Question

Which one of the following whey proteins plays a role in lactose biosynthesis?

• A. Immunoglobulin
• B. \(\beta\)-lactoglobulin
• C. Bovine serum albumin
• D. \(\alpha\)-lactalbumin

Hint:

Lactose Biosynthesis: Regulatory Specifier Protein = $\alpha$-LACTALBUMIN (Whey protein). Forms the Lactose Synthase complex with Galactosyltransferase in the Golgi.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Lactose synthase multienzyme complex: the globular whey protein $lpha$-lactalbumin functions as the regulatory specifier protein (B protein) that lowers the $K_m$ of UDP-galactosyltransferase (A protein) for glucose by 1000-fold to synthesize lactose.
Key Formula or Approach:
\[ \text{UDP-Galactose} + \text{D-Glucose} \xrightarrow{\mathbf{Lactose \text{ } Synthase \text{ [Galactosyltransferase (A) } + \; \alpha\mathbf{\text{-Lactalbumin (B)}]}}} \mathbf{Lactose} + \text{UDP} \]

Step 2: Detailed Explanation:
In lactation biochemistry and mammary gland enzymology:
- Lactose (4-O-$\beta$-D-galactopyranosyl-D-glucopyranose) is synthesized in the lumen of the mammary Golgi apparatus by the heterodimeric Lactose Synthase Enzyme Complex:
1. Component A (Galactosyltransferase): A general glycosylating enzyme that normally attaches galactose to N-acetylglucosamine.
2. Component B ($\alpha$-Lactalbumin) (D): A major, calcium-binding globular whey protein synthesized exclusively in mammary alveolar epithelial cells during lactogenesis.
- Binding of $\alpha$-lactalbumin to galactosyltransferase alters its substrate specificity, lowering the $K_m$ for D-Glucose by 1,000-fold, thus catalyzing the transfer of galactose from UDP-galactose to glucose to produce Lactose.

Step 3: Final Answer:
Hence, \(\alpha\)-lactalbumin plays a vital role in lactose biosynthesis, corresponding to option (D).