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 nonphotochemical quenching 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 photoinduced 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, but the OCP^R dimer forms a more stable and elongated shape than the OCP^O dimer. The TG measurements revealed that upon photoexcitation, OCP^O monomers undergo two structural transitions before associating into OCP^R dimers. In contrast, the OCP^O dimer dissociates prior to the structural rearrangement, highlighting a fundamental difference in their reaction pathways. Moreover, the dimerization was found to moderately reduce the photoreactivity of OCP^O compared to the monomer. We also found that apo-OCP readily forms a heterodimer with OCP^R, potentially altering reaction pathways and changing intrinsic kinetic behavior.