Accepted name: GDP-L-galactose/GDP-D-glucose: hexose 1-phosphate guanylyltransferase
Reaction: (1) GDP-β-L-galactose + α-D-mannose 1-phosphate = β-L-galactose 1-phosphate + GDP-α-D-mannose
(2) GDP-α-D-glucose + α-D-mannose 1-phosphate = α-D-glucose 1-phosphate + GDP-α-D-mannose
Other name(s): VTC2; VTC5; GDP-L-galactose phosphorylase
Systematic name: GDP-β-L-galactose/GDP-α-D-glucose:hexose 1-phosphate guanylyltransferase
Comments: This plant enzyme catalyses the conversion of GDP-β-L-galactose and GDP-α-D-glucose to β-L-galactose 1-phosphate and α-D-glucose 1-phosphate, respectively. The enzyme can use inorganic phosphate as the co-substrate, but several hexose 1-phosphates, including α-D-mannose 1-phosphate, α-D-glucose 1-phosphate, and α-D-galactose 1-phosphate, are better guanylyl acceptors. The enzyme's activity on GDP-β-L-galactose is crucial for the biosynthesis of L-ascorbate.
1.  Linster, C.L., Gomez, T.A., Christensen, K.C., Adler, L.N., Young, B.D., Brenner, C. and Clarke, S.G. Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants. J. Biol. Chem. 282 (2007) 18879–18885. [PMID: 17462988]
2.  Dowdle, J., Ishikawa, T., Gatzek, S., Rolinski, S. and Smirnoff, N. Two genes in Arabidopsis thaliana encoding GDP-L-galactose phosphorylase are required for ascorbate biosynthesis and seedling viability. Plant J. 52 (2007) 673–689. [PMID: 17877701]
3.  Wolucka, B.A. and Van Montagu, M. The VTC2 cycle and the de novo biosynthesis pathways for vitamin C in plants: an opinion. Phytochemistry 68 (2007) 2602–2613. [PMID: 17950389]
4.  Laing, W.A., Wright, M.A., Cooney, J. and Bulley, S.M. The missing step of the L-galactose pathway of ascorbate biosynthesis in plants, an L-galactose guanyltransferase, increases leaf ascorbate content. Proc. Natl. Acad. Sci. USA 104 (2007) 9534–9539. [PMID: 17485667]
5.  Linster, C.L., Adler, L.N., Webb, K., Christensen, K.C., Brenner, C. and Clarke, S.G. A second GDP-L-galactose phosphorylase in arabidopsis en route to vitamin C. Covalent intermediate and substrate requirements for the conserved reaction. J. Biol. Chem. 283 (2008) 18483–18492. [PMID: 18463094]
6.  Muller-Moule, P. An expression analysis of the ascorbate biosynthesis enzyme VTC2. Plant Mol. Biol. 68 (2008) 31–41. [PMID: 18516687]
[EC created 2010, modified 2020]