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ascorbate glutathione cycle

Many metabolic processes, including respiration, photosynthetic electron transport and oxidation of glycolate in photorespiration, generate reactive oxygen species such as singleton oxygen and hydrogen peroxide. For example, the univalent reduction of dioxygen in illuminated chloroplasts produces superoxide anion radicals, which are disproportionate to hydrogen peroxide and dioxygen by the superoxide dismutase enzyme, contained in the stroma and lumen. These highly reactive compounds can damage membrane lipids and certain enzymes and thus interrupt cell function. The ascorbate-glutathione cycle performs the scavenging of |FRAME: HYDROGEN-PEROXIDE|. In this cycle, |FRAME: ASCORBATE| and |FRAME: GLUTATHIONE| transfer reducing power from NADPH to hydrogen peroxide, reducing it to water. The reduction of hydrogen peroxide is catalyzed by |FRAME: MONOMER-5127|, in a reaction that generates |FRAME: CPD-318|, a radical that can be reduced to ascorbate by |FRAME: MONOMER-5129|. If not reduced rapidly, monodehydroascorbate is disproportionated into |FRAME: ASCORBATE| and |FRAME: L-DEHYDRO-ASCORBATE|, and the latter is reduced back to |FRAME: ASCORBATE| by |FRAME: MONOMER-5132| using |FRAME: GLUTATHIONE| as the reducing agent. The oxidized glutathione is reduced by |FRAME: MONOMER-5145| using NADPH (see |FRAME: GLUT-REDOX-PWY|). The ascorbate-glutathione cycle presents in at least four different subcellular locations including the cytosol, chloroplast, mitochondrion and peroxisome. Studies of the Arabidopsis enzymes of the pathway show that the same set of enzymes, including |FRAME: MONOMER-5127|, |FRAME: MONOMER-5129| and |FRAME: MONOMER-5145|, are dual targeted to chloroplast and mitochondrion |CITS: [12954611]|. Ascorbate peroxidase is in fact the major peroxidase in spinach leaves. A similar scavenging system appears to operate in several species of cyanobacteria and Euglena.

from BIOCYC source record: META_PWY-2261
Type: pathway
Taxonomic scope
conserved biosystem

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