Kinetic Insights into Photoinduced Monomer–Dimer Conversion and Activation of Orange Carotenoid Protein

The Orange Carotenoid Protein (OCP) is a blue-green light sensor that regulates non-photochemical quenching (NPQ) in cyanobacteria through reversible transitions between its dark-adapted (OCP ^O ) and light-adapted (OCP ^R ) states. Despite extensive studies, the detailed reaction scheme remains unclear. In this study, we examined the photo-induced reaction dynamics of OCP using size-exclusion chromatography (SEC), small-angle X-ray scattering (SAXS), and transient grating (TG) spectroscopy. We found that OCP ^O and OCP ^R exist in monomer–dimer equilibria, with OCP ^R forming more stable and elongated dimers. TG measurements revealed that upon photoexcitation, OCP ^O monomers undergo two structural transitions before associating into OCP ^R dimers. In contrast, OCP ^O dimers dissociate prior to the structural rearrangement, highlighting a fundamental difference in their reaction pathways. Moreover, dimerization was found to moderately reduce the photo-reactivity of OCP ^O compared to the monomer. We also found that apo-OCP readily forms heterodimers with OCP ^R , potentially altering reaction pathways and masking true kinetic behavior.