Factors Affecting the Rate of Enzyme Reactions: MDCAT Biology notes
Factors affecting the rate of enzyme reactions MDCAT notes: enzyme and substrate concentration, saturation, temperature, denaturation, optimum pH values.
Overview
How fast an enzyme works depends on four main factors: enzyme concentration, substrate concentration, temperature and pH. Inhibitors can also reduce the rate. Each enzyme works fastest under its own optimum conditions.
Enzyme concentration
If there is plenty of substrate, the rate of reaction rises in direct proportion to enzyme concentration. More enzyme means more active sites. Adding enzyme never reduces the rate.
Substrate concentration
When enzyme concentration is fixed, raising substrate concentration increases the rate at first. The rate then levels off once all active sites are occupied. This is saturation. Beyond this point, adding more substrate has no effect, because the enzyme is working at its maximum rate ($V_{max}$). The graph is a curve that flattens, not a straight diagonal line.
Temperature
- Minimum temperature: the lowest temperature at which the enzyme works. Below it the enzyme is inactive but not destroyed. Warming reactivates it.
- As temperature rises, molecules move faster, collide more often, and the rate increases.
- Optimum temperature: the temperature of maximum activity. For human enzymes this is about 37 °C. In MCQ options, the nearest value, around 40 °C, is taken as the optimum for most enzymes.
- Maximum temperature: above the optimum, the enzyme starts to denature and the rate falls sharply.
pH
Each enzyme has an optimum pH. Most intracellular human enzymes work best at around pH 6–8. At very low or very high pH, the charges on the amino acids change, the ionic and hydrogen bonds holding the shape break, and the enzyme is denatured.
| Enzyme | Optimum pH (FSc textbook) | Medium |
|---|---|---|
| Pepsin | 2.00 | Acidic (stomach) |
| Sucrase | 4.50 | Acidic |
| Enterokinase | 5.50 | Acidic |
| Salivary amylase | 6.80 | Near neutral |
| Chymotrypsin | 7.00–8.00 | Neutral to slightly alkaline |
| Catalase | 7.60 | Slightly alkaline |
| Pancreatic lipase | 9.00 | Alkaline (intestine) |
| Arginase | 9.70 | Alkaline |
Trypsin also works in the alkaline small intestine, at around pH 8. When options give only round numbers, choose the closest: pancreatic lipase at pH 8 rather than 2, 4 or 6, and catalase at pH 7.
Denaturation
Denaturation is the loss of the enzyme's three-dimensional (tertiary) structure. It is caused by high temperature, extreme pH or heavy-metal ions. The active site loses its shape, so the substrate can no longer fit. Denaturation by heat is usually permanent. It does not change the number or sequence of amino acids, because peptide bonds stay intact.
Worked example
Question: Doubling the substrate concentration in a reaction produces no change in rate. Why?
Answer: All the active sites are already occupied, so the enzyme is saturated. Only adding more enzyme would raise the rate.
Common MDCAT traps
- "Enzyme concentration reduces the rate" is always the incorrect statement.
- Maximum temperature is where denaturation begins, not where activity is best.
- Lipase needs an alkaline medium and pepsin needs an acidic one.
- Substrate saturation produces a plateau, not a straight line.
- Denaturation destroys tertiary structure, not the amino acid chain.
Quick revision
- Pepsin works best at pH 2.
- Salivary amylase works best at pH 6.8.
- Most human cellular enzymes work at pH 6–8.
- Body enzymes have an optimum temperature of about 37 °C.
- Inhibitors reduce the rate of enzyme reactions.