title: Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants.
abstract: The first committed step in the biosynthesis of L-ascorbate from D-glucose in plants requires conversion of GDP-L-galactose to L-galactose 1-phosphate by a previously unidentified enzyme. Here we show that the protein encoded by VTC2, a gene mutated in vitamin C-deficient Arabidopsis thaliana strains, is a member of the GalT/Apa1 branch of the histidine triad protein superfamily that catalyzes the conversion of GDP-L-galactose to L-galactose 1-phosphate in a reaction that consumes inorganic phosphate and produces GDP. In characterizing recombinant VTC2 from A. thaliana as a specific GDP-L-galactose/GDP-D-glucose phosphorylase, we conclude that enzymes catalyzing each of the ten steps of the Smirnoff-Wheeler pathway from glucose to ascorbate have been identified. Finally, we identify VTC2 homologs in plants, invertebrates, and vertebrates, suggesting that a similar reaction is used widely in nature.
authors.affiliation: Department of Chemistry and Biochemistry, Molecular Biology Institute, UCLA, Los Angeles, California 90095, USA.
authors.forename: Carole L
authors.initials: CL
authors.lastname: Linster
authors.forename: Tara A
authors.initials: TA
authors.lastname: Gomez
authors.forename: Kathryn C
authors.initials: KC
authors.lastname: Christensen
authors.forename: Lital N
authors.initials: LN
authors.lastname: Adler
authors.forename: Brian D
authors.initials: BD
authors.lastname: Young
authors.forename: Charles
authors.initials: C
authors.lastname: Brenner
authors.forename: Steven G
authors.initials: SG
authors.lastname: Clarke
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism*; D001205:Ascorbic Acid / Q000096:biosynthesis* / Q000378:metabolism; D005690:Galactose / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D008969:Molecular Sequence Data; D010744:Phosphoric Monoester Hydrolases / Q000235:genetics* / Q000378:metabolism*; D006005:Phosphorylases / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D013379:Substrate Specificity
chemical_list: D029681:Arabidopsis Proteins; D006153:Guanosine Diphosphate; D006005:Phosphorylases; C522629:GDP-L-galactose phosphorylase, Arabidopsis; D010744:Phosphoric Monoester Hydrolases; D005947:Glucose; D001205:Ascorbic Acid; D005690:Galactose