In lactating mammary gland, the enzyme complex that synthesizes lactose from UDP-galactose and glucose changes its substrate preference during milk production. Which hormonal signal brings about this change?

• A Oxytocin induces expression of a mammary-specific galactosyltransferase isoform
• B Insulin stimulates conversion of galactosyltransferase to lactose synthase by proteolytic cleavage
• C Prolactin increases alpha-lactalbumin, which lowers the Km of galactosyltransferase for glucose ✓
• D Progesterone activates preformed lactose synthase through allosteric modification

Explanation

Galactosyltransferase alone transfers galactose to N-acetylglucosamine-containing proteins and has negligible affinity for glucose. Prolactin upregulates alpha-lactalbumin, a modifier protein that binds the enzyme to form lactose synthase and dramatically lowers its Km for glucose, allowing lactose production. Oxytocin drives milk ejection, progesterone falls before lactogenesis, and no proteolytic activation is involved.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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