Vitamin C of mannose or galactose to ascorbic acid. In animals, the starting material is glucose. In some species that synthesize ascorbate in the liver (including mammals and perching birds), the glucose is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process. In animals that cannot synthesize vitamin C, the enzyme -gulonolactone oxidase (GULO), that catalyses the last step in the biosynthesis, is highly mutated and non-functional. The biosynthesis of ascorbic acid in vertebrates starts with the formation of UDP-glucuronic acid. UDP-glucuronic acid is formed when UDP-glucose undergoes two oxidations catalyzed by the enzyme UDP-glucose 6-dehydrogenase. UDP-glucose 6-dehydrogenase uses the co-factor NAD
Vitamin C -Galactose reacts with the enzyme -galactose dehydrogenase, whereby the lactone ring opens and forms again but with between the carbonyl on C1 and hydroxyl group on the C4, resulting in -galactonolactone. -Galactonolactone then reacts with the mitochondrial flavoenzyme -galactonolactone dehydrogenase. to produce ascorbic acid. -Ascorbic acid has a negative feedback on -galactose dehydrogenase in spinach. Ascorbic acid efflux by embryo of dicots plants is a well-established mechanism of iron reduction, and a step obligatory for iron uptake. All plants synthesize ascorbic acid. Ascorbic acid functions as a cofactor for enzymes involved in photosynthesis, synthesis of plant hormones, as an antioxidant