The pathway of L-ascorbic acid biosynthesis in the colourless microalga Prototheca moriformis

Abstract

When mutant strain UV77-247 of Prototheca moriformis Kruger was fed d-[1-13C]Glc, it synthesized l-ascorbic acid (AA) with approximately three-quarters of the label at the C-1 position and the remaining label at the C-6 position, showing that AA is made by a non-inversion (retention) pathway, i.e. C-1 of Glc becomes C-1 of AA. The label present at C-6 is consistent with the glycolytic conversion of Glc to 3-carbon intermediates and subsequent gluconeogenesis. Compounds suggested as intermediates in inversion-type pathways were not converted to AA. Most strains converted Man to AA at a rate greater than they did Glc. Enzyme activities leading from Fru-6-P to the formation of GDP-Man were identified in all strains, but none of these activities correlated with the mutants' abilities to accumulate AA. However, there was a strong correlation between GDP-Man-3,5-epimerase activity and AA accumulation. Wild-type P. moriformis ATCC 75669 and mutant strains of varying AA-synthesizing abilities rapidly converted l-Gal or l-galactono-1,4-lactone to AA. Based on this data, a biosynthetic pathway from Glc to AA is proposed in which the epimerase is the rate-limiting activity in AA synthesis.

The VTC2 cycle and the de novo biosynthesis pathways for vitamin C in plants: An opinion

“…As mentioned earlier, ascorbic acid is synthesized in Chlorella sp. via a non-inversion pathway that is thought to involve GDP-mannose 3 00 ,5 00 -epimerase (Running et al, 2003). Indeed, the GDP-mannose 3 00 ,5 00 -epimerase protein of C. reinhardtii is very similar to the Arabidopsis enzyme (74% identity) (Table 1).…”

Biochemistry and Physiology of Vitamins in Euglena

“…Some eukaryotic microorganisms contain significant concentrations of cellular AsA however, details of the biosynthetic pathway remain unclear. Chlorella and Prototheca contain appreciable amounts of AsA and the addition of D-Man, L-GalA, or L-GalL to Prototheca medium resulted in a significant increase in the level of AsA suggesting the presence of a pathway similar to the plant pathway (Running et al 2002(Running et al , 2003. The protozoan parasite, Trypanosoma brucei, was found to possess a functional gene encoding D-arabinono-1,4-lactone oxidase which also oxidizes L-GalL however, the exact pathway arriving at the final precursor has not yet been determined (Wilkinson et al 2005).…”

Overexpression of Yeast Arabinono-1,4-Lactone Oxidase Gene (ALO) Increases Tolerance to Oxidative Stress and Al Toxicity in Transgenic Tobacco Plants

“…AsA concentrations were calculated from a standard curve prepared using 2.5 to 12.5 μM of AsA. In some experiments, as indicated in the figure legends, AsA was analyzed by HPLC system (Model HP1100, Alltech, USA) according to the method described by Running et al (2003). Leaf disks (0.2 g) or root tips (0.1 g) were extracted in 2 ml of 10 % trichloroacetic acid (TCA).…”