ACT Science: Understand How Enzymes Work and When They Perform Best

An enzyme is a protein that speeds up a chemical reaction without being consumed. It works by lowering the activation energy (the energy barrier the reactants must overcome). The substrate (reactant) binds to the enzyme's active site, forming an enzyme-substrate complex. The enzyme catalyzes the reaction, releasing the product. The enzyme is then free to bind another substrate. Picture a lock (enzyme) and key (substrate): only the right key fits the lock. If the lock's shape changes (due to heat or pH), the key no longer fits, and the reaction slows. The key insight is that enzymes are shape-dependent; anything that changes their shape affects their function.

Example: Amylase breaks down starch into sugar. Optimal amylase temperature is around 37°C (human body temperature). At 0°C, it barely works (substrate does not fit well). At 100°C, the enzyme denatures (its shape is destroyed permanently) and does not work at all. This pattern holds for all enzymes and is tested repeatedly on the ACT.

Factor 1: Temperature. Enzymes have an optimal temperature (usually 37°C for human enzymes). Below optimal, collisions between enzyme and substrate are slower; above optimal, the enzyme denatures. Factor 2: pH. Enzymes have an optimal pH (varies by enzyme; pepsin works at pH 2, trypsin at pH 8). Extreme pH denatures the enzyme. Factor 3: Substrate concentration. At low substrate levels, increasing substrate increases reaction rate (more collisions). At high substrate levels, the enzyme becomes saturated (all active sites are busy), and adding more substrate does not speed up the reaction. On the ACT, you will see graphs showing how these factors affect enzyme activity; recognize the optimal peak and the decline on either side.

Common ACT pattern: A graph shows enzyme activity (y-axis) vs. temperature or pH (x-axis). The curve peaks at the optimal value and drops on both sides. This bell-curve pattern is the hallmark of enzyme behavior and appears in 1-2 ACT Science questions per test.

Scenario 1: Pepsin (an enzyme that digests protein in the stomach) has optimal pH 2 and optimal temperature 37°C. A student tests pepsin at pH 7 and temperature 37°C. Prediction: Enzyme activity is lower than optimal (pH is off). Scenario 2: A graph shows that enzyme X works fastest at 40°C. At 20°C, it is slower. At 60°C, it is slower still. What happens at 80°C? Prediction: Activity is very low (the enzyme is approaching denaturation). Scenario 3: Enzyme Y can convert 100 molecules of substrate per second when substrate is abundant. What is the maximum rate if substrate is scarce? Prediction: Lower than 100/second (the enzyme will not be fully saturated). For each scenario, identify which factor (temperature, pH, or substrate) is changing and predict how enzyme activity responds.

On the next practice test, find science questions about enzymes and predict the answer before reading the choices. Compare your prediction to the correct answer. This builds pattern recognition and trains you to think like the test-maker.

Enzyme questions appear 2-4 times per ACT Science section, often disguised as digestion, cellular respiration, or photosynthesis questions. Understanding the lock-and-key model and the three factors (temperature, pH, substrate) gives you the foundation to answer all enzyme-related questions, even if the specific enzyme is unfamiliar. Enzyme function is a high-value concept because it connects to many biological processes and appears frequently on tests.

Spend 20 minutes this week learning the characteristics of five common enzymes (amylase, lipase, pepsin, trypsin, lactase). For each, note the optimal pH and temperature. Then answer 10 enzyme questions from old tests, using the lock-and-key model to predict activity changes. By test day, enzyme questions will feel intuitive instead of confusing.