OP20 - The evolution of vitamin C biosynthetic pathways in plants and algae

Abstract

Ascorbic acid (vitamin C) is present in high concentrations in leaves and, in some species, in fruit. In leaves it has a major role in photoprotection of photosynthesis both as scavenger of reactive oxygen species but also as a cofactor for violaxanthin de-epoxidase, an enzyme required for the synthesis of the photoprotective xanthophyll pigment zeaxanthin. Animals use an ER-localised gulonolactone oxidase (GULO) for ascorbate biosynthesis but this also generates hydrogen peroxide. The biosynthetic pathway in plants and photosynthetic protists is distinct from animals in using a mitochondrial galactonolactone dehydrogenase (GLDH) in the last step that does not generate hydrogen peroxide. Genome sequence analysis and biochemical data suggest that GULO has been lost repeatedly throughout eukaryote evolution (resulting in inability to synthesise ascorbate in primates and some other animal groups) and has been functionally replaced by GLDH in all photosynthetic eukaryotes. Green algae and plants derived from primary endosymbiosis with a cyanobacterium synthesise ascorbate by a pathway distinct from animals while photosynthetic protists with chloroplasts derived by secondary endosymbiosis appear to have a “hybrid” pathway combining an animal-like pathway with plant-like GLDH in the final step. It is proposed that unlike the animal pathway, the plant pathway and the “hybrid” pathway provide a high capacity for ascorbate biosynthesis without hydrogen peroxide formation that provides sufficient ascorbate for photoprotection of photosynthesis.