Two-Photon-Driven Photoprotection Mechanism in Echinenone-Functionalized Orange Carotenoid Protein

• DOI: 10.1021/jacs.4c13341
• Publication Date: 2025-01-21T18:41:53Z
• Authors: Stanisław Niziński; Elisabeth Hartmann; Robert L. Shoeman; Mirosław Tarnawski; Adjélé Wilson; Jochen Reinstein; Diana Kirilovsky; Michel Sliwa; Gotard Burdziński; Ilme Schlichting
• Keywords: Kinetics; [CHIM.THEO] Chemical Sciences/Theoretical and/or physical chemistry; Light; Irradiation; Light absorption; Absorption

[Image: see text] Orange carotenoid protein (OCP) is a photoactive protein that mediates photoprotection in cyanobacteria. OCP binds different ketocarotenoid chromophores such as echinenone (ECN), 3′- hydroxyechinenone (hECN), and canthaxanthin (CAN). In the dark, OCP is in an inactive orange form known as OCP(O); upon illumination, a red active state is formed, referred to as OCP(R), that can interact with the phycobilisome. Large gaps still exist in the mechanistic understanding of the events between photon absorption and formation of the OCP(R) state. Recent studies suggested that more than one photon may be absorbed during the photocycle. Using a two-pulse excitation setup with variable time delays between the pulses, we demonstrate that canthaxanthin-functionalized OCP(O) forms the OCP(R) signature after absorption of a single photon. By contrast, OCP(O) complexed with hECN or ECN does not photoconvert to OCP(R) upon single photon absorption. Instead, OCP(R) is formed only upon absorption of a second photon arriving roughly one second after the first one, implying the existence of a metastable light-sensitive OCP(1hv) intermediate. To the best of our knowledge, a sequential 2-photon absorption mechanism in a single biological photoreceptor chromophore is unique. It results in a nonlinear response function with respect to light intensity, effectively generating a threshold switch. In the case of OCP, this prevents down regulation of photosynthesis at low light irradiance.