Enzymes are biological catalysts, usually proteins, that speed up chemical reactions in living organisms. They make processes such as digestion, DNA copying, and cellular respiration fast enough to support life. An enzyme has a specific three-dimensional shape, and part of that shape forms an active site where a substrate can bind.
Understanding enzyme structure helps explain why cells can control thousands of reactions with precision.
The active site fits a substrate because of shape, charge, polarity, and chemical interactions. In the lock-and-key model, the substrate already matches the active site, while in the induced-fit model, the enzyme changes shape slightly as binding occurs. Enzymes lower activation energy by stabilizing the transition state and bringing reactants into the best position for reaction.
Because enzyme shape depends on conditions such as temperature and pH, changes in the environment can reduce activity or cause denaturation.
Key Facts
- Enzyme + substrate forms an enzyme-substrate complex: E + S ⇌ ES.
- Enzymes speed reactions by lowering activation energy: lower Ea means faster reaction rate.
- Enzymes are not consumed in the reaction, so one enzyme can catalyze many substrate molecules.
- Substrate specificity comes from the active site's shape and chemical properties.
- Induced fit means the active site changes shape slightly when the substrate binds.
- Extreme temperature or pH can denature an enzyme by disrupting its three-dimensional structure.
Vocabulary
- Enzyme
- An enzyme is a biological catalyst that increases the rate of a chemical reaction without being used up.
- Active site
- The active site is the specific region of an enzyme where the substrate binds and the reaction occurs.
- Substrate
- A substrate is the reactant molecule that binds to an enzyme and is changed during the reaction.
- Activation energy
- Activation energy is the minimum energy needed for reactants to reach the transition state and form products.
- Denaturation
- Denaturation is the loss of an enzyme's functional shape, often caused by extreme temperature, pH, or chemicals.
Common Mistakes to Avoid
- Saying enzymes add energy to reactions. Enzymes do not supply energy; they lower the activation energy needed for the reaction to proceed.
- Thinking the active site is only a rigid hole. Many enzymes use induced fit, so the active site can shift slightly to grip the substrate and stabilize the transition state.
- Assuming higher temperature always increases enzyme activity. Reaction rate usually rises at first, but too much heat can denature the enzyme and sharply reduce activity.
- Confusing substrate specificity with product specificity. An enzyme recognizes particular substrates at its active site, and the products depend on the reaction that enzyme catalyzes.
Practice Questions
- 1 An uncatalyzed reaction has an activation energy of 80 kJ/mol. An enzyme lowers it to 35 kJ/mol. By how many kJ/mol did the enzyme reduce the activation energy?
- 2 A solution contains 2.0 micromoles of enzyme, and each enzyme molecule catalyzes 500 reactions per second. How many micromoles of substrate are converted per second if substrate is unlimited?
- 3 A mutation changes one charged amino acid in an enzyme's active site to a nonpolar amino acid. Explain how this could affect substrate binding and enzyme activity.