Structural basis of the two-photon photoactivation mechanism of orange carotenoid protein

Rory Munro; Elena A. Andreeva; Elisabeth Hartmann; Quentin Goor; Hosni El Zein; Stanisław Niziński; Adjélé Wilson; Elke De Zitter; Grégory Effantin; Nicolas Coquelle; Ninon Zala; Martin V. Appleby; Shira Bar-Zvi; Camila Bacellar; Emma V. Beale; Emmanuelle Bignon; Bernhard Brutscher; Martin Byrdin; Claudio Cirelli; Florian Dworkowski; L. Foucar; Guillaume Gotthard; A. Gorel; Marie Luise Grünbein; M. Hilpert; Philip J. M. Johnson; Marco Kloos; Gregory Knoop; Karol Nass; Gabriela Nass Kovacs

doi:10.64898/2026.04.21.718960

BiochemistrybioRxivMay 10, 2026

Preprint

Structural basis of the two-photon photoactivation mechanism of orange carotenoid protein

Rory Munro iD, Elena A. Andreeva iD, Elisabeth Hartmann, Quentin Goor iD, Hosni El Zein iD, Stanisław Niziński iD, Adjélé Wilson iD, Elke De Zitter iD, Grégory Effantin iD, Nicolas Coquelle iD, Ninon Zala iD, Martin V. Appleby iD, Shira Bar-Zvi, Camila Bacellar iD, Emma V. Beale iD, Emmanuelle Bignon iD, Bernhard Brutscher iD, Martin Byrdin iD, Claudio Cirelli iD, Florian Dworkowski iD, L. Foucar, Guillaume Gotthard iD, A. Gorel iD, Marie Luise Grünbein, M. Hilpert iD, , Jacques‐Philippe Colletier iD

Abstract

Cyanobacteria have produced Earths oxygen for 2.4 billion years by adapting to fluctuating irradiance. This adaptation relies on orange carotenoid protein (OCP), which mediates light-intensity- dependent photoprotective energy dissipation using a unique two-photon absorption mechanism. Photon absorption by ground-state OCP (OCPO) generates a metastable intermediate (OCP1h{nu}) that either relaxes thermally or, upon absorption of a second photon within [~]1 s, converts to the active photoprotective state (OCPR). By integrating static and time-resolved crystallography, cryo-EM, computation, spectroscopy and biochemistry, we assign the structure of OCP1h{nu}, establish its functional relevance and capture structural snapshots along the OCPO[->]OCP1h{nu} and OCP1h{nu}[->]OCPR photochemical pathways. We elucidate the molecular mechanism of OCP, which serves as a unique biological circuit breaker protecting the photosynthetic machinery from high light flux.

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