Violaxanthin and Diadinoxanthin Cycles as an Important Photoprotective Mechanism in Photosynthesis

The molecular mechanism and regulation of violaxanthin (Vx) and diadinoxanthin (Ddx) cycle are discussed. The influence of lipids on the de-epoxidation of the xanthophyll cycle pigments was investigated. Experiments on the significance of physical properties of the aggregates formed by inverted lipid micelles, which are necessary for de-epoxidation, on the conversion of Vx into antheraxanthin (Ax) and zeaxanthin (Zx) and diadinoxanthin (Ddx) to diatoxanthin (Dtx) were performed. Thickness of the hydrophobic fraction of the aggregates, size of the inverted micelles, suggested by mathematical description of the structures, and solubility of Vx and Ddx in various kinds of lipids were the next tested parameters. Obtained results show that the rate of de-epoxidation was strongly dependent on physicochemical properties of lipids. The key role for enzyme activation play non-bilayer lipids and the parameters of inverted micelles created by them, such as thickness, molecular dynamics of hydrophobic core, and their diameter. Additionally the influence of thylakoid lipids on the de-epoxidation of Vx, associated with the light-harvesting complex of PSII (LHCII) were carried out. Almost complete Vx de-epoxidation in the LHCII fractions containing high amounts of endogenous monogalactosyldiacylglycerol (MGDG) was observed in in vitro assays. LHCII preparations with low concentrations of MGDG exhibited a strongly reduced Vx de-epoxidation, which could be increased by addition of exogenous, pure MGDG. The presented results showed that MGDG and other non-lamellar lipids are necessary for the solubilization of Vx and Ddx and they provide also the three-dimensional structures, which are needed for the binding of de-epoxidases and for the accessibility of Vx and Ddx to these enzymes.