Algae, ascorbate peroxidase

Ascorbate peroxidase is a heme-eontaining enzyme that catalyses the ascorbate-dependent reduction of potentially damaging hydrogen peroxide in plants and algae, Eq. (1),... 

Miyake, C., Michihata, F., and Asada, K., 1991, Scavenging of hydrogen peroxide in prokaryotic and eukaryotic algae acquisition of ascorbate peroxidase during the evolution of cyanobacteria. Plant Cell Physiol. 32 33n43. 

Sauser, K. R., Liu, J. K., and Wong, T.-Y., 1997, Identification of a copper-sensitive ascorbate peroxidase in die unicellular green alga Selenastrwn capricornutum, BioMetals 10 163fi 168. 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

Hydrogen peroxide in higher plants and algae may also be decomposed by ascorbate peroxidase. Ascorbate peroxidase inactivates hydrogen peroxide in the cytosol and chloroplasts by the following mechanism. 

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