L-1,2-propanediol exits more rapidly than L-lactaldehyde from Escherichia coli.
Catabolism of the six-carbon compound L-fucose results in formation of dihydroxyacetone phosphate (C-1-to-C-3 fragment) and L-lactaldehyde (C-4-to-C-6 fragment) as intermediates. The fate of lactaldehyde depends on the respiratory growth conditions. Aerobically, lactaldehyde is oxidized to L-lactate by an NAD-linked dehydrogenase (ald product). L-Lactate, in turn, is converted to pyruvate, which enters the general metabolic pool. Anaerobically, lactaldehyde is reduced to L-1,2-propanediol by an NADH-linked oxidoreductase (fucO product). L-1,2-Propanediol is excreted as a terminal fermentation product. In a previous study, we showed that retention of the C-4-to-C-6 fragment of fucose depended on the competition for lactaldehyde by aldehyde dehydrogenase and propanediol oxidoreductase (Y. Zhu and E.C.C. Lin, J. Bacteriol. 169:785-789, 1987). In this study, we compared the wild-type strain and isogenic mutant strains defective in ald, fucO, or both for ability to accumulate radioactivity when incubated with fucose labeled at either the C-1 or the C-6 position. The results showed that although blocking the oxidation of lactaldehyde prevented its assimilation, rapid exit of the 3-carbon unit occurred only when the compound was reduced to propanediol. Moreover, growth experiments on fucose indicated that a double ald fucO mutant accumulated inhibiting concentrations of lactaldehyde. The inner cell membrane therefore appears to be much more permeable to the 3-carbon alcohol than to the 3-carbon aldehyde. The almost instantaneous exit of propanediol appears to be a facilitated process.
Zhu
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Y
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115.
Lin
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EC
D000447:Aldehydes / Q000378:metabolism*; D001692:Biological Transport; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D005798:Genes, Bacterial; D005809:Genes, Regulator; D005838:Genotype; D009154:Mutation; D010641:Phenotype; D019946:Propylene Glycol; D011409:Propylene Glycols / Q000378:metabolism*; D015183:Restriction Mapping; D013045:Species Specificity
D000447:Aldehydes; D011409:Propylene Glycols; C014634:lactaldehyde; D019946:Propylene Glycol