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Pathway Description
Galactose Metabolism
Escherichia coli 536
Category:
Metabolite Pathway
Sub-Category:
Metabolic
Created: 2024-12-30
Last Updated: 2025-07-18
Galactose can be synthesized through two pathways: melibiose degradation involving an alpha galactosidase and lactose degradation involving a beta galactosidase. Melibiose is first transported inside the cell through the melibiose:Li+/Na+/H+ symporter. Once inside the cell, melibiose is degraded through alpha galactosidase into an alpha-D-galactose and a beta-D-glucose. The beta-D-glucose is phosphorylated by a glucokinase to produce a beta-D-glucose-6-phosphate which can spontaneously be turned into a alpha D glucose 6 phosphate. This alpha D-glucose-6-phosphate is metabolized into a glucose -1-phosphate through a phosphoglucomutase-1. The glucose -1-phosphate is transformed into a uridine diphosphate glucose through UTP--glucose-1-phosphate uridylyltransferase. The product, uridine diphosphate glucose, can undergo a reversible reaction in which it can be turned into uridine diphosphategalactose through an UDP-glucose 4-epimerase.
Galactose can also be produced by lactose degradation involving a lactose permease to uptake lactose from the environment and a beta-galactosidase to turn lactose into Beta-D-galactose.
Beta-D-galactose can also be uptaken from the environment through a galactose proton symporter.
Galactose is degraded through the following process:
Beta-D-galactose is introduced into the cytoplasm through a galactose proton symporter, or it can be synthesized from an alpha lactose that is introduced into the cytoplasm through a lactose permease. Alpha lactose interacts with water through a beta-galactosidase resulting in a beta-D-glucose and beta-D-galactose. Beta-D-galactose is isomerized into D-galactose. D-Galactose undergoes phosphorylation through a galactokinase, hence producing galactose 1 phosphate. On the other side of the pathway, a gluose-1-phosphate (product of the interaction of alpha-D-glucose 6-phosphate with a phosphoglucomutase resulting in a alpha-D-glucose-1-phosphate, an isomer of Glucose 1-phosphate, or an isomer of Beta-D-glucose 1-phosphate) interacts with UTP and a hydrogen ion in order to produce a uridine diphosphate glucose. This is followed by the interaction of galactose-1-phosphate with an established amount of uridine diphosphate glucose through a galactose-1-phosphate uridylyltransferase, which in turn output a glucose-1-phosphate and a uridine diphosphate galactose. The glucose -1-phosphate is transformed into a uridine diphosphate glucose through UTP--glucose-1-phosphate uridylyltransferase. The product, uridine diphosphate glucose, can undergo a reversible reaction in which it can be turned into uridine diphosphategalactose through an UDP-glucose 4-epimerase, and so the cycle can keep going as long as more lactose or galactose is imported into the cell
References
Galactose Metabolism References
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Yamamoto Y, Aiba H, Baba T, Hayashi K, Inada T, Isono K, Itoh T, Kimura S, Kitagawa M, Makino K, Miki T, Mitsuhashi N, Mizobuchi K, Mori H, Nakade S, Nakamura Y, Nashimoto H, Oshima T, Oyama S, Saito N, Sampei G, Satoh Y, Sivasundaram S, Tagami H, Horiuchi T, et al.: Construction of a contiguous 874-kb sequence of the Escherichia coli -K12 genome corresponding to 50.0-68.8 min on the linkage map and analysis of its sequence features. DNA Res. 1997 Apr 28;4(2):91-113. doi: 10.1093/dnares/4.2.91.
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Lemaire HG, Muller-Hill B: Nucleotide sequences of the gal E gene and the gal T gene of E. coli. Nucleic Acids Res. 1986 Oct 10;14(19):7705-11. doi: 10.1093/nar/14.19.7705.
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Wong LJ, Sheu KF, Lee SL, Frey PA: Galactose-1-phosphate uridylyltransferase: isolation and properties of a uridylyl-enzyme intermediate. Biochemistry. 1977 Mar 8;16(5):1010-6. doi: 10.1021/bi00624a032.
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This pathway was propagated using PathWhiz -
Pon, A. et al. Pathways with PathWhiz (2015) Nucleic Acids Res. 43(Web Server issue): W552–W559.
Propagated from SMP0000841
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