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Last Updated: 2018-12-22
L-lyxose is an uncommon sugar in nature, and wild-type E. coli can not utilize it as a sole source of carbon and energy. However, mutations can arise that allow E. coli to metabolize L-lyxose through the use of enzymes of the rhamnose, arabinose and 2,3-diketo-L-gulonate systems. L-lyxose enters the cell through the rhaT-encoded rhamnose transporter. It is then isomerized to L-xylulose by L-rhamnose isomerase. Two types of mutations then allow further utilization of L-xylulose. Not shown here, a mutated from of L-rhamnulose kinase can phosphorylate L-xylulose, yielding L-xylulose-1-phosphate. Further metabolism to dihydroxyacetone phosphate and glycolate occurs through the rhamnulose-1-phosphate aldolase and aldehyde dehydrogenase A, NAD-linked enzymes. These products can enter glycolate degradation and glycolysis. As shown here, L-xylulose can be phosphorylated by L-xylulose kinase in a mutant in which YiaJ, the repressor for the yiaKLMNO-lyxK-sgbHUE operon, has been disrupted. L-xylulose-5-phosphate is then further metabolized by predicted L-xylulose 5-phosphate 3-epimerase (so far only a predicted function) and either of two L-ribulose 5-phosphate 4-epimerases (SgbE or AraD) to D-xylulose-5-phosphate, which then enters the pentose phosphate pathway (non-oxidative branch). The enzymes of L-rhamnose degradation I and L-arabinose degradation I are inducible by L-lyxose as well. (EcoCyc)
L-Lyxose Degradation References
Badia J, Gimenez R, Baldoma L, Barnes E, Fessner WD, Aguilar J: L-lyxose metabolism employs the L-rhamnose pathway in mutant cells of Escherichia coli adapted to grow on L-lyxose. J Bacteriol. 1991 Aug;173(16):5144-50.Pubmed: 1650346
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