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lactose degradation III

In |FRAME:TAX-562| the disaccharide lactose is degraded by hydrolysis of the beta-1,4 glycosidic bond by beta-galactosidase, producing beta-D-glucose and beta-D-galactose. The enzyme can also catalyze conversion of lactose to allolactose (beta-D-galactopyranosyl-(1-6)-D-glucopyranose) by transglycosylation, and can hydrolyze allolactose (in |CITS: [6767683]|). 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 |FRAME:TAX-562|, galactose, glucose and allolactose reach high levels inside the cells and are rapidly effluxed into the medium |CITS: [6767683]|. It has also been shown that an |FRAME:TAX-562| 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 |CITS: [6426769]|. No |FRAME:TAX-562| 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 |CITS: [10438463] [10209755]|. It has been suggested that as glucose reenters the cell it could be phosphorylated to glucose-1-phosphate by the phosphoenolpyruvate-phosphotransferase system (in |CITS: [6767683]|). 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 |CITS: [6767683]|), or active transport systems galP, or mgl (Lin in |CITS: [ColiSalII]|). Galactose can also be converted to glucose-1-phosphate (Fraenkel in |CITS: [ColiSalII]|). (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 beta-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 |CITS: [13718526]|. Lactose and other galactosides are transported into the cell by lactose permease, the product of the second structural gene of the operon |CITS: [12893935]|. 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 |CITS: [11937062]|. A review of biochemical studies of the three lac enzymes by Zabin and Fowler can be found in |CITS: [Operon78]|. |FRAME:TAX-382| is a soil bacterium that exists either free-living, or in a symbiotic nitrogen-fixing relationship in root nodules of leguminous plants such as alfalfa (|FRAME:TAX-3879|) (in |CITS: [11481430]|). R. meliloti requires large amounts of metabolic energy for nitrogen fixation and can utilize many compounds as carbon sources, including alpha-galactosides and beta-galactosides |CITS: [12218025]| and reviewed in |CITS: [3904617]|. Nomenclature note: |FRAME:TAX-382| is also known as |FRAME:TAX-382| (|CITS: [9922248]| and URL cited in |CITS: [11474104]|). In |FRAME:TAX-382| Rm2011 (a derivative of SU47), two distinct beta-galactosidase activities are present, one that is inducible, and another that is constitutive at a low level |CITS: [409467]| . The inducible beta-galactosidase activity is necessary for growth on lactose, as mutants lacking the inducible enzyme cannot use lactose as a sole carbon source. The lactose utilization locus is located on the megaplasmid pRmeSU47b, one of the two megaplasmids present in R. meliloti. There is evidence to suggest that succinate also plays a role in regulating |FRAME:TAX-379| lactose utilization. R. meliloti grown on both lactose and succinate preferentially utilize succinate, repressing the inducible beta-galactosidase activity even in the presence of lactose. Unlike |FRAME:TAX-562|, further catabolism of beta-D-galactose in rhizobia is poorly understood |CITS: [6427192]|. Glucose catabolism in most rhizobia proceeds via the Entner-Doudoroff pathway, as reviewed in |CITS: [3904617]|. In |FRAME:TAX-9606| (humans), lactose is hydrolyzed by the intestinal enzyme |FRAME: CPLX-7457|, EC (click on the link for details). This enzyme differs in substrate specificity from human intestinal beta-galactosidase |CITS: [5822067]|

from BIOCYC source record: META_BGALACT-PWY
Type: pathway
Taxonomic scope
conserved biosystem

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