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: Photoprotection; Chromophore; Canthaxanthin; Photoexcitation

Orange carotenoid protein (OCP) is a photoactive that mediates photoprotection in cyanobacteria. OCP binds different ketocarotenoid chromophores such as echinenone (ECN), 3′- hydroxyechinenone (hECN), and canthaxanthin (CAN). In the dark, an inactive orange form known OCPO; upon illumination, red active state formed, referred to OCPR, can interact with phycobilisome. Large gaps still exist mechanistic understanding of events between photon absorption formation OCPR state. Recent studies suggested more than one may be absorbed during photocycle. Using two-pulse excitation setup variable time delays pulses, we demonstrate canthaxanthin-functionalized OCPO forms signature after single photon. By contrast, complexed hECN or ECN does not photoconvert absorption. Instead, formed only second arriving roughly first one, implying existence metastable light-sensitive OCP1hv intermediate. To best our knowledge, sequential 2-photon mechanism biological photoreceptor chromophore unique. It results nonlinear response function respect light intensity, effectively generating threshold switch. case OCP, this prevents down regulation photosynthesis at low irradiance.