Structure of Photosystem I with SOD reveals coupling between energy conversion and oxidative stress protection

Abstract

In photosynthetic organisms, Photosystem I (PSI) not only converts energy into its chemical form but is also a major source of superoxide (O2-), which can damage cellular components. Its detoxification by superoxide dismutase (SOD) is essential for redox homeostasis. We report the structure of the PSI-SOD supercomplex from Chromera velia (C. velia), revealing a direct coupling between energy conversion and oxidative stress protection. The heterodimeric SOD comprises a catalytic SOD2 and a non-catalytic SOD1 anchoring it to the conserved stromal PSI surface near the ferredoxin-binding site with four contact areas. Structural comparison with plants and algae shows that SOD could occupy a similar position via interactions with PsaD. C. velia PSI displays several distinct features, including a split subunit PsaA linked by PsaV, a unique iso-fucoxanthin bound to PsaB, and an extended PsaF coordinating an extra chlorophyll in the membrane. The antenna includes five fucoxanthin-chlorophyll family proteins (FCPs) stabilized by adaptations unique to each antenna protein. These findings reveal the structural basis for the integration of a photoprotective mechanism with energy metabolism in photosynthetic membranes.