭活和组装错误。灵敏的极谱技术被用来跟踪在闪烁照明下的组装过程，该过程是蓝藻集胞藻属（Sychochocystis sp。）的基因工程膜中的组成Mn ²⁺和Ca ²⁺离子的函数。PCC6803阐明Ca ²⁺和外围蛋白的作用。我们显示，组织此过程的蛋白质支架被装配蛋白Psb27与Ca ²⁺一起变构调节。稳定了光活化中间体，这种功能在光活化闪光之间的间隔较长时尤其明显。结果表明，三个关键的金属结合位点：两个Mn和一个Ca，Ca ²⁺占据Ca的位点对于抑制光灭活至关重要。长期观察到的Mn ²⁺和Ca ²⁺之间的竞争发生在第二个Mn位点，并且通过竞争Ca ²⁺对其进行占领减慢了重新排列的速度。相对低的总体光激活量子效率可以通过正确占据这些金属结合位点以及缓慢地重组蛋白质连接环境来解释，这对于第一稳定组装中间体的光氧化捕获是必不可少的。