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.
|Superclasses:||Degradation/Utilization/Assimilation → Carbohydrates Degradation → Sugars Degradation|
Some taxa known to possess this pathway include : Arabidopsis thaliana col
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]).
Another controversy concerns the enzymes VTC2/VTC5 (named for Vitamin C defficiency). Two activities of VTC2 have been reported, GDP-L-galactose phosphorylase and GDP-L-galactose: hexose-1-P guanylyltransferase. One study suggested that the major activity of VTC2 is the guanylyltransferase activity [Laing07], whereas another study suggested that phosphorylase is the major activity of VTC2 [Linster07a]. The guanylyltransferase activity is critical for the so called VTC2 cycle and extended VTC2 cycle, both of which lead to the formation of L-galactose-1-P, an intermediate in the L-galactose pathway. Arabidopsis double mutants of VTC2/VTC5 are seedling lethal, which can be rescued by supplementation with L-galactose [Dowdle07]. It demonstrates that the L-galactose pathway is a critical source of ascorbate, at least in Arabidopsis.
The extended VTC2 cycle includes an additional (unidentified) enzyme in comparison with the VTC2 cycle. The putative GDP-D-mannose 2"-epimerase catalyzes the reversible conversion of GDP-D-glucose to GDP-D-mannose. GDP-D-glucose, that can be formed by VTC2, strongly inhibits GDP-D-mannose 3",5"-epimerase, another enzyme in the VTC2 cycle. Thus, the conversion of the "toxic" GDP-D-glucose by the extended VTC2 cycle makes the extended pathway more favorable than the original VTC2 cycle (reviewed in [Wolucka07]). In addition, the extended VTC2 cycle can make direct use of D-glucose-1-P which is derived from photosynthesis.
The intermediate GDP-sugars in ascorbate biosynthesis are also substrates for cell wall polysaccharides biosynthesis and protein glycosylation.
Dowdle07: Dowdle J, Ishikawa T, Gatzek S, Rolinski S, Smirnoff N (2007). "Two genes in Arabidopsis thaliana encoding GDP-L-galactose phosphorylase are required for ascorbate biosynthesis and seedling viability." Plant J 52(4);673-89. PMID: 17877701
Laing07: Laing WA, Wright MA, Cooney J, Bulley SM (2007). "The missing step of the L-galactose pathway of ascorbate biosynthesis in plants, an L-galactose guanyltransferase, increases leaf ascorbate content." Proc Natl Acad Sci U S A 104(22);9534-9. PMID: 17485667
Linster07a: Linster CL, Gomez TA, Christensen KC, Adler LN, Young BD, Brenner C, Clarke SG (2007). "Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants." J Biol Chem 282(26);18879-85. PMID: 17462988
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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