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
L-tartrate is found in many plants, particularly in grapes, bananas and tamarinds. It is the principle acid found in wine, contributing important aspects to the taste, mouthfeel and aging potential of wine.
Pelargonium crispum accumulates L-tartrate [Stafford61] which it produces from L-ascorbate with concomitant production of oxalate [Wagner73]. Radio-tracer studies have shown that L-tartrate is produced from L-threonate, which is derived from carbons 3-6 of ascorbate, indicating cleavage between C2 and C3 carbons of ascorbate (unlike the alternative pathway found in grapes, where theronate is derived from carbons 1-4 of ascorbate, cleaved between C4 and C5).
The pathway is believed to exist in many plant families [Williams78a]. While the existence of the pathway has been documented for quite a long time, it has been poorly understood until recent times. A study of ascorbate degradation by cultured cells of Paul's Scarlet Climber Rose revealed that the pathway, which is extracellular, proceeds via the intermediates cyclic-2,3-O-oxalyl-L-threonate, cyclic- 3,4-O-oxalyl-L-threonate, and 4-O-oxalyl-L-threonate [Green05]. While the pathway can proceed non-enzymatically, it was shown that several steps are enzymatically catalyzed, including the initial oxidation of L-ascorbate, the conversion of cyclic- 3,4-O-oxalyl-L-threonate to 4-O-oxalyl-L-threonate, and the hydrolysis of 4-O-oxalyl-L-threonate to L-threonate and oxalate [Green05].
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
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