Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants.
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.
Linster
Carole L
CL
Department of Chemistry and Biochemistry, Molecular Biology Institute, UCLA, Los Angeles, California 90095, USA.
Gomez
Tara A
TA
Christensen
Kathryn C
KC
Adler
Lital N
LN
Young
Brian D
BD
Brenner
Charles
C
Clarke
Steven G
SG
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
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