are disaccharides whose
monomeric units are connected by β(1→ 4)
. 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
.) When lactose is in an aqueous solution, like cellobiose and maltose, it is a
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.