Two-photon driven photoprotection mechanism in echinenone-functionalized Orange Carotenoid Protein

Abstract

AbstractOrange 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 OCPO; upon illumination, a red active state is formed, referred to as OCPR, that can interact with the phycobilisome. Large gaps still exist in the mechanistic understanding of the events between photon absorption and formation of the OCPRstate. Recent studies suggested that more than one photon may be absorbed during the photocycle. Using a two-pulse excitation setup with variable time delays we demonstrate that canthaxanthin-functionalized OCPOforms the OCPRsignature after absorption of a single photon. By contrast, OCPOcomplexed with hECN or ECN does not photoconvert to OCPRupon single photon absorption. Instead, OCPRis formed only upon absorption of a second photon, arriving roughly one second after the first one, implying the existence of a metastable light-sensitive OCP1hvintermediate. To the best of our knowledge, a sequential 2-photon absorption mechanism in a single biological photoreceptor chromophore is unique. It results in a non-linear 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.