Molecular Activation Mechanism and Structural Dynamics of Orange Carotenoid Protein

Like most photosynthetic organisms, cyanobacteria are vulnerable to fluctuations in light intensity, which can damage their machinery. To protect against this, they use a photoprotective mechanism called non-photochemical quenching (NPQ), where excess absorbed photo-energy is dissipated as heat. In cyanobacteria, activation of Orange Carotenoid Protein (OCP) the critical first step NPQ response. OCP also only known photosensitive protein, uses carotenoid for its activation. We summarize current knowledge on induced reactions OCP; different mechanisms that have been proposed; photocycle kinetics and characteristics; reported structural intermediates. discuss possible interpretations experimental results, we formulate important open questions directions future work, reveal molecular basis photosensing by OCP.