Explain how enzymes act as biological catalysts, how the active site and substrate fit, and how temperature and pH affect enzyme activity (NYSSLS LS1, structure and function; analyzing data).

What this topic is asking

NYSSLS LS1 wants you to understand that the reactions of life are controlled by enzymes, and to explain how an enzyme works and why it is so sensitive to conditions. On the Life Science: Biology Regents this almost always comes as a cluster with a graph of enzyme activity against temperature or pH, where you read the optimum and explain the shape of the curve.

What an enzyme does

Reactions need a minimum input of energy to start, the activation energy. An enzyme works by lowering that activation energy, so the reaction proceeds far faster at the moderate temperatures inside a cell than it otherwise could. Because the enzyme is not consumed, a single enzyme molecule can catalyze the same reaction over and over.

The active site and specificity

An enzyme is specific: its active site has a particular shape that fits only its substrate, rather like a key fitting one lock. When the substrate binds, the enzyme holds it in the right position for the reaction, then releases the product and is free to act again. Because the active site fits only a complementary substrate, one enzyme catalyzes only one reaction (or one type of reaction), which is why cells need thousands of different enzymes.

Temperature

As temperature rises, molecules move faster and collide more often, so enzyme activity increases up to an optimum (about 37 degrees Celsius for human enzymes). Above the optimum, the rate falls sharply because the heat denatures the enzyme: the protein unfolds, the active site changes shape, and the substrate no longer fits. Denaturation is usually permanent, so a denatured enzyme does not recover on cooling. At low temperature the enzyme is not damaged but works slowly because collisions are rare; warming restores activity.

pH

Each enzyme also has an optimum pH at which its active site holds the correct shape. Move too far from that pH and the active site is distorted (the enzyme denatures), so activity falls. The optimum reflects where the enzyme works in the body: pepsin in the acidic stomach has a low optimum pH, while many other enzymes work best near neutral.

Try this

Q1. Explain how an enzyme speeds up a reaction. [2]

• Cue. It lowers the activation energy needed for the reaction, so the reaction proceeds faster; the enzyme is not used up.

Q2. Explain why heating an enzyme well above its optimum stops it working permanently. [2]

• Cue. The heat denatures the enzyme, permanently changing the shape of the active site so the substrate can no longer bind.