Enzymes
When the substrate fits into the active site, they form an enzyme-substrate complex. The enzyme catalyses the reaction to form products, which are then released. The enzyme remains unchanged and can be reused.
Specificity: Because the active site shape is specific to one substrate (like a lock and key), an enzyme can only catalyse one specific reaction. For example, lipase breaks down fats but cannot break down proteins because the protein substrate does not fit into the lipase active site.
Scenario: You have a solution containing both fat molecules and protein molecules, and you add the enzyme lipase.
Outcome: Only the fat molecules are broken down into fatty acids and glycerol. The protein molecules remain unchanged.
Reasoning:
- Lipase has an active site complementary to the shape of fat molecules.
- Fat molecules fit into the active site, forming an enzyme-substrate complex.
- Protein molecules have a different shape that is not complementary to the lipase active site.
- Therefore, proteins cannot bind to the active site, and no reaction occurs.
Correction: The active site is complementary (fits like a puzzle piece), not identical. Also, enzymes are not used up; they are released unchanged after the reaction and can catalyse further reactions.
Correction: While many intracellular enzymes have an optimum pH near 7, digestive enzymes vary. For example, pepsin (in the stomach) has an optimum pH of ~2 (acidic), while trypsin (in the small intestine) has an optimum pH of ~8 (alkaline).
When to use: When asked to explain the effect of increasing temperature on enzyme activity, especially below and above the optimum.
Why examiners accept this: You must link kinetic energy to collision frequency and then address structural integrity for denaturation. Examiners look for specific terminology regarding bond breaking.
Correct Explanation Structure:
- Below Optimum: As temperature increases, enzyme and substrate molecules gain more kinetic energy. This leads to a higher frequency of effective collisions, increasing the rate of reaction.
- At Optimum: The rate is at its maximum because the frequency of collisions is high while the active site shape is still intact.
- Above Optimum (Denaturation): At high temperatures, the heat energy breaks the weak bonds (such as hydrogen bonds) that maintain the enzyme's tertiary structure. This causes a permanent change in the shape of the active site. The substrate can no longer fit into the active site, so the enzyme-substrate complex cannot form, and activity drops to zero.
When to use: When asked to explain why enzyme activity decreases at extreme pH levels.
Why examiners accept this: You must explicitly mention the disruption of ionic/hydrogen bonds and the resulting change in active site shape, not just 'it stops working'.
Correct Explanation Structure:
- Changes in pH alter the concentration of H^+ or OH^- ions in the solution.
- These ions interfere with the hydrogen bonds and ionic bonds that hold the enzyme's tertiary structure together.
- This causes a change in the shape of the active site (denaturation).
- The substrate no longer fits into the active site, so the reaction rate decreases.
- pH
- Substrate concentration