Explain enzyme action: active site, enzyme-substrate complex, substrate and product.

5.1 Enzymes

Objective (AO1 + AO2)

Explain how enzymes work by describing the active site, the enzyme‑substrate complex, the substrate and the product. Include why enzymes are essential for life, the influence of temperature, pH and denaturation, and the practical investigation required by the Cambridge IGCSE 0610 syllabus.

Why Enzymes Are Essential

• Biochemical reactions in living organisms would be far too slow to sustain life without a catalyst.
• Enzymes increase the reaction rate by lowering the activation energy, allowing reactions to proceed rapidly at the mild temperatures and neutral pH found in cells.
• Because they are not consumed, a single enzyme molecule can catalyse many cycles of the reaction.

What Is an Enzyme?

• Catalyst – a protein that increases the rate of a chemical reaction without being permanently altered.
• Functions under mild conditions (≈ room temperature, neutral pH) that would be impossible for most inorganic catalysts.
• All enzymes are proteins (some have non‑protein cofactors, but the catalytic component is always a protein).

Common IGCSE Enzyme Examples

Active Site & Specificity

• The active site is a three‑dimensional pocket formed by the folding of the protein chain.
• It contains specific amino‑acid residues that bind the substrate through weak forces (hydrogen bonds, ionic bonds, van‑der‑Waals interactions).
• Specificity – only substrates with a complementary shape, charge and chemical environment fit the active site.
  • Lock‑and‑key model: the substrate fits a rigid active site exactly.
  • Induced‑fit model (preferred): binding induces a slight change in enzyme shape, improving complementarity and further lowering activation energy.

Enzyme‑Substrate Complex (ES)

When a substrate (S) binds to the active site of an enzyme (E) an enzyme‑substrate complex (ES) is formed. This temporary association aligns the reactant(s) in the optimal orientation for the reaction to proceed.

Step‑by‑Step Mechanism

The overall reaction can be written as:

$$E + S \;\rightleftharpoons\; ES \;\xrightarrow{\text{catalysis}}\; E + P$$

1. Binding: Substrate fits into the active site → ES complex.
2. Induced‑fit: Enzyme undergoes a conformational change, lowering the activation energy.
3. Conversion: Bonds in the substrate are broken/formed while it remains bound, producing product(s) (P).
4. Release: Product leaves the active site; the enzyme returns to its original shape and can bind another substrate molecule.

Substrate vs. Product

Factors Affecting Enzyme Activity (Core Syllabus)

Practical Investigation (Syllabus Requirement)

Investigating the effect of temperature or pH on enzyme activity is a compulsory IGCSE experiment. The following outline uses amylase and starch:

1. Prepare a starch solution (substrate) and a fixed volume of amylase solution.
2. Divide the mixture into several test tubes and place each at a different temperature (e.g., 0 °C, 25 °C, 37 °C, 50 °C) or in buffered solutions of different pH (pH 3, 5, 7, 9, 11).
3. After a set incubation time (e.g., 5 min), stop the reaction by placing the tube in ice.
4. Add a few drops of iodine solution. Iodine forms a blue‑black complex with any remaining starch; the intensity of the colour indicates how much substrate is left.
5. Measure colour intensity (visually or with a colourimeter) and plot temperature (or pH) against % starch remaining. The peak of the curve shows the optimum condition.

Key points for the report:

• State the hypothesis (e.g., “Enzyme activity will be highest at the optimum temperature/pH”).
• Identify variables: independent (temperature or pH), dependent (amount of starch remaining), controlled (enzyme concentration, substrate concentration, reaction time).
• Explain how the results illustrate the effect of temperature/pH on the shape of the active site.

Link to Assessment Objectives

Key Points to Remember

• Enzymes are **proteins** that act as **catalysts** – they speed up reactions without being consumed.
• The **active site** determines **specificity** (lock‑and‑key or induced‑fit).
• Formation of the **enzyme‑substrate complex** is the first step in lowering the activation energy.
• After conversion, the **product** is released and the enzyme is unchanged, ready for another cycle.
• Each enzyme has an **optimum temperature and pH**; deviations reduce the rate, and extreme conditions cause **denaturation** (usually irreversible).
• Investigating temperature and pH effects is a required practical; results must be plotted and interpreted in terms of enzyme structure.

Common Misconceptions

• Enzymes are **not** used up in the reaction – they re‑appear unchanged.
• Enzymes do **not** change the position of equilibrium; they only help the system reach equilibrium faster.
• Temperature and pH affect the **shape** of the enzyme (and thus the active site), not its chemical composition.
• Denaturation is normally **irreversible**; once the active site is destroyed the enzyme can no longer function.

Suggested Diagram (for classroom use)