Lactose Synthase

The enzymes shown in Figure 13.13 participate in mammary gland in the synthesis of lactose in milk. Lactose is formed from UDP-Gal plus glucose by lactose synthase, in the presence of the protein -lactalbumin. The endergonic synthesis of UDP-Gal in this tissue proceeds smoothly because of its high rate of conversion to lactose.

An enzyme called galactosyltransferase is closely related to lactose synthase. It catalyzes the reaction below.

> UDP-Galactose + N-Acetylglucosamine -> UDP + N-Acetyllactosamine

The common form of the enzyme contains a single polypeptide subunit. The presence of an additional subunit, -lactalbumin, changes the specificity of the enzyme so that lactose is synthesized instead.

> UDP-Galactose + Glucose -> UDP + Lactose

This modified enzyme is lactose synthase. Synthesis of -lactalbumin is activated hormonally in mothers shortly after giving birth. The -lactalbumin protein combines with preexisting galactosyltransferase, changes its specificity, and activates the large amount of lactose synthesis needed for milk production.

See also: Figure 9.17