A mathematical model describing kinetics of conversion of violaxanthin to zeaxanthin via intermediate antheraxanthin by the xanthophyll cycle enzyme violaxanthin de-epoxidase

The xanthophyll cycle is one of the mechanisms protecting the photosyntheti c apparatus against the light energy excess. Its action is still not well u nderstood on the molecular level. Our model makes it possible to follow ind ependently the kinetics of the two de-epoxidation steps occurring in the xa nthophyll cycle: the conversion of violaxanthin into antheraxanthin and the conversion of antheraxanthin into zeaxanthin. Using a simple form of the t ransition rates of these two conversions, we model the time evolution of th e concentration pattern of violaxanthin, antheraxanthin and zeaxanthin duri ng the de-epoxidation process. The model has been applied to describe the r eactions of de-epoxidation in a system of liposome membranes composed of ph osphatidylcholine and monogalactosyldiacylglycerol. Results obtained within the model fit very well with the experimental data. Values of the transiti on probabilities of the violaxanthin conversion into antheraxanthin and the antheraxanthin conversion into zeaxanthin calculated by means of the model indicate that the first stage of the de-epoxidation process is much slower than the second one. (C) 2000 Academic Press.