Photoprotection is essential for cyanobacteria to prevent light-induced damage, and the orange carotenoid protein (OCP) plays a central role in this process. Although the structural changes associated with OCP photoactivation have been characterized, the underlying molecular mechanisms have remained elusive. Using atomistic simulations that integrate excited-state dynamics and enhanced sampling techniques, we uncover a new photoactivation mechanism that requires a trans-to-cis photoisomerization of the bound carotenoid. We show that the presence of the cis isomer enables the subsequent steps of the activation mechanisms, in addition to matching experimental spectroscopic signals. Moreover, we uncover a multiphoton mechanism responsible for regenerating the all-trans configuration observed in the active form of OCP. These findings provide a mechanistic framework for OCP photoactivation, linking photochemistry with large-scale conformational changes.
Arcidiacono, A., Bondanza, M., Cupellini, L. & Mennucci, B.
Atomistic simulations reveal the photoactivation mechanism of a carotenoid-binding photoreceptor
PNAS, 123 (11) e2515214123 (2026)
Read the full paper here: https://doi.org/10.1073/pnas.2515214123
