The Carbonyl Group in ß2 of the Carotenoid Tunes the Photocycle Kinetics in Orange Carotenoid Protein.

ABSTRACT

Adaptation to rapid environmental changes is crucial for maintaining optimal photosynthetic efficiency and is ultimately key to the survival of all photosynthetic organisms. Like most of them, cyanobacteria protect their photosynthetic apparatus against rapidly increasing light intensities by nonphotochemical quenching (NPQ). In cyanobacteria, NPQ is controlled by Orange Carotenoid Protein (OCP) photocycle. OCP is the only known photoreceptor that uses carotenoid for its light activation. How carotenoid drives and controls this unique photoactivation process is still unknown. However, understanding and potentially controlling the OCP photocycle may open up new possibilities for improving photosynthetic biomass. Here we investigate the effect of the carbonyl group in the ß2 ring of the carotenoid on the OCP photocycle. We report microsecond to minute OCP light activation kinetics and Arrhenius plots of the two OCP forms Canthaxanthin-bound OCP (OCPCAN) and echinenone-bound OCP (OCPECH). The difference between the two carotenoids is the presence of a carbonyl group in the ß2-ring located in the N-terminal domain of the protein. A combination of temperature-dependent spectroscopy, flash photolysis, and pump-probe transient absorption allows us to report the previously unresolved OCP intermediate associated primarily with the absorption bleach (OCPB). OCPB dominates the photokinetics in the µs to subms time range for OCPCAN and in the µs to ms range for OCPECH. We show that in OCPCAN the OCP photocycle steps are always faster than in OCPECH from 2 to almost 20 times depending on the step. These results suggest that the presence of the carbonyl group in the ß2-ring of the carotenoid accelerates the OCP photocycle.