l-Ascorbate biosynthesis in higher plants: the role of VTC2

In the past year, the last missing enzyme of the l-galactose pathway, the linear form of which appears to represent the major biosynthetic route to l-ascorbate (vitamin C) in higher plants, has been identified as a GDP-l-galactose phosphorylase. This enzyme catalyzes the first committed step in the synthesis of that vital antioxidant and enzyme cofactor. Here, we discuss how GDP-l-galactose phosphorylase enzymes, encoded in Arabidopsis by the paralogous VTC2 and VTC5 genes, function in concert with the other enzymes of the l-galactose pathway to provide plants with the appropriate levels of l-ascorbate. We hypothesize that regulation of l-ascorbate biosynthesis might occur at more than one step and warrants further investigation to allow for the manipulation of vitamin C levels in plants.