Enzymes are biological catalysts that speed up chemical reactions in living systems without being used up. They make reactions fast enough for cells to grow, repair, move materials, and release energy. Most enzymes are proteins with a specific three-dimensional shape that helps them recognize the molecules they act on.
Understanding enzyme catalysis connects chemistry, biology, medicine, and biotechnology.
Understanding Chemistry: Enzymes and Catalysis
At the active site, an enzyme does more than hold a substrate in place. It positions particular atoms so that bonds can break or form more easily. Some enzymes bring two reactants close together.
Others stretch a bond, transfer a hydrogen ion, or create a small charged region that makes a reaction pathway easier. The enzyme does not supply unlimited energy. It provides a route with a smaller energy barrier.
This is important because many useful cell reactions would otherwise be far too slow at normal body temperature. Enzymes can make a reaction selective, so the cell produces the needed product instead of many unwanted products.
The fit between a substrate and an active site depends on weak chemical attractions. These include attractions between charges, hydrogen bonding, and regions that avoid water. A small change in the substrate can prevent binding.
This explains why enzymes are often highly specific. For example, the enzyme lactase helps split lactose, the sugar in milk. People who make little lactase may have difficulty digesting dairy foods because lactose reaches the large intestine without being fully broken down.
Bacteria there use it and produce gases. Enzyme specificity therefore has direct effects on digestion, nutrition, and health.
Temperature and pH change enzyme activity because they affect molecular motion and protein shape. At low temperatures, particles move more slowly, so successful collisions happen less often. At a higher temperature, collisions become more frequent up to a useful range.
Too much heat can disrupt the weak attractions that maintain an enzyme's shape. The active site then no longer works properly. This loss of shape is called denaturation.
Extreme pH can cause similar damage by changing electrical charges on parts of the enzyme or substrate. Pepsin works in the acidic stomach, while many enzymes in the small intestine work best in less acidic conditions.
Inhibitors show why enzyme chemistry matters in medicine and industry. A competitive inhibitor resembles a substrate and occupies the active site. Increasing substrate concentration can sometimes reduce its effect.
A noncompetitive inhibitor binds elsewhere and changes the enzyme's working shape, so extra substrate may not help. Some drugs are designed to block enzymes linked to disease. Organophosphate pesticides can block enzymes needed for normal nerve signals, which makes them dangerous.
In laboratories, students should distinguish a slower rate from a completely stopped reaction. They should control one variable at a time, measure equal time intervals, and remember that an enzyme can still be present even when its shape has been damaged.
Key Facts
- Enzymes lower activation energy, so reactions occur faster at the same temperature.
- E + S ⇌ ES ⇌ E + P represents enzyme, substrate, enzyme-substrate complex, and product.
- Reaction rate usually increases with substrate concentration until the enzyme becomes saturated.
- Lock-and-key model: the substrate fits into a pre-shaped active site.
- Induced-fit model: the active site changes shape slightly when the substrate binds.
- Temperature, pH, substrate concentration, enzyme concentration, and inhibitors all affect enzyme activity.
Vocabulary
- Enzyme
- An enzyme is a biological catalyst that speeds up a chemical reaction without being permanently changed.
- Substrate
- A substrate is the reactant molecule that binds to an enzyme and is converted into product.
- Active site
- The active site is the specific pocket or groove on an enzyme where the substrate binds and the reaction occurs.
- Activation energy
- Activation energy is the minimum energy needed for reactant molecules to reach the transition state and react.
- Inhibitor
- An inhibitor is a molecule that decreases enzyme activity by blocking binding or changing the enzyme's shape.
Common Mistakes to Avoid
- Saying enzymes add energy to a reaction is wrong because enzymes lower the activation energy barrier rather than supplying the reaction energy.
- Assuming enzymes are consumed during reactions is wrong because enzymes are regenerated after product release and can catalyze many cycles.
- Thinking every substrate fits every enzyme is wrong because active site shape, charge, and chemical groups make enzyme binding highly specific.
- Ignoring pH and temperature is wrong because changes in these conditions can alter enzyme shape and reduce or destroy catalytic activity.
Practice Questions
- 1 An uncatalyzed reaction has an activation energy of 80 kJ/mol. With an enzyme, the activation energy is 35 kJ/mol. By how many kJ/mol did the enzyme lower the activation energy?
- 2 An enzyme converts 240 substrate molecules into product in 30 seconds. What is the average reaction rate in molecules per second?
- 3 A student says the lock-and-key model and induced-fit model mean the same thing. Explain the difference between the two models and why induced fit often gives a more realistic description of enzyme action.