Abstract
Temperature dependence of violaxanthin de-epoxidase was measured using the system of isolated violaxanthin de-epoxidase enzyme (VDE) and unilamellar vesicles composed of various proportions of monogalactosyldiacylglycerol (MGDG) and egg yolk phosphatidylcholine (PC). It has been shown that at the constant ratio of VDE and MGDG in the assay system, the kinetics of de-epoxidation depended greatly on proportion of MGDG to PC as well as on temperature. A mathematical model of violaxanthin de-epoxidation was constructed and applied to calculate the probability of violaxanthin to antheraxanthin and to zeaxanthin conversion at different phases of deepoxidation process. Measuremants of de-epoxidation rate and spin label mobility at different temperatures revealed that membrane fluidity is an important factor controlling conversion of violaxanthin to antheraxanthin. A model of the xanthophyll cycle is proposed in which VDE binding and the de-epoxidation reactions occur in MGDG rich domains. Availability of violaxanthin for de-epoxidation is a lateral diffusion-dependent process controlled by membrane fluidity. Significance of the presented results for understanding the mechanism of violaxanthin de-epoxidation in native thylakoid membranes is discussed.
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