Escherichia coli K-12 substr. MG1655 Pathway: lactose degradation III
Inferred from experiment

Pathway diagram: lactose degradation III

If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Locations of Mapped Genes:

Schematic showing all replicons, marked with selected genes

Genetic Regulation Schematic

Genetic regulation schematic for lactose degradation III

Synonyms: lactose degradation 3

Superclasses: Degradation/Utilization/AssimilationCarbohydrates DegradationSugars DegradationLactose Degradation

In E. coli the disaccharide lactose is degraded by hydrolysis of the β-1,4 glycosidic bond by β-galactosidase, producing β-D-glucose and β-D-galactose. The enzyme can also catalyze conversion of lactose to allolactose (β-D-galactopyranosyl-(1-6)-D-glucopyranose) by transglycosylation, and can hydrolyze allolactose (in [Huber80]). Allolactose is the physiological inducer of this pathway.

Further metabolism of glucose and galactose inside the cell is thought to proceed by their initial transport out of the cell, followed by reentry. It has been shown that when lactose is added to a growing culture of E. coli, galactose, glucose and allolactose reach high levels inside the cells and are rapidly effluxed into the medium [Huber80]. It has also been shown that an E. coli mutant defective in the uptake of glucose and galactose grew poorly with lactose as a sole carbon source. Additional transport rate and radiotracer studies supported the efflux mechanism [Huber84]. No E. coli genes specifically involved in sugar efflux during lactose metabolism have been conclusively identified. However, SetA and SetB, members of the SET (sugar efflux transporter) family may have a role [Liu99, Liu99a].

It has been suggested that as glucose reenters the cell it could be phosphorylated to glucose-1-phosphate by the phosphoenolpyruvate-phosphotransferase system (in [Huber80]). Glucose-1-phosphate could then be converted to glucose-6-phosphate by phosphoglucomutase and enter glycolysis. Galactose could reenter the cell by facilitated diffusion (in [Huber80]), or active transport systems galP, or mgl (Lin in [Neidhardt96]). Galactose can also be converted to glucose-1-phosphate (Fraenkel in [Neidhardt96]). (See EcoCyc pathways: glucose and glucose-1-phosphate degradation; galactose degradation I; and superpathway of glycolysis and Entner-Doudoroff).

The E. coli lacZ gene coding for β-galactosidase is the first of three structural genes of the historically significant lac operon. The study of this operon provided the primary basis for the original operon concept [Jacob61]. Lactose and other galactosides are transported into the cell by lactose permease, the product of the second structural gene of the operon [Abramson03]. The third structural gene codes for galactoside acetyltransferase (thiogalactoside transacetylase). The proposed function of this enzyme is acetylation of potentially toxic pyranosides that are exported from the cell, thereby preventing their reentry [Wang02a]. A review of biochemical studies of the three lac enzymes by Zabin and Fowler can be found in [Miller78].

Created 01-Aug-1995 by Riley M, Marine Biological Laboratory


Abramson03: Abramson J, Smirnova I, Kasho V, Verner G, Kaback HR, Iwata S (2003). "Structure and mechanism of the lactose permease of Escherichia coli." Science 301(5633);610-5. PMID: 12893935

Huber80: Huber RE, Lytton J, Fung EB (1980). "Efflux of beta-galactosidase products from Escherichia coli." J Bacteriol 141(2);528-33. PMID: 6767683

Huber84: Huber RE, Hurlburt KL (1984). "Escherichia coli growth on lactose requires cycling of beta-galactosidase products into the medium." Can J Microbiol 30(3);411-5. PMID: 6426769

Jacob61: Jacob F, Monod J (1961). "Genetic regulatory mechanisms in the synthesis of proteins." J Mol Biol 3;318-56. PMID: 13718526

Liu99: Liu JY, Miller PF, Willard J, Olson ER (1999). "Functional and biochemical characterization of Escherichia coli sugar efflux transporters." J Biol Chem 274(33);22977-84. PMID: 10438463

Liu99a: Liu JY, Miller PF, Gosink M, Olson ER (1999). "The identification of a new family of sugar efflux pumps in Escherichia coli." Mol Microbiol 31(6);1845-51. PMID: 10209755

Miller78: Miller JH, Reznikoff WS (eds) "The Operon." Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1978.

Neidhardt96: Neidhardt FC, Curtiss III R, Ingraham JL, Lin ECC, Low Jr KB, Magasanik B, Reznikoff WS, Riley M, Schaechter M, Umbarger HE "Escherichia coli and Salmonella, Cellular and Molecular Biology, Second Edition." American Society for Microbiology, Washington, D.C., 1996.

Wang02a: Wang XG, Olsen LR, Roderick SL (2002). "Structure of the lac operon galactoside acetyltransferase." Structure 10(4);581-8. PMID: 11937062

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Bartesaghi14: Bartesaghi A, Matthies D, Banerjee S, Merk A, Subramaniam S (2014). "Structure of β-galactosidase at 3.2-Å resolution obtained by cryo-electron microscopy." Proc Natl Acad Sci U S A 111(32);11709-14. PMID: 25071206

Bartesaghi15: Bartesaghi A, Merk A, Banerjee S, Matthies D, Wu X, Milne JL, Subramaniam S (2015). "Electron microscopy. 2.2 Å resolution cryo-EM structure of β-galactosidase in complex with a cell-permeant inhibitor." Science 348(6239);1147-51. PMID: 25953817

Beckwith67: Beckwith JR (1967). "Regulation of the lac operon. Recent studies on the regulation of lactose metabolism in Escherichia coli support the operon model." Science 156(3775);597-604. PMID: 5337175

Bourgeois65: Bourgeois S, Cohn M, Orgel LE (1965). "Suppression of and complementation among mutants of the regulatory gene of the lactose operon of Escherichia coli." J Mol Biol 14(1);300-2. PMID: 5327656

Case73: Case GS, Sinnott ML, Tenu JP (1973). "The role of magnesium ions in beta-galactosidase hydrolyses. Studies on charge and shape of the beta-galactopyranosyl binding site." Biochem J 1973;133(1);99-104. PMID: 4721625

Cohn51: Cohn M, Monod J (1951). "[Purification and properties of the beta-galactosidase (lactase) of Escherichia coli.]." Biochim Biophys Acta 7(1);153-74. PMID: 14848081

COHN52: Cohn M, Torriani AM (1952). "Immunochemical studies with the beta-galactosidase and structurally related proteins of Escherichia coli." J Immunol 69(5);471-91. PMID: 13011306

COHN53: Cohn M, Torriani AM (1953). "The relationships in biosynthesis of the beta-galactosidase- and Pz-proteins in Escherichia coli." Biochim Biophys Acta 10(2);280-9. PMID: 13051404

COHN57: Cohn M (1957). "Contributions of studies on the beta-galactosidase of Escherichia coli to our understanding of enzyme synthesis." Bacteriol Rev 21(3);140-68. PMID: 13471456

Cohn89: Cohn M (1989). "The way it was: a commentary on 'Studies on the Induced Synthesis of beta-galactosidase in Escherichia coli: the Kinetics and Mechanism of Sulfur Incorporation'." Biochim Biophys Acta 1000;109-12. PMID: 2505844

Cook62: Cook A, Lederberg J (1962). "Recombination studies of lactose nonfermenting mutants of Escherichia coli K-12." Genetics 47;1335-53. PMID: 14022758

Craig12: Craig DB, Schwab T, Sterner R (2012). "Random mutagenesis suggests that sequence errors are not a major cause of variation in the activity of individual molecules of β-galactosidase." Biochem Cell Biol 90(4);540-7. PMID: 22475386

Craven65: Craven GR, Steers E, Anfinsen CB (1965). "Purification, Composition, and Molecular Weight of the β-Galactosidase of Escherichia coli K12." J Biol Chem 240;2468-77. PMID: 14304855

Gallagher97: Gallagher CN, Huber RE (1997). "Monomer-dimer equilibrium of uncomplemented M15 beta-galactosidase from Escherichia coli." Biochemistry 36(6);1281-6. PMID: 9063875

Gallagher98: Gallagher CN, Huber RE (1998). "Studies of the M15 beta-galactosidase complementation process." J Protein Chem 17(2);131-41. PMID: 9535275

Gallagher99: Gallagher CN, Huber RE (1999). "Stabilities of uncomplemented and complemented M15 beta-galactosidase (Escherichia coli) and the relationship to alpha-complementation." Biochem Cell Biol 77(2);109-18. PMID: 10438145

Gilbert66: Gilbert W, Muller-Hill B (1966). "Isolation of the lac repressor." Proc Natl Acad Sci U S A 56(6);1891-8. PMID: 16591435

GOA01: GOA, MGI (2001). "Gene Ontology annotation based on Enzyme Commission mapping." Genomics 74;121-128.

GOA01a: GOA, DDB, FB, MGI, ZFIN (2001). "Gene Ontology annotation through association of InterPro records with GO terms."

GOA06: GOA, SIB (2006). "Electronic Gene Ontology annotations created by transferring manual GO annotations between orthologous microbial proteins."

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Please cite the following article in publications resulting from the use of EcoCyc: Nucleic Acids Research 41:D605-12 2013
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