The origins of photosynthetic systems: Clues from the phosphorus and sulphur chemical scenarios.

Photosynthesis is the predominant biochemical process of carbon dioxide assimilation in the biosphere. To reduce carbon dioxide into organic compounds, photosynthetic organisms have one or two distinct photochemical reaction centre complexes with which they capture solar energy and generate ATP and reducing power. The core polypeptides of the photosynthetic reaction centres show low homologies but share overlapping structural folds, overall architecture, similar functional properties and highly conserved positions in protein sequences suggesting a common ancestry. However, the other biochemical components of photosynthetic apparatus appear to be a mosaic resulting from different evolutionary trajectories. The current proposal focusses on the nature and biosynthetic pathways of some organic redox cofactors that participate in the photosynthetic systems: quinones, chlorophyll and heme rings and their attached isoprenoid side chains, as well as on the coupled proton motive forces and associated carbon fixation pathways. This perspective highlights clues about the involvement of the phosphorus and sulphur chemistries that would have shaped the different types of photosynthetic systems.