Official Project Description
To adapt to shifting lighting conditions, plants modify their photosynthetic activity.
The xanthophyll cycle, in which the carotenoid violaxanthin is transformed into zeaxanthin in strong light, plays a key role in controlling photosynthesis.
This process activates the disposal of excess absorbed energy as heat and the scavenging of reactive oxygen species.
When photosynthetic electron transport exceeds the capacity of assimilatory reactions, violaxanthin deepoxidase (VDE), the enzyme responsible for zeaxanthin synthesis, is activated by the acidification of the thylakoid lumen.
At neutral pH, VDE is a soluble and inactive enzyme, but at acidic pH, it attaches to the thylakoid membrane where it binds its substrate.
Ascorbate is used by VDE as a cosubstrate as well, and its pH-dependent Km may indicate a preference for ascorbic acid.
At neutral and acidic pH, we established the structures of the central lipocalin domain of VDE (VDEcd).
VDEcd is monomeric and has its active site blocked by a lipocalin barrel at neutral pH.
The barrel unwinds and the enzyme emerges as a dimer during acidification.
The two violaxanthin beta-ionone rings may be deep oxidized simultaneously in two channels that connect the two active sites of the dimer, giving VDE an ideal example of an asymmetric enzyme's adaptation to its symmetric substrate.