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 GDP-mannose 3'',5'' epimerase an energy-conserving 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.