Enzymes: Mechanism and Properties

This lesson covers the nature of enzymes, the lock and key and induced fit models of enzyme action, and the factors affecting enzyme activity, as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand how enzymes work as biological catalysts, their specificity, and the effects of temperature and pH on their activity.

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What Are Enzymes?

Enzymes are biological catalysts — they speed up the rate of chemical reactions in living organisms without being permanently changed or consumed in the reaction.

Key Properties of Enzymes

Property	Detail
Chemical nature	Almost all enzymes are globular proteins (a few catalytic RNA molecules, called ribozymes, also exist)
Specificity	Each enzyme catalyses only one specific reaction or type of reaction
Not consumed	Enzymes are not used up — they can be reused many times
Work in small amounts	A single enzyme molecule can catalyse thousands of reactions per second
Affected by conditions	Enzyme activity is influenced by temperature, pH, substrate concentration, and the presence of inhibitors
Intracellular and extracellular	Some enzymes work inside cells (intracellular, e.g. catalase, ATP synthase); others are secreted and work outside cells (extracellular, e.g. digestive enzymes like trypsin, amylase)

Key Definition: A catalyst is a substance that increases the rate of a chemical reaction without being permanently altered. It does this by lowering the activation energy of the reaction.

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Activation Energy

Every chemical reaction requires a minimum amount of energy to get started — this is called the activation energy (Eₐ). It represents the energy needed to break the existing bonds in the reactants so that new bonds can form in the products.

Enzymes lower the activation energy by providing an alternative reaction pathway with a lower energy barrier. This means that more molecules have sufficient energy to react at any given temperature, so the reaction occurs faster.

Without an enzyme: The activation energy is high → reaction is slow (or does not occur at body temperature).

With an enzyme: The activation energy is lowered → reaction is fast at body temperature (37 °C in humans).

Exam Tip: When explaining how enzymes work, always state that they lower the activation energy of the reaction. Do not say they "provide energy" — this is a common error. The substrate molecules already have kinetic energy; the enzyme simply lowers the barrier they need to overcome.

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The Active Site

Each enzyme has a region called the active site — a small, specific three-dimensional cleft or pocket on the enzyme's surface where the substrate (the molecule the enzyme acts on) binds.

Properties of the Active Site

• It is formed by a small number of amino acid residues (typically 3–12) that are brought together by the folding of the polypeptide chain (tertiary structure).
• The R-groups of these amino acids are precisely positioned to bind the substrate and catalyse the reaction.
• The active site has a specific shape and charge distribution that is complementary to the substrate — like a lock and key.
• The active site makes up only a small fraction of the total enzyme surface.

Key Definition: The active site is the region of an enzyme molecule where the substrate binds and the catalytic reaction takes place. Its shape is complementary to the shape of the substrate.

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Models of Enzyme Action

The Lock and Key Model (Emil Fischer, 1894)

In this model:

1. The enzyme has a rigid active site with a fixed shape.
2. The substrate has a complementary shape to the active site — it fits perfectly, like a key into a lock.
3. The substrate binds to the active site, forming an enzyme–substrate (ES) complex.
4. The reaction occurs, and products are released.
5. The enzyme is unchanged and ready to catalyse another reaction.

Enzyme + Substrate → Enzyme–Substrate Complex → Enzyme + Product(s)

The following diagram illustrates the enzyme catalysis cycle, showing how the enzyme is recycled after each reaction:

graph LR
    A["Enzyme + Substrate"] --> B["Enzyme-Substrate<br/>Complex"]
    B --> C["Enzyme-Product<br/>Complex"]
    C --> D["Enzyme + Products"]
    D -->|"Enzyme recycled"| A

This model explains enzyme specificity — only a substrate with the correct shape can fit the active site.

Limitation: This model suggests the active site is completely rigid and unchanging, which does not fully explain experimental observations.

The Induced Fit Model (Daniel Koshland, 1958)

This more accurate model proposes that:

1. The active site is not perfectly complementary to the substrate initially.
2. When the substrate approaches the active site, the enzyme changes shape slightly — the active site moulds around the substrate.
3. This conformational change ensures a tighter fit between the enzyme and substrate.
4. The induced fit places strain on the substrate bonds, helping to lower the activation energy further.
5. After the reaction, the products no longer fit the active site and are released. The enzyme returns to its original shape.

Feature	Lock and Key	Induced Fit
Active site shape	Rigid, fixed	Flexible, changes shape on substrate binding
Complementarity	Perfect fit before binding	Approximate fit that improves on binding
Explains	Enzyme specificity	Specificity and catalytic mechanism
Supported by	Early enzyme studies	X-ray crystallography and molecular dynamics

Exam Tip: The induced fit model is considered more scientifically accurate than the lock and key model. In exam questions, describe both models but emphasise induced fit. Explain that the conformational change helps lower the activation energy by placing strain on the substrate bonds or by correctly positioning catalytic groups in the active site.

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How Enzymes Lower Activation Energy

Enzymes use several mechanisms to lower activation energy:

1. Proximity and orientation — The enzyme holds the substrates in the correct position and orientation for the reaction, increasing the chance of successful collisions between reactants.
2. Strain on bonds — The induced fit of the enzyme distorts the substrate, weakening bonds that need to be broken.
3. Providing a microenvironment — The active site may be hydrophobic or have a different pH from the surrounding solution, favouring the reaction.
4. Temporary covalent intermediates — Some enzyme active sites form brief covalent bonds with the substrate during catalysis.
5. Acid–base catalysis — R-groups in the active site donate or accept protons (H⁺), facilitating bond breaking and formation.

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Factors Affecting Enzyme Activity

Effect of Temperature

Temperature Range	Effect on Enzyme Activity
Low temperatures	Molecules have low kinetic energy → few successful enzyme–substrate collisions → reaction rate is slow
Increasing temperature	Kinetic energy increases → more frequent and energetic collisions between enzyme and substrate → reaction rate increases
Optimum temperature	The rate is at its maximum. For human enzymes, this is typically around 37 °C
Above optimum	Excess thermal energy causes vibrations that break hydrogen bonds, ionic bonds and hydrophobic interactions maintaining the tertiary structure → the active site changes shape → the substrate can no longer fit → the enzyme is denatured → rate decreases sharply

The relationship between temperature and enzyme activity produces a characteristic asymmetric curve: a gradual increase to the optimum, followed by a sharp decline as denaturation occurs.

Temperature Coefficient (Q₁₀)

The Q₁₀ value describes how much the rate of a reaction increases for every 10 °C rise in temperature:

Q₁₀ = Rate at (T + 10) °C / Rate at T °C

For most enzyme-catalysed reactions, Q₁₀ is approximately 2 (the rate roughly doubles for every 10 °C rise) — but only up to the optimum temperature.

Effect of pH

• Each enzyme has an optimum pH at which its activity is greatest.
• Most human enzymes have an optimum pH around 7–8 (e.g. salivary amylase ≈ pH 7).
• Pepsin (stomach) has an optimum pH of approximately 2.
• Alkaline phosphatase has an optimum near pH 10.

pH Condition	Effect
At optimum pH	R-groups are ionised correctly → active site shape is optimal → maximum rate
Above or below optimum	Changes in H⁺ concentration alter the ionisation of R-groups → ionic bonds and hydrogen bonds are disrupted → active site shape changes → substrate cannot bind → rate decreases
Extreme pH	The enzyme is denatured — the tertiary structure is permanently disrupted

Exam Tip: When explaining the effect of pH on enzymes, be specific. State that pH changes alter the ionisation of R-groups (e.g. –NH₂ may become –NH₃⁺, or –COO⁻ may become –COOH). This disrupts ionic bonds and hydrogen bonds in the tertiary structure, changing the shape of the active site so that the substrate can no longer form an enzyme–substrate complex. Vague answers like "pH changes the active site" will not score full marks.

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Cofactors and Coenzymes

Some enzymes require additional non-protein molecules to function:

Term	Definition	Examples
Cofactor	A non-protein substance required for an enzyme to function. May be inorganic ions or organic molecules.	General term — includes coenzymes and prosthetic groups
Inorganic ion cofactors	Metal ions that bind to the enzyme or substrate	Zn²⁺ in carbonic anhydrase; Mg²⁺ in hexokinase; Fe²⁺ in catalase; Cl⁻ activates salivary amylase
Coenzyme	An organic, non-protein molecule that binds temporarily to the enzyme active site to assist catalysis. Often derived from vitamins.	NAD⁺ (from niacin/vitamin B3); FAD (from riboflavin/vitamin B2); Coenzyme A (from pantothenic acid/vitamin B5)
Prosthetic group	A cofactor that is permanently bound to the enzyme	Haem group in catalase; FAD in succinate dehydrogenase

Exam Tip: Know the distinction between a cofactor (general term), a coenzyme (organic, binds temporarily) and a prosthetic group (permanently bound). This is a common short-answer question worth 2–3 marks.

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Summary

• Enzymes are globular protein biological catalysts that lower the activation energy of reactions.
• The active site is a specific region complementary to the substrate.
• The lock and key model explains specificity; the induced fit model also explains how catalysis occurs through conformational changes.
• Temperature affects enzyme activity: rate increases to an optimum then falls sharply due to denaturation.
• pH affects enzyme activity by altering the ionisation of R-groups, changing the active site shape.
• Cofactors (inorganic ions), coenzymes (organic, temporary) and prosthetic groups (permanent) assist enzyme function.

Exam Tip: In practical-based questions, you may be asked to design an experiment to investigate the effect of temperature or pH on enzyme activity. Include a controlled variable list (enzyme concentration, substrate concentration, volume, time), a method for measuring rate (e.g. volume of gas produced per minute, time for colour change), and a description of how to control temperature (water bath) or pH (buffer solutions).

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A-Level Deep Dive: Enzymes — Mechanism and Properties

Spec mapping

This lesson sits in Edexcel 9BI0 Topic 1 — Biological Molecules, on enzymes as biological catalysts: their globular protein nature, the active site as a specific 3-D arrangement of R-groups arising from tertiary fold, the lock-and-key and induced-fit models of substrate binding, transition-state stabilisation as the mechanism by which activation energy is lowered, and the effects of temperature and pH on rate (refer to the official Pearson Edexcel 9BI0 specification for exact wording). Content statements paraphrase to: define enzyme, active site, substrate, enzyme-substrate complex (ES), enzyme-product complex (EP), activation energy and turnover number; contrast lock-and-key (rigid complementary fit) with induced-fit (reversible distortion of active site on substrate binding) and explain why induced-fit is the modern accepted model; predict rate vs temperature (asymmetric — rises to optimum, then sharp denaturation cliff) and rate vs pH (bell-shaped about an enzyme-specific optimum); name cofactors, coenzymes and prosthetic groups. The lesson is examined directly on Paper 1 and reactivates synoptically on the next lesson (lesson 8 — Vmax, Km, competitive vs non-competitive inhibition), Topic 5 (respiratory enzymes — pyruvate dehydrogenase, ATP synthase, photosynthetic enzymes), Topic 7 (digestive cascades — pepsinogen → pepsin, trypsinogen → trypsin) and Topic 8 (inborn errors of metabolism — phenylketonuria, Tay-Sachs, lactose intolerance).

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Worked example with full mark scheme

Question (8 marks): Catalase (optimum pH 7.4) catalyses the breakdown of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂. A student measures initial rate of O₂ evolution at pH 5.4, 7.4 and 9.4, holding [substrate] saturating, [enzyme] constant and temperature at 25 °C. Rate at pH 5.4 and pH 9.4 is approximately 30% of the rate at pH 7.4.

(a) Explain, in molecular terms, why a 2-unit pH shift from the optimum reduces the rate to approximately 30%. (4)

(b) The student then warms the pH 7.4 sample from 25 °C to 35 °C and measures rate again. Predict the rate change using a Q₁₀ of 2.0, and state the molecular limit at which this prediction breaks down. (4)

Solution with mark scheme:

(a) M1 (AO1) — pH change alters R-group ionisation. At pH 5.4, excess H⁺ protonates carboxylates (–COO⁻ → –COOH); at pH 9.4, –NH₃⁺ deprotonates to –NH₂ — loss of charge in both directions.

A1 (AO2) — loss of charged R-groups breaks ionic bonds (e.g. lysine –NH₃⁺ ↔ aspartate –COO⁻) and disrupts H-bonds in the tertiary fold, deforming the active site.

A1 (AO2) — fewer enzyme molecules have a productive active site; ES formation falls and so does initial rate. Mild shifts are reversible; extreme shifts denature.

A1 (AO3) — the bell-shape reflects the population of correctly-folded enzymes; a 30% residual rate at ±2 pH units indicates partial deformation, not full denaturation.

(b) M1 (AO2) — Q₁₀ = rate(T+10) / rate(T). At Q₁₀ = 2.0 and ΔT = 10 °C, predicted rate at 35 °C = 2 × rate at 25 °C.

A1 (AO1) — higher temperature raises kinetic energy, so a greater fraction of E + S collisions clear the activation barrier, increasing productive ES formation.

A1 (AO3) — prediction breaks down at the denaturation threshold (catalase ~45–50 °C). Above this, weak-bond disruption deforms the active site and rate falls sharply — the asymmetric "denaturation cliff."

A1 (AO3) — Q₁₀ assumes a single collision-limited step. Once denaturation contributes, rate is a competition between collision-frequency gain and active-site loss, and a single Q₁₀ no longer applies.

Total: 8 marks (a: M1 A1 A1 A1; b: M1 A1 A1 A1). A* candidates link R-group ionisation → ionic-bond loss → active-site deformation, and frame Q₁₀ as a kinetic model that fails at denaturation.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Compare the lock-and-key and induced-fit models of enzyme action, and explain why induced-fit is regarded as the better model for understanding catalysis.

Mark scheme decomposition by AO: