The Multiple Roles of the Protein in the Photoactivation of Orange Carotenoid Protein

During their life, photosynthetic organisms are exposed to widely varying light conditions, which can not only compromise the light harvesting function but also induce photodamage. To adapt, the various organisms have developed different regulatory mechanisms that allow them to modulate the amount of energy harvested and dissipate the excess in heat. In cyanobacteria, photoregulation is mediated by the orange carotenoid protein (OCP), a protein that non-covalently binds a carotenoid chromophore. When photoactivated, OCP opens up into two separated but linked domains allowing the carotenoid to translocate in one domain and to interact with the light-harvesting antenna. Here, we have designed a multiscale computational approach to reveal the molecular origin of the different spectral features observed in the active and inactive forms of OCP and explain their thermodynamic stability. This understanding is fundamental to obtaining a detailed picture of the photoactivation mechanism.