title: Recent progress on the characterization of aldonolactone oxidoreductases.
abstract: L-Ascorbic acid (ascorbate, AsA, vitamin C) is essential for animal and plant health. Despite our dependence on fruits and vegetables to fulfill our requirement for this vitamin, the metabolic network leading to its formation in plants is just being fully elucidated. There is evidence supporting the operation of at least four biosynthetic pathways leading to AsA formation in plants. These routes use D-mannose/L-galactose, L-gulose, D-galacturonate, and myo-inositol as the main precursors. This review focuses on aldonolactone oxidoreductases, a subgroup of the vanillyl alcohol oxidase (VAO; EC 1.1.3.38) superfamily, enzymes that catalyze the terminal step in AsA biosynthesis in bacteria, protozoa, animals, and plants. In this report, we review the properties of well characterized aldonolactone oxidoreductases to date. A shared feature in these proteins is the presence of a flavin cofactor as well as a thiol group. The flavin cofactor in many cases is bound to the N terminus of the enzymes or to a recently discovered HWXK motif in the C terminus. The binding between the flavin moiety and the protein can be either covalent or non-covalent. Substrate specificity and subcellular localization differ among the isozymes of each kingdom. All oxidases among these enzymes possess dehydrogenase activity, however, exclusive dehydrogenases are also found. We also discuss recent evidence indicating that plants have both L-gulono-1,4-lactone oxidases and L-galactono-1,4-lactone dehydrogenases involved in AsA biosynthesis.
authors.affiliation: Arkansas Biosciences Institute, Arkansas State University, P.O. Box 639, State University, AR 72467, USA.
authors.forename: Siddique I
authors.initials: SI
authors.lastname: Aboobucker
authors.affiliation: Arkansas Biosciences Institute, Arkansas State University, P.O. Box 639, State University, AR 72467, USA; Department of Chemistry and Physics, Arkansas State University, P.O. Box 419, State University, AR 72467, USA. Electronic address: alorence@astate.edu.
authors.forename: Argelia
authors.initials: A
authors.lastname: Lorence
mesh_terms: D000429:Alcohol Oxidoreductases / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D001205:Ascorbic Acid / Q000378:metabolism; D003067:Coenzymes; D005415:Flavins / Q000378:metabolism*; D005690:Galactose / Q000378:metabolism; D007783:Lactones / Q000378:metabolism; D008358:Mannose / Q000378:metabolism; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D010944:Plants / Q000201:enzymology* / Q000235:genetics; D011994:Recombinant Proteins; D016415:Sequence Alignment; D013379:Substrate Specificity; D013400:Sugar Acids / Q000378:metabolism
chemical_list: D003067:Coenzymes; D005415:Flavins; D007783:Lactones; D011994:Recombinant Proteins; D013400:Sugar Acids; C534014:galactono-1,4-lactone; D010088:Oxidoreductases; D000429:Alcohol Oxidoreductases; C076865:vanillyl-alcohol oxidase; D008358:Mannose; D001205:Ascorbic Acid; D005690:Galactose
keywords: Aldonolactone oxidoreductase; Ascorbate; Galactonolactone dehydrogenase; Gulonolactone oxidase; Vanillyl alcohol oxidase family