Question

Which of the following is not an intermediate compound in the biosynthesis of lactose

• A. Lactose -6-phosphate
• B. UDP-Galactose
• C. Glucose-1-phosphate
• D. UDP-Glucose

Hint:

Lactose synthesis requires active nucleotide-sugars, specifically UDP-Galactose, which serves as the glycosyl donor. The final product is free lactose, meaning no phosphorylated intermediates of lactose (like lactose-6-phosphate) are synthesized in mammalian cells.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Lactose is the primary disaccharide found in milk, consisting of one molecule of D-galactose and one molecule of D-glucose linked by a \(\beta\)-1,4-glycosidic bond.
In the mammary gland, lactose biosynthesis occurs within the Golgi apparatus of secretory epithelial cells, catalyzed by the enzyme lactose synthase.

Step 2: Detailed Explanation:
Let us review the biochemical pathway of lactose synthesis in the mammary gland to evaluate the intermediates:
- Blood glucose enters the mammary secretory cell. A portion of this glucose is phosphorylated to Glucose-6-phosphate and isomerized to Glucose-1-phosphate.
- Glucose-1-phosphate reacts with UTP to form UDP-Glucose (uridine diphosphate glucose).
- UDP-Glucose is then epimerized by the enzyme UDP-glucose 4-epimerase to form UDP-Galactose.
- Finally, the enzyme lactose synthase (composed of galactosyltransferase and \(\alpha\)-lactalbumin) catalyzes the transfer of galactose from UDP-Galactose to free D-glucose, releasing lactose and UDP:
\[ \text{UDP-Galactose} + \text{D-Glucose} \rightarrow \text{Lactose} + \text{UDP} \]
- Evaluating the options, Glucose-1-phosphate, UDP-Glucose, and UDP-Galactose are key intermediates in this pathway.
- Lactose-6-phosphate, however, does not participate in this mammalian biosynthetic pathway. It is only encountered as an intermediate in bacterial transport and metabolism pathways (such as the PEP-PTS pathway in certain lactic acid bacteria).

Step 3: Final Answer:
Lactose-6-phosphate is not an intermediate compound in the biosynthesis of lactose.