The main route for galactose utilization is conversion to glucose-6-phosphate, as shown in Figure 13.13.

1. This pathway begins with the ATP-dependent conversion of galactose to galactose-1-phosphate, catalyzed by galactokinase (Figure 13.13, reaction 1).

2. Conversion to UDG-galactose (Figure 13.13, reaction 2), by a transferase reaction with a nucleotide-linked sugar (catalyzed by UDP-glucose:-D-galactose-1-phosphate uridylyltransferase). In this reaction, galactose-1-phosphate is swapped for the glucose-1-phosphate on UDP-glucose.

3. The NAD⁺-linked enzyme UDP-galactose 4-epimerase can then convert UDP-galactose to UDP-Glucose (reaction 3)* .
Details of this epimerization reaction are shown in Figure 13.14.

4. The glucose-1-phosphate formed in reaction 2 is then converted to glucose-6-phosphate by phosphoglucomutase (reaction 4), an enzyme involved also in glycogen biosynthesis. Nucleoside diphosphate sugars such as UDP-glucose are widely used intermediates in polysaccharide biosynthesis.

*Note in Figure 13.13, reaction 3 that UDP-glucose is formed from glucose-1-phosphate and UTP by UDP-glucose pyrophosphorylase. This enzyme is named from its reverse reaction, involving cleavage of the phosphoric acid anhydride bond in UDP-glucose by addition across that bond of the elements of pyrophosphoric acid.

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