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 → Carboxylates Degradation → L-Ascorbate Degradation|
Expected Taxonomic Range: Viridiplantae
L-ascorbate, also known as vitamin C, fulfils multiple essential roles in both plants and animals. Being a strong reducing agent, it functions as an antioxidant and a redox buffer. It is also a cofactor for several enzymes, which are involved in many important pathways, including collagen hydroxylation, carnitine biosynthesis, norepinephrine biosynthesis, and hormone and tyrosine metabolism. In plants L-ascorbate is also implicated in defense against pathogens and in control of plant growth and development. A significant proportion of a plant's ascorbate is found in the apoplast (the aqueous solution permeating the cell walls) [Green05].
Under aerobic conditions L-ascorbate is oxidized in cells to dehydroascorbate (via the radical monodehydroascorbate radical), which can be recycled back to ascorbate by the ascorbate glutathione cycle. However, once formed, dehydroascorbate can be further broken down in vivo by irreversible reactions, escaping the ascorbate glutathione cycle.
Several pathways for the irreversible catabolism of ascorbate have been described. Facultatively aerobic bacteria such as Escherichia coli and Klebsiella pneumoniae degrade L-ascorbate by different pathways under aerobic and anaerobic conditions (see L-ascorbate degradation II (bacterial, aerobic) and L-ascorbate degradation I (bacterial, anaerobic)). The anaerobic pathway begins with phosphorylation of ascorbate (mediated by a PTS-type transporter), while the aerobic pathway proceeds via 2,3-dioxo-L-gulonate. Both pathways produce D-xylulose 5-phosphate, a centeral metabolite that is fed into the pentose phosphate pathway [Campos08].
Plants from the Vitaceae family (e.g. grapes) metabolize ascorbate to L-tartrate via the intermediates 2-keto-L-gulonate and L-idonate (see pathway L-ascorbate degradation IV). The tartrate skeleton is derived from carbons 1-4 of L-ascorbate, indicating a cleavage between carbons 4 and 5 [Loewus99, DeBolt06].
The geraniaceous plant Pelargonium crispum metabolizes ascorbate to L-tartrate and oxalate via a different pathway, with L-threonate, rather than L-idonate, as an intermediate (see pathway L-ascorbate degradation III). In this case the tartrate skeleton is derived from carbons 3-6 of L-ascorbate, indicating a cleavage between carbons 2 and 3 [Loewus99, Franceschi05]. Grapes are also known to accumulate oxalate [DeBolt04], and thus may be using both pathways to generate tartrate.
About This Pathway
2,3-dioxo-L-gulonate (DKG) is a well established degradation product of L-dehydro-ascorbate (DHA), although the pathway(s) it is involved in are not well characterized. A study of ascorbate degradation by human lens homogenates found that under non-oxidative conditions DHA was hydrolyzed to DKG, which was subsequently converted to oxalate and L-erythrulose [Simpson00]. These findings contradicted an earlier study of L-ascorbate degradation into oxalate in several plants, which found that DKG was a very poor substrate for oxalate production (although it is possible that the pathways differ among plants and animals) [Yang75].
This pathway describes the findings of a careful study that evaluated the different chemical reactions involved in DHA catabolism in the plant apoplast [Parsons11]. The study determined that DHA is a branch point in ascorbate catabolism - under oxidative conditions it is oxidized to oxalate and its esters (see L-ascorbate degradation III), while under non-oxidative conditions it is hydrolyzed to DKG. The oxidation:hydrolysis ratio is determined by the concentration of reactive oxygen species.
Under non-enzymatic, non-oxidative conditions, DKG was formed slowly and converted to 2-carboxy-L-xylonolactone, which was reversibly de-lactonized to 2-carboxy-L-threo-pentonate [Parsons11]. There was little evidence for enzyme activity in the catabolism of DKG. The presence of either L-ascorbate or hydrogen peroxide supressed the formation of 2-carboxy-L-xylonolactone, suggesting that DKG was diverted under these conditions to competing oxidative pathways.
The fact that the pathway proceeds non-enzymatically suggests that it may occur in the apoplast of most, if not all plants.
Campos08: Campos E, de la Riva L, Garces F, Gimenez R, Aguilar J, Baldoma L, Badia J (2008). "The yiaKLX1X2PQRS and ulaABCDEFG gene systems are required for the aerobic utilization of L-ascorbate in Klebsiella pneumoniae strain 13882 with L-ascorbate-6-phosphate as the inducer." J Bacteriol 190(20);6615-24. PMID: 18708499
DeBolt04: DeBolt, S., Hardie, J., Tyerman, S., Ford, C. M. (2004). "Composition and synthesis of raphide crystals and druse crystals in berries of Vitis vinifera L. cv. Cabernet Sauvignon: ascorbic acid as precursor for both oxalic and tartaric acids as revealed by radiolabelling studies." Aust. J. Grape Wine Res. 10: 134-142.
Parsons11: Parsons HT, Yasmin T, Fry SC (2011). "Alternative pathways of dehydroascorbic acid degradation in vitro and in plant cell cultures: novel insights into vitamin C catabolism." Biochem J. PMID: 21846329
Ishikawa96: Ishikawa T, Sakai K, Yoshimura K, Takeda T, Shigeoka S (1996). "cDNAs encoding spinach stromal and thylakoid-bound ascorbate peroxidase, differing in the presence or absence of their 3'-coding regions." FEBS Lett 384(3);289-93. PMID: 8617374
Ishikawa98: Ishikawa T, Yoshimura K, Sakai K, Tamoi M, Takeda T, Shigeoka S (1998). "Molecular characterization and physiological role of a glyoxysome-bound ascorbate peroxidase from spinach." Plant Cell Physiol 39(1);23-34. PMID: 9516999
Leonardis00: Leonardis, Silvana, Dipierro, Nunzio, Dipierro, Silvio "Purification and characterization of an ascorbate peroxidase from potato tuber mitochondria." Plant Physiology and Biochemistry, 2000, 38(10):773-779.
Shigeoka02: Shigeoka S, Ishikawa T, Tamoi M, Miyagawa Y, Takeda T, Yabuta Y, Yoshimura K (2002). "Regulation and function of ascorbate peroxidase isoenzymes." J Exp Bot 53(372);1305-19. PMID: 11997377
Yoshimura98: Yoshimura K, Ishikawa T, Nakamura Y, Tamoi M, Takeda T, Tada T, Nishimura K, Shigeoka S (1998). "Comparative study on recombinant chloroplastic and cytosolic ascorbate peroxidase isozymes of spinach." Arch Biochem Biophys 353(1);55-63. PMID: 9578600
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