This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: vitamin C biosynthesis, L-ascorbic acid biosynthesis II
|Superclasses:||Biosynthesis → Cofactors, Prosthetic Groups, Electron Carriers Biosynthesis → Vitamins Biosynthesis → Ascorbate Biosynthesis|
Expected Taxonomic Range: Viridiplantae
Ascorbate (vitamin C) is an important antioxidant and an enzyme cofactor. Higher plants and higher animals (but not humans) can synthesize ascorbate. Plants provide the major dietary vitamin C source for humans. The plant ascorbate biosynthesis pathways have only been recently proposed. They differ from what was found in mammals.
About the different routes of ascorbate biosynthesis in plants:
The existence of alternative ascorbate biosynthesis routes in plants and the contribution of each route to ascorbate biosynthesis in vivo have been under debate. Wheeler et al originally proposed a so called linear L-galactose pathway (see L-ascorbate biosynthesis I (L-galactose pathway)) that requires the enzymes PMI (phosphomannose isomerase), PMM (phosphomannomutase) and VTC1 (GDP-D-mannose pyrophosphorylase) among the others [Wheeler98]. The failure to detect PMI activity in many plants in the past has prompted the search for alternative routes and the subsequent proposal of the L-gulose pathway (see L-ascorbate biosynthesis II (L-gulose pathway)) [Wolucka03]. However, the recent detection of PMI activity in Arabidopsis, the lower ascorbate contents observed in a PMI silenced line of tobacco, the higher ascorbate contents obtained in a PMI over-expressing line, and the lower ascorbate levels in the Arabidopsis VTC1 mutants all argue in favor of the linear L-galactose pathway (reviewed in [Linster08]).
About This Pathway
GDP-D-mannose 3",5"-epimerase converts GDP-D-mannose not only to GDP-L-galactose, an intermediate in the L-galactose pathway, but also to GDP-L-gulose. This finding prompted the proposal of the L-gulose pathway. Further more, feeding Arabidopsis cells with L-gulose or L-gulose-1,4-lactone increased ascorbate level. It again supports the existence of the L-gulose route [Wolucka03]. The L-galactose dehydrogenase purified from pea was shown active with L-gulose, although the enzyme's substrate specificity with L-gulose (Km= 3.7 mM) was very low comparing with its preferred substrate L-galactose (Km= 0.43 mM) [Gatzek02]. It remains to be determined whether it is has a biological role in the L-gulose pathway. L-gulose-1,4-lactone dehydrogenase activity was detected in potato tubers, in both the mitochondrial and cytosolic cell fractions [Wolucka03].
Gatzek02: Gatzek S, Wheeler GL, Smirnoff N (2002). "Antisense suppression of l-galactose dehydrogenase in Arabidopsis thaliana provides evidence for its role in ascorbate synthesis and reveals light modulated l-galactose synthesis." Plant J 30(5);541-53. PMID: 12047629
Wolucka03: Wolucka BA, Van Montagu M (2003). "GDP-mannose 3',5'-epimerase forms GDP-L-gulose, a putative intermediate for the de novo biosynthesis of vitamin C in plants." J Biol Chem 278(48);47483-90. PMID: 12954627
Major05: Major LL, Wolucka BA, Naismith JH (2005). "Structure and function of GDP-mannose-3',5'-epimerase: an enzyme which performs three chemical reactions at the same active site." J Am Chem Soc 127(51);18309-20. PMID: 16366586
Wolucka01: Wolucka BA, Persiau G, Van Doorsselaere J, Davey MW, Demol H, Vandekerckhove J, Van Montagu M, Zabeau M, Boerjan W (2001). "Partial purification and identification of GDP-mannose 3",5"-epimerase of Arabidopsis thaliana, a key enzyme of the plant vitamin C pathway." Proc Natl Acad Sci U S A 98(26);14843-8. PMID: 11752432
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