The pathway of L-ascorbic acid biosynthesis in the colourless microalga Prototheca moriformis.
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.
Running
Jeffrey Allan
JA
Bio-Technical Resources, 1035 South 7th Street, Manitowoc, WI 54220, USA. jeff@biotechresources.com
Burlingame
Richard Paul
RP
Berry
Alan
A
D001205:Ascorbic Acid / Q000096:biosynthesis*; D002238:Carbohydrate Epimerases / Q000378:metabolism; D002247:Carbon Isotopes / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009154:Mutation; D011525:Prototheca / Q000145:classification / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*
D002247:Carbon Isotopes; D002238:Carbohydrate Epimerases; C019192:GDPmannose 3,5-epimerase; D005947:Glucose; D001205:Ascorbic Acid