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: ascorbic acid degradation I
|Superclasses:||Degradation/Utilization/Assimilation → Carboxylates Degradation → L-Ascorbate Degradation|
Expected Taxonomic Range:
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
The ula regulon encodes all of the proteins involved in this pathway and is formed by two operons, ulaG and ulaA-F. ulaG encodes an L-ascorbate 6-phosphate lactonase, and ulaA-F encodes the three components of the L-ascorbate PTS permease (L-ascorbate PTS permease - UlaA subunit, L-ascorbate PTS permease - UlaB subunit, and L-ascorbate PTS permease) and three catabolic enzymes (UlaDEF).
L-ascorbate is imported and converted to L-ascorbate 6-phosphate by the L-ascorbate PTS permease [Zhang03c]. The intracellular L-ascorbate-6-phosphate is subsequently metabolized by UlaG, UlaD, UlaE and UlaF to D-D-xylulose 5-phosphate, which can enter central metabolism via the non-oxidative branch of the pentose phosphate pathway [Yew02].
Under aerobic conditions an additional operon, yiaK-S, is required to catabolize L-ascorbate (see L-ascorbate degradation II (bacterial, aerobic)).
Unification Links: EcoCyc:PWY0-301
Campos04: Campos E, Baldoma L, Aguilar J, Badia J (2004). "Regulation of expression of the divergent ulaG and ulaABCDEF operons involved in LaAscorbate dissimilation in Escherichia coli." J Bacteriol 186(6);1720-8. PMID: 14996803
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.
Garces08: Garces F, Fernandez FJ, Gomez AM, Perez-Luque R, Campos E, Prohens R, Aguilar J, Baldoma L, Coll M, Badia J, Vega MC (2008). "Quaternary structural transitions in the DeoR-type repressor UlaR control transcriptional readout from the L-ascorbate utilization regulon in Escherichia coli." Biochemistry 47(44);11424-33. PMID: 18844374
Yew02: Yew WS, Gerlt JA (2002). "Utilization of L-ascorbate by Escherichia coli K-12: assignments of functions to products of the yjf-sga and yia-sgb operons." J Bacteriol 2002;184(1);302-6. PMID: 11741871
Campos02: Campos E, Aguilar J, Baldoma L, Badia J (2002). "The gene yjfQ encodes the repressor of the yjfR-X regulon (ula), which is involved in L-ascorbate metabolism in Escherichia coli." J Bacteriol 184(21);6065-8. PMID: 12374842
DiazMejia09: Diaz-Mejia JJ, Babu M, Emili A (2009). "Computational and experimental approaches to chart the Escherichia coli cell-envelope-associated proteome and interactome." FEMS Microbiol Rev 33(1);66-97. PMID: 19054114
Fernandez11: Fernandez FJ, Garces F, Lopez-Estepa M, Aguilar J, Baldoma L, Coll M, Badia J, Vega MC (2011). "The UlaG protein family defines novel structural and functional motifs grafted on an ancient RNase fold." BMC Evol Biol 11;273. PMID: 21943130
Garces08a: Garces F, Fernandez FJ, Perez-Luque R, Aguilar J, Baldoma L, Coll M, Badia J, Vega MC (2008). "Overproduction, crystallization and preliminary X-ray analysis of the putative L-ascorbate-6-phosphate lactonase UlaG from Escherichia coli." Acta Crystallogr Sect F Struct Biol Cryst Commun 64(Pt 1);36-8. PMID: 18097099
Garces10: Garces F, Fernandez FJ, Montella C, Penya-Soler E, Prohens R, Aguilar J, Baldoma L, Coll M, Badia J, Vega MC (2010). "Molecular architecture of the Mn2+-dependent lactonase UlaG reveals an RNase-like metallo-beta-lactamase fold and a novel quaternary structure." J Mol Biol 398(5);715-29. PMID: 20359483
Ibanez00a: Ibanez E, Gimenez R, Pedraza T, Baldoma L, Aguilar J, Badia J (2000). "Role of the yiaR and yiaS genes of Escherichia coli in metabolism of endogenously formed L-xylulose." J Bacteriol 2000;182(16);4625-7. PMID: 10913097
Kuznetsova05: Kuznetsova E, Proudfoot M, Sanders SA, Reinking J, Savchenko A, Arrowsmith CH, Edwards AM, Yakunin AF (2005). "Enzyme genomics: Application of general enzymatic screens to discover new enzymes." FEMS Microbiol Rev 29(2);263-79. PMID: 15808744
Samuel01: Samuel J, Luo Y, Morgan PM, Strynadka NC, Tanner ME (2001). "Catalysis and binding in L-ribulose-5-phosphate 4-epimerase: a comparison with L-fuculose-1-phosphate aldolase." Biochemistry 40(49);14772-80. PMID: 11732896
Shi08: Shi R, Pineda M, Ajamian E, Cui Q, Matte A, Cygler M (2008). "Structure of L-xylulose-5-Phosphate 3-epimerase (UlaE) from the anaerobic L-ascorbate utilization pathway of Escherichia coli: identification of a novel phosphate binding motif within a TIM barrel fold." J Bacteriol 190(24);8137-44. PMID: 18849419
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