Enzymes
- A catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction.
- Enzymes as proteins that are involved in all metabolic reactions, where they function as biological catalysts
Why enzymes are important
- In living organisms, metabolism must happen very quickly to maintain life processes like respiration, digestion, and excretion. At normal body temps., these reactions would occur far too slowly to sustain life.
- Enzymes lower the activation energy required for these reactions, allowing them to happen fast enough to keep organisms alive.
Enzyme action
Key words
- A substrate is the molecule(s) entering the reaction (the “key”).
- An active site a specifically shaped region on the surface of the enzyme where the substrate binds (the “lock”).
- Enzyme-substrate complex: The temporary structure formed when the substrate binds tightly to the enzyme’s active site.
- Product: The molecule(s) released from the active site at the end of the reaction.
Process
- The enzyme and substrate collide.
- The substrate binds to the active site because their shapes are complementary.
- An enzyme-substrate complex is formed, and the reaction takes place.
- The products are released. The enzyme emerges completely unchanged and is free to catalyze another reaction.
Enzyme specificity
- Enzymes are highly specific. This means one enzyme can only catalyze one specific reaction.
- The reason for this is entirely due to the complementary shape of the active site. If the substrate’s three-dimensional shape does not perfectly match the fit of the active site, no enzyme-substrate complex can form, and no reaction will occur.
Factors affecting enzyme activity
Temperature
| Temp. stage | What happens to the molecules | Effect on reaction rate |
|---|---|---|
| Low temps. | Kinetic energy is low. Molecules move slowly. | Very slow rate. Low frequency of effective collisions between enzymes and substrates. |
| Rising temps. | Kinetic energy increases. Molecules move faster. | Rate increases. The frequency of effective collisions increases. |
| Optimum temp. | The temp. at which the enzyme works at its maximum possible rate (approx. 37°C in humans). | Peak reaction rate. |
| High temps. | Excessive thermal energy breaks the weak bonds holding the protein structure together. | Rate drops rapidly to zero. The enzyme is denatured. |
- When an enzyme denatures, the shape of its active site is permanently altered. The substrate can no longer fit into the active site, meaning no effective collisions can happen and the reaction stops.
pH
- Every enzyme has an optimum pH where it functions best. For most cellular enzymes, this is around pH 7 (neutral), but there are exceptions (e.g. pepsin in the acidic stomach works best at a low pH, like pH 2).
- Going too far or below the optimum pH alters the chemical bonds within the enzyme protein, resulting in the change of the shape and fit of the active site. This is denaturation.