Describe enzyme action: active site shape is complementary to its substrate and product formation.

5.1 Enzymes

Learning Objective

Explain how enzymes act as biological catalysts. In particular, describe how the three‑dimensional shape of the active site is complementary to its substrate, how this complementarity lowers the activation energy, and why the enzyme is unchanged and can be reused after product formation.

Key Definitions (Cambridge wording)

• Catalyst: a substance that speeds up a chemical reaction without being consumed or permanently altered.
• Enzyme: a protein (the IGCSE focus) that acts as a catalyst for a specific biochemical reaction.
• Active site: a specialised region of the enzyme whose precise three‑dimensional shape (and charge distribution) binds the substrate.
• Substrate: the reactant that fits into the active site.
• Enzyme–substrate complex (ES): the temporary association of enzyme and substrate before product formation.
• Product: the molecule(s) produced when the substrate is converted.
• Activation energy (Eₐ): the minimum energy required for a reaction to proceed; enzymes lower Eₐ by stabilising the transition state.

Enzyme as a Catalyst

All catalysts, including enzymes, are unchanged after the reaction and can therefore act repeatedly. Enzymes achieve this by providing an alternative reaction pathway with a lower activation energy.

Active‑Site Complementarity

• Lock‑and‑key model: the active site (the “lock”) has a fixed shape that exactly matches the substrate (the “key”).
• Induced‑fit model: binding of the substrate induces a slight conformational change in the enzyme, improving the fit and further stabilising the transition state.

Both models illustrate that the active‑site shape is complementary to the substrate, allowing precise binding and efficient catalysis.

Reaction Scheme (Catalytic Cycle)

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

• E – enzyme
• S – substrate
• ES – enzyme–substrate complex
• P – product

Step‑by‑Step Enzyme‑Catalysed Reaction

1. Binding (ES formation): substrate fits into the active site, forming the ES complex.
2. Transition‑state stabilisation: the enzyme’s shape and charge environment lower the activation energy, making bond breaking/forming easier.
3. Product formation: bonds are rearranged; product(s) are created while the enzyme itself remains unchanged.
4. Release: product(s) leave the active site, restoring the enzyme’s original conformation so it can bind another substrate molecule.
5. Reuse: because the enzyme is unchanged, it can catalyse many successive reaction cycles.

Enzyme Specificity

The active site is shaped (and often charged) to match a particular substrate, so an enzyme can catalyse only one type of reaction and produce a specific product. This high specificity underlies the tight regulation of metabolic pathways.

Factors Influencing Enzyme Activity (IGCSE focus)

Investigating Temperature & pH (IGCSE Practical)

• Prepare a series of reaction tubes containing identical amounts of enzyme and substrate.
• Vary the temperature (e.g., 0 °C, 20 °C, 37 °C, 50 °C) **or** the pH (e.g., pH 4, 7, 9) for each tube.
• Measure the rate of product formation (colour change, gas evolution, etc.) over a fixed time period.
• Plot rate versus temperature or pH. The peak of the curve shows the optimum; the sharp decline beyond the optimum indicates denaturation.

Example: Catalase Acting on Hydrogen Peroxide

• Enzyme: Catalase (a protein found in potato, liver, blood).
• Substrate: H₂O₂ (hydrogen peroxide).
• Reaction: 2 H₂O₂ → 2 H₂O + O₂ (visible as bubbles of oxygen).
• Active‑site feature: contains an iron ion that assists in breaking the O–O bond, lowering the activation energy.
• Optimum temperature: ≈ 37 °C; activity falls sharply above ≈ 50 °C due to denaturation.
• Typical IGCSE practical: place a small piece of potato in H₂O₂ at different temperatures and record the volume of O₂ produced in 1 min.

Summary

• Enzymes are protein catalysts; they speed up reactions without being consumed.
• The active site’s three‑dimensional shape (and charge) is complementary to the substrate – the lock‑and‑key/induced‑fit concept.
• Binding forms an ES complex; the enzyme stabilises the transition state, lowering the activation energy and accelerating product formation.
• After the reaction the product is released and the enzyme is unchanged, allowing it to act repeatedly.
• Maximum activity occurs at each enzyme’s optimum temperature and pH; beyond these limits the protein denatures and activity falls.
• Reaction rates increase with substrate concentration (up to V_max) and enzyme concentration, and can be reduced by competitive or non‑competitive inhibitors.