Structural Insights and Challenges in the Supercomplexes Formed by Photosystem I in Phototrophs.

Photosystem I (PSI) is a central component of photosynthesis, driving essential processes such as light energy conversion and energy metabolism. This review addresses several knowledge gaps regarding PSI by providing comprehensive insights into the structural diversity of PSI supercomplexes across various evolutionary groups, including cyanobacteria, algae and land plants. It clarifies the oligomerization states of PSI and its interactions with light-harvesting complexes (LHCs) and other protein complexes such as NDH and Cyt bf. Environmental factors, including light intensity, iron availability and pH, significantly influence PSI's structure and function. These factors drive PSI's adaptability through conformational changes and the formation of specialized supercomplexes. For example, under iron deficiency, cyanobacteria form PSI-IsiA complexes to compensate for reduced PSI content. The diversity of PSI's light-harvesting antenna components, such as Lhca proteins in land plants and Lhcr proteins in red algae, is crucial for optimizing light absorption and energy transfer under varying light conditions. Structural evidence also supports the existence of PSI-PSII supercomplexes, revealing direct interactions that facilitate energy transfer between photosystems and protect them from photodamage. These findings highlight the evolutionary significance of PSI's structural diversity and its role in photosynthetic efficiency and environmental adaptation.

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