Pepsin functions optimally in the human stomach at approximately pH 2. A student gradually increases the pH of a solution containing pepsin and its substrate from pH 2 to pH 8. Which of the following would most likely occur?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Pepsin is an aspartyl protease engineered by natural selection to cleave peptide bonds within the highly acidic environment of the gastric lumen, where hydrogen ion concentration reaches approximately 10⁻² M (pH 2). The catalytic mechanism of pepsin depends on two aspartate residues in its active site—Asp32 and Asp215 (using porcine pepsin numbering)—that function as a proton-donating dyad. At pH 2, one aspartate remains protonated (COOH) while the other is deprotonated (COO⁻), creating an electrostatic environment that polarizes the carbonyl carbon of the substrate's peptide bond, enabling nucleophilic attack by a water molecule. This precise protonation state exists only within a narrow acidic window.

Why Other Options Are Wrong

Beyond the active site, pepsin's tertiary structure is stabilized by a network of ionic interactions, hydrogen bonds, and disulfide bridges that are exquisitely sensitive to proton concentration. At pH 2, specific glutamate and aspartate residues along the protein surface and interior remain protonated, allowing favorable van der Waals contacts and preventing electrostatic repulsion between negatively charged carboxylate groups. As pH rises toward 8, these acidic residues lose their protons (pKa values for Asp ~3.9, Glu ~4.3), generating negative charges that repel neighboring carboxylates. The resulting electrostatic repulsion forces apart β-sheet domains that maintain the active site cleft, and the catalytic dyad loses its asymmetric protonation. Hydrogen-bond geometry throughout the protein backbone distorts because donor-acceptor distances shift as side chains ionize, disrupting the precise three-dimensional fold required for substrate binding. The protein undergoes irreversible conformational denaturation at extremes far from its optimum, as hydrophobic core residues become exposed to the aqueous solvent.

PILLAR 2 — STEP-BY-STEP LOGIC

The stimulus describes a gradual pH increase from 2 to 8—a six-order-of-magnitude decrease in proton concentration. At pH 2, pepsin operates at its Vmax with minimal Km for peptide substrates. As the student raises pH past 3, the catalytic aspartates begin losing their required protonation states, and the reaction velocity declines according to the bell-shaped pH-activity profile characteristic of enzymes. By pH 5–6, pepsin retains negligible catalytic activity; the active site geometry has collapsed. Approaching pH 8, the accumulated ionization of dozens of surface and interior residues has fully denatured the enzyme, unfolding the β-barrel scaffold. The substrate, being a small peptide, does not gain or lose function—it simply waits for a catalyst that no longer exists in a competent conformation. Therefore, the correct answer (B) states that enzymatic activity progressively decreases and eventually ceases entirely, which directly reflects the thermodynamic and structural cascade triggered by charge repulsion and active-site protonation loss.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A likely suggests that reaction rate increases as pH rises, perhaps because students confuse the stomach environment with the small intestine, where trypsin and chymotrypsin function optimally near pH 8. This reflects a conceptual flaw: attributing the optima of one enzyme (trypsin, a serine protease with catalytically essential histidine requiring higher pH for proper charge state) to a completely different enzyme class. Each enzyme's pH optimum emerges from its unique active-site residue composition, not from the general cellular location.

Option C probably claims that pepsin remains partially active across the full pH range or that activity remains constant until an abrupt threshold. This reflects misunderstanding of the continuous, graded nature of protein denaturation curves and pH-rate profiles. Students selecting this option may believe enzyme structure is binary—fully folded or fully unfolded—rather than appreciating that partial unfolding progressively reduces catalytic efficiency through intermediate conformational states with compromised active-site geometry.

Option D likely invokes an increase in substrate concentration or a change in substrate structure compensating for enzyme inactivation. This reflects confusion between enzyme properties and substrate properties; pH alters the ionization state of both enzyme and substrate, but the peptide substrate has no catalytic capability itself. No amount of substrate accumulation can overcome the loss of a competent catalyst—Michaelis-Menten kinetics dictates that even infinite substrate cannot surpass the Vmax dictated by the concentration of functional, properly folded enzyme.