Light activation of Orange Carotenoid Protein reveals initial C8′-C7′ double bond trans/cis photoisomerization

Orange Carotenoid protein (OCP) is the only known photoreceptor which uses carotenoid for its activation. It is found exclusively in cyanobacteria, where it functions to control light-harvesting of the photosynthetic machinery. The mechanism of photoactivation includes H-bond rupture between the carotenoid keto group and the protein moiety in the C-terminal domain (CTD) but details of the primary photochemical reactions and structural dynamics are not yet resolved. Here we present data from second-to-minute time-resolved crystallography under bright illuminations capturing the primary photoproduct and structure of subsequent reaction intermediates. The initial photoproduct shows carotenoid trans/cis photoisomerization at the C7′-C8′ double bond and structural changes in the N-terminal domain with minute timescale kinetics. These are followed by a thermally-driven cis/trans isomerization that recovers to the dark state carotenoid configuration. In addition, the structural changes are propagated to the CTD resulting in the eventual H-bond rupture. The photoisomerization and its transient nature are confirmed in OCP crystals and solutions by FTIR and UV/VIS spectroscopy. This study reveals the photoisomerization of the carotenoid and subsequent thermal structural reactions as the basis primary events of OCP photoreception. It is an initial step in understanding and potentially controlling OCP dynamics, offering the prospect of novel applications in biomass engineering as well as advancing optogenetics and bioimaging. ### Competing Interest Statement The authors have declared no competing interest.