If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
Synonyms: lactose degradation 3
|Superclasses:||Degradation/Utilization/Assimilation → Carbohydrates Degradation → Sugars Degradation → Lactose 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 [Wang02]. A review of biochemical studies of the three lac enzymes by Zabin and Fowler can be found in [Miller78].
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.
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. PMID: 25071206
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
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
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
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
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