Lactose is a disaccharide that consists of a molecule of β-galactose attached by a β(1→4) linkage to glucose. Therefore, lactose is galactosyl β(1→4)-glucose. Lactose, known as “milk sugar,” is made by lactating (milk-producing) mammary glands.
LACTOSE SYNTHESIS
Lactose is a
disaccharide that consists of a molecule of β-galactose attached by a β(1→4)
linkage to glucose. Therefore, lactose is galactosyl β(1→4)-glucose. Lactose,
known as “milk sugar,” is made by lactating (milk-producing) mammary glands.
Therefore, milk and other dairy products are the dietary sources of lactose.
Lactose is synthesized in the Golgi b y lactose synthase (UDP-galactose:glucose
galactosyltransferase), which transfers galactose from UDP-galactose to
glucose, releasing UDP (Figure 12.7). This enzyme is composed of two proteins,
A and B. Protein A is a β-D-galactosyltransferase and is found in a number of
body tissues. In tissues other than the lactating mammary gland, this enzyme
transfers galactose from UDP-galactose to N-acetyl-D-glucosamine, forming the
same β(1→4) linkage found in lactose, and producing N-acetyllactosamine, a
component of the structurally important N-linked glycoproteins. In contrast,
protein B is found only in lactating mammary glands. It is α-lactalbumin, and
its synthesis is stimulated by the peptide hormone prolactin. Protein B forms a
complex with the enzyme, protein A, changing the specificity of that
transferase so that lactose, rather than N-acetyllactosamine, is produced (see
Figure 12.7).
Figure 12.7 Lactose synthesis.
UDP = uridine diphosphate.
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