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Published on February 25, 2009; 10.1104/pp.108.133223

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Received November 26, 2008
Accepted February 18, 2009

COMPUTER SIMULATION OF THE DYNAMIC BEHAVIOR OF THE GLUTATHIONE-ASCORBATE REDOX CYCLE IN CHLOROPLASTS

Edelmira Valero *, Maria I. Gonzalez-Sanchez , Hermenegilda Macia , and Francisco Garcia-Carmona

Departamento de Quimica-Fisica, Escuela de Ingenieros Industriales, Universidad de Castilla-La Mancha, Campus Universitario, s/n, E-02071, Albacete, Spain; Departamento de Matematicas, Escuela Superior de Ingenieria Informatica, Universidad de Castilla-La Mancha, Campus Universitario, s/n, E-02071, Albacete, Spain; and Departamento de Bioquimica y Biologia Molecular A, Facultad de Biologia, Universidad de Murcia, E-30100 Murcia, Spain

* Corresponding author; email: Edelmira.Valero{at}uclm.es.

The glutathione-ascorbate redox pathway in chloroplasts is a complex network of spontaneous, photochemical and enzymatic reactions for detoxifying H2O2. This paper presents a comprehensive sensitivity analysis of the system. A model has been constructed to simulate oxidative stress conditions, enabling steady-state concentrations of the metabolites involved in the pathway and photochemical and enzymatic fluxes to be calculated. The model includes an electron source whose flux is distributed among three competitive routes (photogeneration of O2-, photoreduction of NADP+ to NADPH and photoreduction of monodehydroascorbate to ascorbate), and which allows the simulation of variations in NADPH concentration with time. Each enzyme considered is introduced in the model taking into account its particular catalytic mechanism, including the inactivation of ascorbate peroxidase in the presence of low ascorbate concentrations. Computer simulations pointed to the great sensitivity of the system to the ratio among fluxes corresponding to ascorbate and NADPH photoproduction and NADPH consumption by the Calvin cycle. Under oxidative stress conditions, the model shows a sequential depletion of antioxidant power in chloroplasts in the order NADPH, glutathione, ascorbate and their recovery in the reverse order. Decreasing levels of glutathione reductase, ascorbate peroxidase and superoxide dismutase led to the irreversible photoinactivation of ascorbate peroxidase and the subsequent increase in H2O2 concentration, preceded by a maximum in dehydroascorbate reductase activity.






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