Molecular and Functional Characterization of Monodehydro-ascorbate and Dehydroascorbate Reductases

Abstract

When ascorbate (AsA) plays a role in cells as an antioxidant or a substrate for some enzymes including ascorbate peroxidase (APX), it is primarily oxidized to the monodehydroascorbate (MDHA) radical and then dehydroascorbate (DHA) is formed via the disproportionation of MDHA. The regeneration of AsA from these oxidation products is essential to maintain cellular levels of AsA. Plant cells contain enzymes catalyzing the reductions of MDHA and DHA, known as MDHA reductase (MDHAR) and DHA reductase (DHAR), respectively. Both reductases are components of the ascorbate-glutathione (AsA-GSH) cycle. MDHAR and DHAR use NADH and GSH as electron donors, respectively. MDHAR is an FAD-containing enzyme that forms a charge-transfer complex with NADH, and DHAR contains a conserved catalytic cysteine residue that forms a mixed disulfide with GSH. Several paralogous genes of MDHAR and DHAR can be found in plant genomes. The isoforms of MDHAR and DHAR, encoded by these genes, show diverse subcellular localization and biochemical properties. Recently, several structures of MDHAR and DHAR from plants were determined independently. These results and the biochemical data obtained represent the reaction mechanism of AsA regeneration by these reductases. This chapter focuses on the molecular properties of these AsA-regenerating enzymes and their biochemical functions.