Vitamin C (ascorbate) is a vitamin found in food and used as a dietary supplement. The vast majority of animals and plants are able to synthesize vitamin C, through a sequence of enzyme-driven steps, which convert monosaccharides to vitamin C. In plants, this is accomplished through the conversion of mannose or galactose to ascorbic acid starting in the cytosol and ending in the mitochondrial matrix . First, GDP-mannose 3,5-epimerase catalyzes the reversible epimerization of GDP-D-mannose into either GDP-L-gulose or GDP-L-galactose. It also can reversibly epimerize GDP-L-gulose into GDP-L-galactose and vice versa. It requires NAD as a cofactor. Second, GDP-L-galactose phosphorylase catalyzes the conversion of GDP-L-galactose into L-galactose 1-phosphate. Third, L-galactose 1-phosphate phosphatase catalyzes the conversion of L-galactose 1-phosphate into L-galactose. It requires magnesium ion as a cofactor. Fourth, L-galactose dehydrogenase catalyzes the conversion of L-galactose into L-galactono-1,4-lactone. L-galactono-1,4-lactone must then be imported into the mitochondrial matrix by a predicted innermitochondrial membrane transporter to complete ascorbate synthesis. L-galactono-1,4-lactone dehydrogenase, localized to the innermitochondrial membrane (coloured dark green in the image), catalyzes two reactions in ascorbate metabolism: the conversion of L-galactono-1,4-lactone into L-ascorbate and the subsequent conversion of L-ascorbate into L-dehydroascorbate. It requires FAD as a cofactor. Ascorbate can then be converted into monodehydroascorbate radical by the mitochondrial L-ascorbate peroxidase S (this plays a key role in hydrogen peroxide removal). Monodehydroascorbate reductase 5 then can convert monodehydroascorbate radical back into L-ascorbate.

Ascorbate Metabolism References