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 (OCPO) and light-adapted (OCPR) 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 OCPO and OCPR exist in monomer-dimer equilibria, with OCPR forming more stable and elongated dimers. TG measurements revealed that upon photoexcitation, OCPO monomers undergo two structural transitions before associating into OCPR dimers. In contrast, OCPO 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 OCPO compared to the monomer. We also found that apo-OCP readily forms heterodimers with OCPR, potentially altering reaction pathways and masking true kinetic behavior.