Panel 1: Lactose
(β-D-galactopyranosyl (1→4) D-glucopyranose)
Panel 2: Cellobiose
(β-D-glucopyranosyl (1→4) D-glucopyranose)
Lactose and Cellobiose are disaccharides whose monomeric units are connected by β(1→ 4) glycosidic bonds. As with cellobiose, the glucose unit of lactose has been rotated 180 °. Look closely at the two structures to determine the one stereogenic center that is different in the pyranosyl units. Do not be distracted by differences that are present in the location of hydroxyl groups that are on nonchiral carbons and by hydrogens that project differently from oxygens on which they are bonded. (Hint: Remeber that glucose and galactose are epimers.) When lactose is in an aqueous solution, like cellobiose and maltose, it is a reducing sugars because the aldehyde is in equilbrium with the α and β anomers of the glucopyranose.

Even though both of these sugars have β(1→ 4) glycosidic bonds, lactose is the only one of the two that is catalytically hydrolyzed by lactase. Lactase distingishes between the two because of the difference in the one stereogenic center in the pyranosyl unit mentioned above.
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