Galactoside, lactose

Fig. 27.9. Lactase activity. Lactase is a (J-galactosidase. It cleaves the (3-galactoside lactose, the major sugar in milk, forming galactose and glucose.

In bacteria, accumulation of substrates against a concentration gradient can occur through two main classes of transport systems (see [30] for a summary). The prototype of the first class of transporters is the /3-galactoside permease of Escherichia coli (see [31]). It is a relatively simple system involving only a single membrane-bound protein. It catalyzes a lactose-H symport. Other transporters... 

The Y gene, which encodes a galactoside permease, the transport protein required for entry of lactose into the cell... 

Galactoside 2-L-fucosyltransferase [EC 2.4.1.69], also known as blood group H a-2-fucosyltransferase, catalyzes the reaction of GDP-L-fucose with /8-D-galacto-syl-R to produce GDP and a-L-fucosyl-l,2-/3-D-galac-tosyl-R. Lactose can also act as the acceptor substrate. 

The lactose (toe) operon (Fig. 28-7a) includes the genes for jS-galactosidase (Z), galactoside permease (F), and thiogalactoside transacetylase (A). The last of these enzymes appears to modify toxic galactosides to facilitate their removal from the cell. Each of the three genes is preceded by a ribosome binding site (not shown in Fig. 28-7) that independently directs the translation... 

FIGURE 28-6 Lactose metabolism in /. coli. Uptake and metabolism of lactose require the activities of galactoside permease and /3-galactosidase. Conversion of lactose to allolactose by transglycosyla-tion is a minor reaction also catalyzed by /3-galactosidase. 

In a more recent study (Asp et al. 1980), six oligosaccharides from hydrolyzed lactose milk which had been treated with K. lactis lactase were isolated from their structure, it was concluded that the enzyme had high transglycosylation activity with specificity for the formation of /3-(l,6)-galactosidic bonds. 

The bacterial lactose-transport protein (lactose permease) transports j3-galactosides, such as lactose, o-nitrophenyl-jS-galactoside, and isopropyl-jS-thiogalactoside. It does not transport galactosides with an a-glycosidic linkage. 

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