If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Degradation/Utilization/Assimilation → Amino Acids Degradation → Proteinogenic Amino Acids Degradation → L-threonine Degradation|
Microorganisms and mammals share two of the major, initial routes for threonine degradation. In the first route threonine is catabolized by catabolic threonine dehydratase (EC 22.214.171.124) to ammonia and 2-oxobutanoate. A biosynthetic version of this enzyme also occurs (see threonine deaminase) [Umbarger57]. In the second route threonine is catabolized by threonine dehydrogenase (EC 126.96.36.199) to form 2-amino-3-oxobutanoate, which is mainly cleaved by 2-amino-3-ketobutyrate CoA ligase, forming glycine and acetyl-CoA. The 2-amino-3-oxobutanoate can also be spontaneously converted to aminoacetone, which may be further metabolized to methylglyoxal (see L-threonine degradation III (to methylglyoxal)). A third route has been demonstrated in several bacteria and fungi. This route is based on the enzyme low-specificity L-threonine aldolase (EC 188.8.131.52), which cleaves threonine directly into glycine and acetaldehyde.
Escherichia. coli has been shown to assimilate nitrogen from some (but not all) amino acids, as well as agmatine, γ-aminobutyrate and the polyamines putrescine and spermidine. These nitrogen sources are used to generate glutamate and glutamine, the major intracellular nitrogen donors. Some nitrogen sources, such as aspartate, can generate glutamate by transamination (see aspartate aminotransferase, PLP-dependent). Others, such as proline and arginine, produce glutamate as end products (glutamate generating amino acids) (see proline degradation and L-arginine degradation II (AST pathway)). Other nitrogen sources, such as serine, require ammonia production for glutamate synthesis (ammonia generating amino acids) (see L-serine degradation). Ammonia generation is required for glutamine synthesis (see L-glutamine biosynthesis I).
In E. coli a low intracellular level of ammonia results in low intracellular glutamine and induction of the nitrogen-regulated (Ntr) response that involves response regulators NtrC transcriptional dual regulator and NtrB sensory histidine kinase. The Ntr response functions in ammonia assimilation, nitrogen scavenging and metabolic coordination.
E. coli has three systems that can transport threonine: serine / threonine:Na+ symporter [Kim02b], branched chain amino acid ABC transporter [Robbins73], and serine / threonine:H+ symporter TdcC [Sumantran90]. Although E. coli can use threonine, glycine, or serine as a nitrogen source, efficient serine or threonine utilization requires amino acid supplementation. Leucine supplementation is required for the use of threonine as a nitrogen source in pathways utilizing threonine dehydrogenase (TDH) which is induced by leucine [Potter77] (see L-threonine degradation II and L-threonine degradation III (to methylglyoxal)). TDH is is a major route for threonine degradation in E. coli. A minor pathway is shown in L-threonine degradation IV and an anaerobic pathway is shown in L-threonine degradation I.
About This Pathway
Many organisms, both prokaryotic and eukaryotic, possess an L-threonine aldolase that can catalyze the interconversion of threonine and glycine [Morris69, Yamada70, Liu97a, Kataoka97, Kataoka97a, Liu98a, Liu98, Joshi06]. Two types of the enzyme have been described, a low-specificity L-threonine aldolase which accepts both L-threonine and L-allo-threonine [Liu97a] (see Escherichia coli low-specificity L-threonine aldolase), and an L-allo-threonine aldolase characterized in Aeromonas jandaei [Kataoka97] (see L-allo-threonine aldolase).
Despite the wide distribution of L-threonine aldolase, its physiological significance is uncertain. It is important for the synthesis of cellular glycine in yeast [Monschau97], but not in wild-type E. coli [Liu98]. In E. coli, if the major glycine biosynthetic pathway is disrupted by a glyA mutation, low-specificity L-threonine aldolase may provide an alternative glycine biosynthetic pathway [Liu98] (see glycine biosynthesis I). Although purified GlyA was also shown to cleave threonine, it did so at a very slow rate [Schirch85] (see serine hydroxymethyltransferase). Overall, the low-specificity L-threonine aldolase pathway is considered to be a minor pathway of threonine degradation in E. coli.
Review: Reitzer, L. (2005) "Catabolism of Amino Acids and Related Compounds" EcoSal 3.4.7 [ECOSAL]
Superpathways: superpathway of L-threonine metabolism
Joshi06: Joshi V, Laubengayer KM, Schauer N, Fernie AR, Jander G (2006). "Two Arabidopsis threonine aldolases are nonredundant and compete with threonine deaminase for a common substrate pool." Plant Cell 18(12);3564-75. PMID: 17172352
Kataoka97: Kataoka M, Wada M, Nishi K, Yamada H, Shimizu S (1997). "Purification and characterization of L-allo-threonine aldolase from Aeromonas jandaei DK-39." FEMS Microbiol Lett 151(2);245-8. PMID: 9228760
Kataoka97a: Kataoka M, Ikemi M, Morikawa T, Miyoshi T, Nishi K, Wada M, Yamada H, Shimizu S (1997). "Isolation and characterization of D-threonine aldolase, a pyridoxal-5'-phosphate-dependent enzyme from Arthrobacter sp. DK-38." Eur J Biochem 248(2);385-93. PMID: 9346293
Kim02b: Kim YM, Ogawa W, Tamai E, Kuroda T, Mizushima T, Tsuchiya T (2002). "Purification, reconstitution, and characterization of Na(+)/serine symporter, SstT, of Escherichia coli." J Biochem (Tokyo) 132(1);71-6. PMID: 12097162
Liu97a: Liu JQ, Nagata S, Dairi T, Misono H, Shimizu S, Yamada H (1997). "The GLY1 gene of Saccharomyces cerevisiae encodes a low-specific L-threonine aldolase that catalyzes cleavage of L-allo-threonine and L-threonine to glycine--expression of the gene in Escherichia coli and purification and characterization of the enzyme." Eur J Biochem 245(2);289-93. PMID: 9151955
Liu98: Liu JQ, Dairi T, Itoh N, Kataoka M, Shimizu S, Yamada H (1998). "Gene cloning, biochemical characterization and physiological role of a thermostable low-specificity L-threonine aldolase from Escherichia coli." Eur J Biochem 1998;255(1);220-6. PMID: 9692922
Liu98a: Liu JQ, Ito S, Dairi T, Itoh N, Kataoka M, Shimizu S, Yamada H (1998). "Gene cloning, nucleotide sequencing, and purification and characterization of the low-specificity L-threonine aldolase from Pseudomonas sp. strain NCIMB 10558." Appl Environ Microbiol 64(2);549-54. PMID: 9464392
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