Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Ascorbate peroxidase is a hydrogen peroxide-scavenging enzyme that is specific to plants and algae and is indispensable to protect chloroplasts and other cell constituents from damage by hydrogen peroxide and hydroxyl radicals produced from it. In this review, first, the participation of ascorbate peroxidase in the scavenging of hydrogen peroxide in chloroplasts is briefly described. Subsequently, the phylogenic distribution of ascorbate peroxidase in relation to other hydrogen peroxide-scavenging peroxidases using glutathione, NADH and cytochrome c is summarized. Chloroplastic and cytosolic isozymes of ascorbate peroxidase have been found, and show some differences in enzymatic properties. The basic properties of ascorbate peroxidases, however, are very different from those of the guaiacol peroxidases so far isolated from plant tissues. Amino acid sequence and other molecular properties indicate that ascorbate peroxidase resembles cytochrome c peroxidase from fungi rather than guaiacol peroxidase from plants, and it is proposed that the plant and yeast hydrogen peroxide-scavenging peroxidases have the same ancestor.

Ascorbate peroxidase – a hydrogen peroxide-scavenging enzyme in plants

Desiccation tolerance of prokaryotes.

Even under optimal conditions, many metabolic processes, including the chloroplastic, mitochondrial, and plasma membrane-linked electron transport systems of higher plants, produce active oxygen species (AOS). Furthermore, the imposition of biotic and abiotic stress conditions can give rise to excess concentrations of AOS, resulting in oxidative damage at the cellular level. Therefore, antioxidants and antioxidant enzymes function to interrupt the cascades of uncontrolled oxidation in each organelle. Ascorbate peroxidase (APX) exists as isoenzymes and plays an important role in the metabolism of H(2)O(2) in higher plants. APX is also found in eukaryotic algae. The characterization of APX isoenzymes and the sequence analysis of their clones have led to a number of investigations that have yielded interesting and novel information on these enzymes. Interestingly, APX isoenzymes of chloroplasts in higher plants are encoded by only one gene, and their mRNAs are generated by alternative splicing of the gene's two 3'-terminal exons. Manipulation of the expression of the enzymes involved in the AOS-scavenging systems by gene-transfer technology has provided a powerful tool for increasing the present understanding of the potential of the defence network against oxidative damage caused by environmental stresses. Transgenic plants expressing E. coli catalase to chloroplasts with increased tolerance to oxidative stress indicate that AOS-scavenging enzymes, especially chloroplastic APX isoenzymes are sensitive under oxidative stress conditions. It is clear that a high level of endogenous ascorbate is essential effectively to maintain the antioxidant system that protects plants from oxidative damage due to biotic and abiotic stresses.

/pdf/regulation-and-function-of-ascorbate-peroxidase-isoenzymes-5fgj02sckc.pdf

Regulation and function of ascorbate peroxidase isoenzymes

summary The gradual but huge increase in atmospheric O # concentration that followed the evolution of oxygenic photosynthesis is one consequence that marks this event as one of the most significant in the earth’s history. The high redox potential of the O # }water couple makes it an extremely powerful electron sink that enables energy to be transduced in respiration. In addition to the tetravalent interconversion of O # and water, there exist a plethora of reactions that involve the partial reduction of O # or photodynamic energy transfer to produce active oxygen species (AOS). All these redox reactions have become integrated during evolution into the aerobic photosynthetic cell. This review considers photosynthesis as a whole-cell process, in which O # and AOS are involved in reactions at both photosystems, enzyme regulation in the chloroplast stroma, photorespiration, and mitochondrial electron transport in the light. In addition, oxidants and antioxidants are discussed as metabolic indicators of redox status, acting as sensors and signal molecules leading to acclimatory responses. Our aim throughout is to assess the insights gained from the application of mutagenesis and transformation techniques to studies of the role of O # and related redox components in the integrated regulation of photosynthesis.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00667.x

Tansley Review No. 112

Cyanobacteria can be divided into two groups, those that has and those that lacks AsA peroxidase. The first group scavenges hydrogen peroxide with the peroxidase using a photoreductant as the electron donor, and the second group only scavenges hydrogen peroxide with catalase

Fumi Michihata

Papers

Scavenging of Hydrogen Peroxide in Prokaryotic and Eukaryotic Algae: Acquisition of Ascorbate Peroxidase during the Evolution of Cyanobacteria