Short answer
The rate of lactose breakdown will decrease (much less lactose is broken down) after the acid is added.
Why this happens (enzyme structure and function)
- Enzyme structure is sensitive to pH. Lactase has a specific three-dimensional shape (its active site) that fits lactose like a lock and key.
- Optimal pH for lactase. Lactase works best around neutral pH (near 7). At neutral pH, the enzyme’s active site and overall shape are properly arranged to bind lactose and catalyze the reaction to glucose and galactose.
- Acidic pH denatures the enzyme. Adding strong acid lowers the pH to 2. Protons (H+) disrupt chemical bonds and interactions (such as ionic bonds and hydrogen bonds) that maintain lactase’s shape.
- Active site changes or loss of structure. When the enzyme’s shape is disrupted, the active site may no longer fit lactose, or the catalytic residues may not function properly.
- Resulting effect on reaction rate. With the active site misfolded or altered, fewer lactose molecules can be bound and converted per unit time, so the rate of lactose breakdown decreases significantly.
In summary: acidifying to pH 2 denatures or disrupts lactase’s structure, reducing its ability to catalyze the reaction, so the rate drops.