Structure and regulation of violaxanthin de-epoxidase, a key enzyme in the photoprotection of photosynthesis

The xanthophyll cycle is a set of light dependent reactions that regulate the conversion of violaxanthin into zeaxanthin, representing a key mechanism in the regulation of photosynthesis and the response to excess light in eukaryotes. Optimisation of zeaxanthin accumulation has also been identified as a promising target for increasing photosynthetic productivity in crops, balancing the trade-off between photoprotection and photosynthetic efficiency. The activation of the xanthophyll cycle depends on violaxanthin de-epoxidase (VDE), the enzyme that catalyses the conversion of violaxanthin into zeaxanthin, activated under conditions of light excess. Photosynthetic eukaryote genomes contain other genes with similarity to VDE, called VDL and VDR, that show conserved structural properties but have different biological roles. The available knowledge on VDE structure and catalytic mechanism is reviewed here, including the identification of key amino acids involved in the catalytic mechanisms and conformational changes during protein activation. VDE activity is also shown to be modulated by multiple mechanisms, such as transcriptional control, redox sensitivity, and pH-dependence. All these regulatory mechanisms have an essential role in modulating VDE protein activity in vivo, and their impact should be considered in efforts to optimize photosynthetic productivity.