AP Biology: Unit 3, Enzymes (Drill 9)

Question 1. Based on the data in Table 1, at what pH is amylase activity greatest, and what does this represent?

• A) pH 6.0; this is the optimal pH because it represents the midpoint of the tested range.
• B) pH 8.0; activity is highest at basic conditions because amylase is a basic protein.
• C) pH 7.0; this is the optimal pH at which the enzyme's active site conformation best complements the substrate, maximizing catalytic rate. ✓
• D) pH 7.0; this is the optimal pH because all enzymes function best at neutral pH regardless of their environment.

Explanation: Correct answer: C. The data show peak activity (88% starch hydrolyzed) at pH 7.0, with activity declining on both sides. This pH is the enzyme's optimum -- the H+ concentration at which the three-dimensional shape of the active site is most complementary to the substrate. The active site geometry is maintained by weak interactions (hydrogen bonds, ionic interactions) among R-groups of amino acids. At pH values away from the optimum, altered H+ concentrations disrupt these interactions, subtly changing active site shape and reducing substrate binding. (A) is incorrect; the optimum is determined by the data, not by the midpoint of the tested range -- the data clearly show peak activity at pH 7.0, not 6.0. (B) is incorrect; pH 8.0 shows 47% hydrolysis, well below pH 7.0's 88%. (D) is incorrect as a rationale; while pH 7.0 is the correct value, the justification is wrong. Enzymes do not all function optimally at neutral pH -- pepsin functions best at ~2.0, as the passage notes. Optimal pH is enzyme-specific based on the protein's amino acid sequence and environment.

Question 2. Why does amylase activity decrease at pH 3.0 compared to its activity at pH 7.0?

• A) At pH 3.0, the substrate starch molecules are chemically destroyed by the acid before the amylase enzyme can bind to them and catalyze hydrolysis.
• B) At pH 3.0, the concentration of substrate is lower because starch dissolves poorly in acidic conditions.
• C) At pH 3.0, the high H+ concentration breaks the covalent peptide bonds of amylase, permanently destroying the enzyme.
• D) At pH 3.0, altered H+ concentrations disrupt the hydrogen bonds and ionic interactions that maintain the enzyme's three-dimensional shape, changing the active site conformation and reducing its complementarity to the substrate. ✓

Explanation: Correct answer: D. Enzyme function depends on the precise three-dimensional shape of the active site, which is maintained by non-covalent interactions (hydrogen bonds, ionic bonds, van der Waals forces) among the R-groups of amino acid residues. Many of these interactions are pH-sensitive: ionic bonds between oppositely charged R-groups, and hydrogen bonds involving -NH and -OH groups, are disrupted when H+ concentration changes. At pH 3.0, excess protons protonate basic R-groups and alter charge distributions, distorting the active site and reducing its fit with the substrate. This is denaturation of the active site (though at moderate pH extremes, the effect may be reversible). (A) is incorrect; starch is stable under mildly acidic conditions. (B) is incorrect; starch solubility is not affected by pH 3.0. (C) is a plausible but incorrect distractor -- peptide bonds are covalent and are not broken by pH 3.0. Extremely harsh acid hydrolysis (concentrated acid, high temperature) is needed to cleave peptide bonds; the mild acid in this experiment disrupts non-covalent interactions, not covalent bonds.

Question 3. A second trial used pepsin instead of amylase, with the same starch substrate and the same pH range. Pepsin is a protease with an optimal pH of approximately 2.0. What would the data table most likely show?

• A) Peak starch hydrolysis near pH 2.0, declining sharply at higher pH values, because pepsin functions optimally in acidic conditions.
• B) The data table would show the same activity pattern as amylase, peaking at pH 7.0, because pH affects every digestive enzyme in an identical way within this experimental context.
• C) Uniform starch hydrolysis across all pH values, because pepsin is more thermally stable than amylase.
• D) Near-zero starch hydrolysis at all pH values, because pepsin is a protease and does not catalyze the breakdown of starch regardless of pH conditions. ✓

Explanation: Correct answer: D. Enzyme specificity is determined by the complementary shape of the active site and its substrate -- a concept sometimes called the induced fit model. Pepsin is a protease: its active site is shaped to bind and cleave peptide bonds in proteins. Starch is a polysaccharide, not a protein. No matter how favorable the pH conditions are for pepsin's general activity, its active site cannot bind starch, so near-zero hydrolysis is expected at every pH tested. (A) describes the pattern pepsin would show on a protein substrate, but the experiment uses starch. Even though pepsin does function best near pH 2.0, that optimum applies only to its natural substrates. (B) is incorrect; different enzymes have different optimal pH values based on their amino acid composition and active site structure -- pH effects are not uniform across enzymes. (C) is incorrect; thermal stability is a separate property from substrate specificity and does not allow an enzyme to catalyze a reaction on an incompatible substrate.

Question 4. A student proposes that increasing the concentration of amylase at pH 3.0 would restore enzyme activity to levels comparable to pH 7.0. Is this prediction supported by biological reasoning?

• A) Yes, because adding more enzyme molecules would compensate for the reduced efficiency of each active site at pH 3.0 and restore overall activity.
• B) No, because at pH 3.0, the altered H+ concentration affects all enzyme molecules; adding more enzyme does not change the pH or restore active site conformation -- all additional molecules would also be in suboptimal conformation. ✓
• C) Yes, because at pH 3.0, only 2% of the enzyme molecules are active; adding more enzyme adds more active molecules.
• D) No, because at pH 3.0, the substrate (starch) is limiting, not the enzyme concentration.

Explanation: Correct answer: B. The same unfavorable pH affects every amylase molecule present in the solution -- this is not a property of individual molecules but of the H+ concentration of the entire environment. Adding more enzyme adds more copies of the enzyme in the same suboptimal pH environment; each additional molecule is equally impaired. The rate-limiting factor is active site conformation, not enzyme count. Restoring activity requires returning to the optimal pH, not adding more enzyme. (A) and (B) share the same error: they assume a fixed fraction of enzyme molecules are "active" at pH 3.0 and that adding more enzyme adds more active copies. In reality, pH acts on every molecule through the same mechanism. (C) is incorrect because the experiment is designed with excess substrate to isolate the enzyme variable; substrate is not the limiting factor at any pH.

Question 5. The data show that amylase activity at pH 8.0 (47%) is substantially lower than at pH 7.0 (88%), but substantially higher than at pH 3.0 (2%). What does this pattern suggest about the effect of pH on amylase?

• A) pH changes on the basic side of the optimum cause less disruption to amylase's active site than equivalent changes on the acidic side, suggesting the enzyme's active site interactions are more sensitive to excess H+ than to H+ deficiency. ✓
• B) Amylase is completely and irreversibly denatured below pH 5.0 but only partially denatured above pH 7.0, explaining the difference in residual activity through permanent loss of every acidic-site enzyme molecule rather than changes in active-site chemistry.
• C) The substrate starch is unstable in acidic conditions, causing the low activity readings at pH 3.0 and 4.0.
• D) Amylase performs optimally over a wide basic range (pH 7.0-9.0) and should be used industrially at pH 8.0 to reduce enzyme consumption.

Explanation: Correct answer: A. The data are asymmetric: moving 4 pH units below the optimum (pH 7.0 -> pH 3.0) reduces activity from 88% to 2% (an ~86 percentage point drop), while moving 2 pH units above the optimum (pH 7.0 -> pH 9.0) reduces activity to 11% -- a comparable drop over fewer units. At pH 8.0 (1 unit above optimum), activity is still 47%, far above pH 5.0 (22%), which is only 2 units below optimum. The pattern suggests that acidic conditions disrupt amylase's active site more strongly than comparably basic conditions within this tested range -- consistent with the active site being more sensitive to excess H+ than to H+ deficiency, though the exact molecular basis would require further investigation. (B) is an overgeneralization: "complete denaturation" implies irreversibility, which the data alone cannot confirm. (C) is incorrect; starch is stable at mildly acidic pH in short-term experiments. (D) is incorrect; pH 8.0 gives substantially reduced activity (47% vs. 88% at pH 7.0), making it a poor choice for industrial optimization.