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Fucose and Rhamnose Degradation
Last Updated: 2018-12-21
In E. coli, L-fucose and L-rhamnose are metabolized through parallel pathways. The pathways converge after their corresponding aldolase reactions yielding the same products: lactaldehye. Via reactions catalyzed by proteins encoded in linked operons comprising a regulon, the methylpentose, alpha-L-rhamnopyranose and/or beta-L-rhamnopyranose, is taken into the cell through a proton symporter and metabolized, enabling E. coli to grow on it as a total source of carbon and energy. For alpha-L-rhamnopyranose, it is isomerized by a l-rhamnose mutarotase resulting in a beta-L-rhamnopyranose which is then isomerized into a keto-L-rhamnulose by a l-rhamnose isomerase. The keto-L-rhamnulose spontaneously changes into a L-rhamnulofuranose which is phosphorylated by a rhamnulokinase resulting in a L-rhamnulose 1-phosphate. This compound reacts with a rhamnulose-1-phosphate aldolase resulting in a dihydroxyacetone phosphate and a lactaldehyde. For beta-L-rhamnopyranose, it is isomerized by a L-fucose mutarotase resulting in a alpha-L-fucopyranose. This compound is then isomerized by an L-fucose isomerase resulting in a L-fuculose which in turn gets phosphorylated into an L-fuculose 1-phosphate through an L-fuculokinase. The compound L-fuculose 1-phosphate reacts with an L-fuculose phosphate aldolase through a dihydroxyacetone phosphate and a lactaldehyde. Two pathways can be used for degradation of L-lactaldehyde. Aerobically, it is converted via lactate to pyruvate, also an intermediate of glycolysis. Anaerobically, lactaldehyde reductase is induced which converts lactaldehyde into propane-1,2-diol. Under aerobic conditions, L-lactaldehyde is oxidized in two steps to pyruvate, thereby channeling all the carbons from fucose or rhamnose into central metabolic pathways. Under anaerobic conditions, L-lactaldehyde is reduced to L-1,2-propanediol, which is secreted into the environment.
Fucose and Rhamnose Degradation References
Escherichia coli and Salmonella: Cellular and Molecular Biology (EcoSal). Online edition.
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