The VTC2 cycle and the de novo biosynthesis pathways for vitamin C in plants:: An opinion

The recent identification of the VTC2 enzyme (GDP-L-galactose: hexose 1-phosphate guanylyltransferase) that forms with the GDPmannose 3 '',5 '' epimerase an energy-con serving hub for the production of GDP-hexoses and L-galactose 1-phosphate [Laing et al., Proc. Natl. Acad. Sci. USA 104, 2007, 9534-9539], is a major breakthrough in our understanding of the biosynthesis of L-ascorbic acid (vitamin C) in plants. The observation that the VTC2 enzyme can use glucose 1-phosphate and GDP-D-glucose as substrates, and the long-known existence of an enigmatic GDP-D-mannose 2 ''-epimerase activity, have led us to the proposal of an extended VTC2 cycle that links photosynthesis with the biosynthesis of vitamin C and the cell-wall metabolism in plants. An evolutionary scenario is discussed for the acquisition of genes of eubacterial origin for the de novo synthesis Of L-ascorbic acid in green algae and plants. (C) 2007 Elsevier Ltd. All rights reserved.