TY - JOUR T1 - Light activation of Orange Carotenoid Protein reveals initial C8’-C7’ double bond <em>trans/cis</em> isomerization JF - bioRxiv DO - 10.1101/2022.01.17.475858 SP - 2022.01.17.475858 AU - V.U. Chukhutsina AU - J.M. Baxter AU - A. Fadini AU - R.M. Morgan AU - M.A. Pope AU - K. Maghlaoui AU - C. Orr AU - A. Wagner AU - J.J. van Thor Y1 - 2022/01/01 UR - http://biorxiv.org/content/early/2022/03/02/2022.01.17.475858.abstract N2 - Orange Carotenoid protein (OCP) is the only known photoreceptor which uses carotenoid for its activation1,2. It is found exclusively in cyanobacteria, where it functions to control light-harvesting of the photosynthetic machinery. However, the photochemical reactions and structural dynamics of this unique photosensing process are not yet resolved. We present time-resolved crystal structures at second-to-minute delays under bright illumination, capturing the early photoproduct and structures of the subsequent reaction intermediates. The first stable photoproduct shows carotenoid trans/cis isomerization 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. Structural changes propagate to the C-terminal domain, resulting, at later time, in the H-bond rupture of the carotenoid keto group with protein residues. The isomerization and its transient nature are confirmed in OCP crystals and solution by FTIR and UV/Vis spectroscopy. This study reveals the isomerization of the carotenoid and subsequent thermal structural reactions as the basis of OCP photoreception. Understanding and potentially controlling the OCP dynamics offers the prospect of novel applications in biomass engineering3 as well as in optogenetics and bioimaging.Competing Interest StatementThe authors have declared no competing interest. ER -