A mutation in the enzyme lactase results in the substitution of alanine with threonine at a crucial active site, leading to a significant decrease in the rate of lactose digestion. Explain why this single amino acid substitution causes such a drastic change in the function of lactase.

• A. Threonine decreases the size of the enzyme, reducing the enzyme's overall surface area for lactose binding.
• B. Threonine has a larger side chain and introduces a polar hydroxyl group, which disrupts the active site's ability to bind lactose.
• C. Alanine and threonine are both nonpolar amino acids, so the substitution should not affect the enzyme's function.
• D. Threonine is unable to form covalent bonds with lactose, preventing the enzyme from breaking down the sugar.

Answer: (B)