Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 98, Issue 26, Pages 14843-14848Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.011578198
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The first step in the biosynthetic pathway of vitamin C in plants is the formation, at the level of sugar nucleotide, Of L-galactosyl residues, catalyzed by a largely unknown GDP-D-mannose 3 ,5 -epimerase. By using combined conventional biochemical and mass spectrometry methods, we obtained a highly purified preparation of GDP-D-mannose 3 ,5 -epimerase from an Arabidopsis thaliana cell suspension. The native enzyme is an 84-kDa dimer, composed of two apparently identical subunits. In-gel tryptic digestion of the enzyme subunit, followed by peptide sequencing and a BLAST search, led to the identification of the epimerase gene. The closest homolog of the plant epimerase is the BlmG gene product of Streptomyces sp., a putative NDP-D-mannose 5 -epimerase. The plant GDP-D-mannose 3 ,5 -epimerase is, to our knowledge, a novel member of the extended short-chain dehydrogenase/reductase family. The enzyme was cloned and expressed in Escherichia coli cells.
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