ACT Science: Enzyme Kinetics and How Catalysts Speed Up Reactions

An enzyme is a protein that speeds up a chemical reaction by lowering the activation energy (the energy needed to start the reaction). Without the enzyme, the reaction happens slowly. With it, the reaction happens much faster. Example: The enzyme amylase breaks down starch into glucose. Without amylase, this would take days or weeks. With amylase (in your saliva and digestive system), it happens in minutes. On the ACT, you don't need to memorize specific enzymes, just understand the concept: enzymes lower activation energy without being consumed (they're reusable). Key insight: Enzymes speed reactions but don't change the starting point (reactants) or ending point (products)—they just provide a faster path between them.

Enzyme-substrate interaction: The substrate (reactant) fits into the enzyme's active site like a key in a lock. This binding, breaking, and releasing of the substrate is what speeds the overall reaction.

Factor 1: Temperature. Higher temperature usually increases enzyme speed (more molecular motion = more collisions), but extreme heat denatures (breaks) the enzyme. Factor 2: pH. Enzymes work best at their optimal pH. Too acidic or basic, and they denature. Factor 3: Enzyme concentration. More enzyme = faster reaction (more active sites available). Factor 4: Substrate concentration. More substrate = faster reaction, up to a point. Once all enzyme active sites are occupied, adding more substrate doesn't help. Factor 5: Cofactors/Coenzymes. Some enzymes need these helper molecules to function. On ACT science questions, you'll see graphs showing how reaction rate changes with temperature or pH. Identify the optimal point and recognize when the enzyme is damaged or saturated.

Common prediction: If temperature increases from 20°C to 50°C, enzyme speed increases. If it goes from 50°C to 80°C, enzyme speed may decrease (denaturation). The graph shows an optimal peak, not a straight line.

Graph 1: Enzyme speed vs temperature. Curve rises from 0°C to ~37°C, peaks, then falls from 37°C to 80°C. Question: "At what temperature does the enzyme work fastest?" Answer: ~37°C (optimal). Graph 2: Enzyme speed vs substrate concentration. Curve rises steeply, then plateaus. Question: "Why does the curve plateau?" Answer: Enzyme saturation—all active sites are occupied. Graph 3: Enzyme speed vs pH. Peak at pH 7, lower on both sides. Question: "Why is the enzyme slower at pH 2 or pH 12?" Answer: Denaturation at extreme pH. For each graph, practice: (1) Identify which variable is on which axis. (2) Describe the shape of the curve. (3) Predict why the shape matches that variable's effect on enzymes.

Daily drill: Find one graph from practice science tests showing enzyme kinetics. Analyze it using the framework above. By week's end, enzyme graphs will feel intuitive.

Enzyme questions appear in 1-2 chemistry or biology passages per ACT Science section. These questions combine graph reading, conceptual understanding, and prediction—multiple skills in one. If you understand enzymes deeply, you can handle complex enzyme-related passages that intimidate other students. Enzyme questions are often high-value: they're worth the same points but many students find them scary, so getting them right boosts your relative score.

Spend 2-3 days on enzyme basics and graph reading. Practice predicting enzyme speed under different conditions. By test day, enzyme questions will feel manageable and you'll pick up free points others miss.