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A Structural Basis for the pH-Dependent Xanthophyll Cycle in Arabidopsis thaliana^[C],[W]

^a Commissariat à l'Energie Atomique, Direction des Sciences du Vivant, Institut de Biologie Environnementale et de Biotechnologie, Laboratoire de Bioénergétique Cellulaire, Saint-Paul-lez-Durance, F-13108, France
^b Centre National de la Recherche Scientifique, Unité Mixte de Recherche Biologie Vegetale et Microbiologie Environnementale, Saint-Paul-lez-Durance, F-13108, France
^c Aix-Marseille Université, Saint-Paul-lez-Durance, F-13108, France
^d Dipartimento di Biologia, Università degli Studi di Padova, 35121, Padova, Italy
^e Dipartimento di Biotecnologie, Università degli Studi di Verona, 37134, Verona, Italy

² Address correspondence to tomas.morosinotto{at}unipd.it.

Plants adjust their photosynthetic activity to changing light conditions. A central regulation of photosynthesis depends on the xanthophyll cycle, in which the carotenoid violaxanthin is converted into zeaxanthin in strong light, thus activating the dissipation of the excess absorbed energy as heat and the scavenging of reactive oxygen species. Violaxanthin deepoxidase (VDE), the enzyme responsible for zeaxanthin synthesis, is activated by the acidification of the thylakoid lumen when photosynthetic electron transport exceeds the capacity of assimilatory reactions: at neutral pH, VDE is a soluble and inactive enzyme, whereas at acidic pH, it attaches to the thylakoid membrane where it binds its violaxanthin substrate. VDE also uses ascorbate as a cosubstrate with a pH-dependent K_m that may reflect a preference for ascorbic acid. We determined the structures of the central lipocalin domain of VDE (VDE_cd) at acidic and neutral pH. At neutral pH, VDE_cd is monomeric with its active site occluded within a lipocalin barrel. Upon acidification, the barrel opens up and the enzyme appears as a dimer. A channel linking the two active sites of the dimer can harbor the entire carotenoid substrate and thus may permit the parallel deepoxidation of the two violaxanthin β-ionone rings, making VDE an elegant example of the adaptation of an asymmetric enzyme to its symmetric substrate.

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