A researcher studying the enzyme lactase notes that the enzyme's activity is most optimal at a pH of 6.8. If the pH of the reaction mixture is increased to 7.4, what will happen to the enzyme's activity?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Enzymes such as lactase (β-galactosidase) depend upon the precise three-dimensional architecture of their active site to bind substrate and catalyze the hydrolysis of the β-1,4-glycosidic bond in lactose, yielding glucose and galactose. This architecture is maintained through a network of noncovalent interactions—hydrogen bonds, ionic (salt bridge) interactions, van der Waals contacts, and hydrophobic packing—among the enzyme's amino acid residues. The ionization state of each residue is governed by the surrounding hydrogen ion concentration, quantified by pH. Amino acid side chains with titratable functional groups—particularly the carboxyl groups of aspartate and glutamate (pKa ≈ 3.9–4.3), the imidazole ring of histidine (pKa ≈ 6.0), the thiol of cysteine (pKa ≈ 8.3), and the amino groups of lysine (pKa ≈ 10.5) and arginine (pKa ≈ 12.5)—gain or lose protons as the pH shifts, altering their charge distribution. When the pH deviates from the enzyme's optimum, the resulting changes in ionization disrupt hydrogen-bond geometry, collapse salt bridges, and induce subtle conformational rearrangements in the active site. These rearrangements reduce the enzyme's ability to form the enzyme-substrate (ES) complex with proper geometric alignment, decreasing the rate at which the substrate is converted to product. The pH optimum of lactase (approximately 6.0–6.8 in the human small intestine) reflects the specific protonation pattern required for catalytic glutamate and histidine residues in the active site to function as acid/base catalysts during glycosidic bond cleavage.

Why Other Options Are Wrong

PILLAR 2 — STEP-BY-STEP LOGIC

The stimulus establishes that lactase operates with maximal catalytic efficiency at pH 6.8. This value represents the point at which the highest proportion of active-site residues exist in their catalytically competent ionization states, yielding the greatest Vmax and the lowest apparent Km. The researcher then raises the pH of the reaction mixture to 7.4—an increase of 0.6 pH units in the alkaline direction. At pH 7.4, the concentration of free hydrogen ions has decreased by approximately fourfold compared to pH 6.8. This deprotonation shift affects residues with pKa values near the transition zone, most notably histidine. A catalytic histidine that was partially or fully protonated at pH 6.8 may become deprotonated at pH 7.4, eliminating its ability to donate a proton during the acid hydrolysis of the glycosidic bond. Simultaneously, any salt bridge networks that stabilize the active-site cleft become destabilized as charge distributions shift. The net consequence is a measurable decline in catalytic rate—lactase's activity decreases because the active-site geometry and the proton transfer chemistry are no longer optimal. The enzyme is not fully denatured at pH 7.4, so some residual activity persists, but it is significantly below the peak rate observed at pH 6.8.

PILLAR 3 — DISTRACTOR ANALYSIS

Option (A), "The enzyme's activity will increase," exploits a common misconception that biological systems universally favor neutral or slightly basic conditions (pH 7.0–7.4) because that is the physiological range for blood and cytosol. Students who associate pH 7.4 with the "normal" environment of the human body may incorrectly assume that moving toward neutrality must enhance any enzyme's function. The flaw here is failing to recognize that each enzyme possesses a unique pH optimum dictated by its local environment and active-site chemistry; lactase evolved to function in the slightly acidic lumen of the small intestine, not at blood pH.

Option (B), "The enzyme's activity will remain the same," traps students who misunderstand the concept of enzyme stability. They may reason that a pH shift of only 0.6 units is too small to produce any measurable effect. This reflects an imprecise understanding of pH as a logarithmic scale: each whole-number change represents a tenfold difference in hydrogen ion concentration, so even a 0.6-unit shift represents an approximately fourfold change—sufficient to alter the protonation state of histidine and other residues near their pKa values.

Option (D), "The enzyme's activity will become more stable," conflates thermodynamic stability of the overall protein fold with catalytic activity. A protein may remain structurally intact (folded) across a broad pH range, yet still lose catalytic efficiency because the active-site microenvironment is exquisitely sensitive to ionization changes. Students selecting this option fail to distinguish between the maintenance of tertiary structure and the preservation of the precise active-site geometry required for substrate binding and transition-state stabilization.