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 K2 by this vitamin group were revealed to be much more complex. K vitamins play an essential role as cofactor for the enzyme γ-glutamyl carboxylase, which is involved in vitamin K-dependent carboxylation of the gla domain in "Gla proteins" (i.e., in conversion of peptide-bound glutamic acid (Glu) to γ-carboxy glutamic acid (Gla) in these proteins). Carboxylation of these vitamin K-dependent Gla-proteins, besides being essential for the function of the protein, is also an important vitamin recovery mechanism since it serves as a recycling pathway to recover vitamin K from its epoxide metabolite (KO) for reuse in carboxylation. Several human Gla-containing proteins