🌿 Biology • Year 11 • Module 1

Enzymes — Structure & Action

Understanding biological catalysts, their structure, and how they accelerate chemical reactions in living cells

Premium Lesson 13 ~50 min
💭 Think First

Clinical Scenario: Lactose Intolerance

A 28-year-old patient experiences bloating, abdominal cramps, and diarrhoea after consuming dairy products. Their doctor explains that they have lactose intolerance due to insufficient production of the enzyme lactase in their small intestine.

Key Clinical Questions:
  • Why does the absence of a single enzyme (lactase) cause such significant digestive symptoms?
  • How do enzymes normally catalyse the breakdown of lactose?
  • What properties of enzymes make them essential for life processes?

As you work through this lesson, consider how enzymes function as biological catalysts and what happens when their structure or activity is compromised.

🎯 Learning Intentions

By the end of this lesson, you will be able to:

1

What Are Enzymes?

Enzymes are biological catalysts—proteins that speed up chemical reactions in living organisms without being consumed in the process. Virtually every metabolic reaction in cells requires an enzyme to proceed at a biologically useful rate.

Key Properties of Enzymes

Property Description Significance
Catalytic Speed up reactions without being used up One enzyme molecule can catalyse thousands of reactions per second
Specific Each enzyme acts on specific substrates Prevents unwanted side reactions; enables precise metabolic control
Protein nature Made of amino acid chains with 3D structure Structure determines function; sensitive to conditions affecting protein folding
Reusable Released unchanged after reaction Cells need only small amounts of each enzyme
Reversible Can catalyse forward and reverse reactions Direction depends on substrate/product concentrations

The Catalytic Power of Enzymes

Enzymes can increase reaction rates by factors of 10⁶ to 10¹² compared to uncatalysed reactions. For example, the enzyme carbonic anhydrase converts CO₂ and H₂O to carbonic acid at a rate of 600,000 molecules per second—so fast that the reaction is essentially instantaneous in red blood cells.

Quick Check

Q: Why is it advantageous for cells to use enzymes rather than relying on heat or pressure to speed up reactions?

2

Enzyme Structure & The Active Site

Enzymes are globular proteins with a specific three-dimensional shape that is essential for their function. This shape is maintained by various bonds and interactions within the protein molecule.

Levels of Protein Structure in Enzymes

  • Primary structure: Linear sequence of amino acids linked by peptide bonds
  • Secondary structure: Local folding into α-helices and β-sheets, stabilised by hydrogen bonds
  • Tertiary structure: Overall 3D shape of the polypeptide, maintained by disulfide bridges, hydrogen bonds, ionic bonds, and hydrophobic interactions
  • Quaternary structure (some enzymes): Association of multiple polypeptide subunits (e.g., haemoglobin, DNA polymerase)

The Active Site

The active site is a specific region on the enzyme surface where substrate molecules bind and chemical transformation occurs. Key features include:

  • Specific pocket or cleft formed by the folding of the polypeptide chain
  • Complementary shape to the substrate molecule(s)
  • Specific amino acid residues that interact chemically with the substrate
  • Usually relatively small compared to the overall enzyme size (typically 3-12 amino acids)

The Importance of Tertiary Structure

The active site depends entirely on the correct tertiary structure of the enzyme. If this 3D shape is disrupted (denaturation), the active site is destroyed and the enzyme loses all catalytic activity—even though the primary structure (amino acid sequence) remains intact.

Cofactors and Coenzymes

Many enzymes require additional non-protein components to function:

Component Nature Examples
Cofactors Inorganic ions (metal ions) Zn²⁺ (carbonic anhydrase), Fe²⁺ (cytochromes), Mg²⁺ (DNA polymerase)
Coenzymes Organic molecules (often vitamins) NAD⁺, FAD, coenzyme A, vitamins B1, B2, B3, B12
Prosthetic groups Tightly bound cofactors/coenzymes Haem group in catalase and peroxidase
3

Models of Enzyme Action

Two major models describe how enzymes interact with their substrates: the lock-and-key model and the induced fit model.

🔒 Lock-and-Key Model (1894)

Proposed by Emil Fischer, this model suggests that the active site has a rigid, pre-formed shape that exactly complements the substrate—like a lock and key.

Key points:

  • Active site shape is fixed
  • Only correctly shaped substrates fit
  • No change to enzyme structure during binding
🤲 Induced Fit Model (1958)

Proposed by Daniel Koshland, this model describes the active site as flexible, changing shape slightly to better fit the substrate upon binding.

Key points:

  • Active site is flexible and dynamic
  • Substrate binding induces conformational change
  • Better explains enzyme promiscuity and regulation

Modern Understanding

The induced fit model is now accepted as more accurate. Evidence from X-ray crystallography shows that enzymes do undergo conformational changes upon substrate binding. This flexibility is essential for:

  • Catalytic efficiency: Induced strain on substrate bonds lowers activation energy
  • Specificity: Only the correct substrate can induce the proper fit
  • Regulation: Allosteric modulators can bind away from the active site and affect its shape
HSC-Style Question

Q: Compare the lock-and-key and induced fit models of enzyme action. Explain why the induced fit model provides a better explanation of enzyme specificity. (4 marks)

4

The Enzyme Action Cycle

Enzyme-catalysed reactions follow a consistent cycle that can be summarised in several key steps:

  1. Substrate binding: The substrate molecule(s) collide with the enzyme and bind to the active site, forming an enzyme-substrate (ES) complex. This involves weak interactions such as hydrogen bonds and ionic bonds.
  2. Induced fit: The enzyme undergoes a conformational change that brings catalytic residues into optimal position and may place strain on substrate bonds.
  3. Catalysis: The reaction occurs at the active site. The enzyme may facilitate the reaction by: (a) orienting substrates correctly, (b) straining substrate bonds, (c) providing a favourable microenvironment, or (d) participating directly in the reaction.
  4. Product formation: The reaction reaches completion, converting substrate to product(s) within the enzyme-product (EP) complex.
  5. Product release: The product(s) have lower affinity for the active site and are released, freeing the enzyme to bind another substrate molecule.

The Overall Reaction

E + S ⇌ ES → EP ⇌ E + P

Where E = enzyme, S = substrate, ES = enzyme-substrate complex, EP = enzyme-product complex, and P = product. The double arrows indicate that each step is reversible, though the overall direction depends on relative concentrations.

Activation Energy and the Transition State

Enzymes accelerate reactions by lowering the activation energy (Eₐ)—the energy barrier that must be overcome for a reaction to proceed. They do this by:

  • Stabilising the transition state (the high-energy intermediate configuration)
  • Providing an alternative reaction pathway with lower Eₐ
  • Bringing reactants into close proximity and correct orientation

Key Point: Enzymes do NOT change the overall free energy change (ΔG) of a reaction, nor do they change the equilibrium position. They simply allow equilibrium to be reached faster by lowering the activation energy barrier.

5

Factors Affecting Enzyme Activity

Several environmental factors influence enzyme activity. Understanding these is crucial for both biological systems and industrial applications.

1. Temperature

  • As temperature increases, kinetic energy increases, leading to more frequent enzyme-substrate collisions
  • Rate approximately doubles for every 10°C rise (Q₁₀ ≈ 2), up to an optimum
  • Optimum temperature: Usually around 37°C for human enzymes (body temperature)
  • Above optimum: increased vibration disrupts weak bonds holding the tertiary structure; enzyme denatures
  • Thermophilic organisms have enzymes with higher optimum temperatures (e.g., 70-80°C)

2. pH

  • Affects ionisation of amino acid residues in the active site
  • Each enzyme has an optimum pH where activity is maximal
  • Most human enzymes: optimum pH ~7 (neutral), e.g., amylase in saliva
  • Pepsin (stomach): optimum pH ~2 (acidic environment)
  • Trypsin (small intestine): optimum pH ~8 (alkaline environment)
  • Extreme pH disrupts ionic bonds, causing denaturation

3. Substrate Concentration

  • At low [S], reaction rate is proportional to substrate concentration
  • As [S] increases, rate increases but progressively less
  • At high [S], rate plateaus—enzyme becomes saturated; all active sites occupied
  • Maximum rate = Vₘₐₓ; at half Vₘₐₓ, [S] = Kₘ (Michaelis constant, a measure of enzyme affinity)

4. Enzyme Concentration

  • Directly proportional to reaction rate (when substrate is not limiting)
  • More enzyme molecules = more active sites available

5. Inhibitors

Type Mechanism Effect Example
Competitive Reversible binding to active site Competes with substrate; increased [S] can overcome Malonate (inhibits succinate dehydrogenase)
Non-competitive Binding to allosteric site Changes enzyme shape; cannot be overcome by [S] Cyanide (inhibits cytochrome oxidase)
6

Enzyme Denaturation

Denaturation is the irreversible loss of an enzyme's three-dimensional structure, resulting in loss of biological activity. Unlike inhibitors which temporarily block function, denaturation permanently destroys the active site.

⚠️ Causes of Denaturation

  • High temperature: Disrupts hydrogen bonds and hydrophobic interactions; thermal agitation exceeds bond strength
  • Extreme pH: Disrupts ionic bonds by changing charge states of amino acid side chains
  • Organic solvents: Disrupt hydrophobic interactions in the protein core
  • Heavy metal ions: Disrupt disulfide bonds (e.g., Hg²⁺, Pb²⁺, Ag⁺)
  • Detergents/surfactants: Disrupt hydrophobic interactions
  • High salt concentration: Can disrupt ionic bonds

Mechanism of Denaturation

  1. Weak bonds maintaining tertiary structure are disrupted
  2. Protein unfolds from its globular shape
  3. The specific 3D shape of the active site is lost
  4. Substrate can no longer bind effectively
  5. Catalytic activity is abolished

Important Note on Reversibility

While most denaturation is irreversible, some enzymes can renature if conditions return to optimal before complete disruption. However, for HSC purposes, denaturation is generally considered irreversible. Ribonuclease is a classic example of an enzyme that can spontaneously renature when cooled.

Biological and Practical Significance

Context Example Application/Consequence
Fever High body temperature (>40°C) Can denature essential enzymes, leading to organ dysfunction
Food preservation Refrigeration, pasteurisation Low temperatures slow enzymes; high heat denatures microbial enzymes
Stain removal Biological washing powders Enzymes (lipases, proteases) work at moderate temperatures; hot water denatures them
Biotechnology PCR reactions Taq polymerase from thermophilic bacteria withstands high temperatures
7

Clinical & Industrial Applications

Understanding enzyme structure and function has led to numerous practical applications in medicine and industry.

Medical Applications

Enzyme Replacement Therapy

Lactase supplements: For lactose intolerance, oral lactase tablets provide the missing enzyme to digest dairy. Pancreatic enzyme replacement: For cystic fibrosis patients, pancreatic enzymes (lipase, protease, amylase) are given as medications.

Diagnostic Enzymes

Cardiac enzymes: Troponin, creatine kinase (CK-MB) are released during heart muscle damage—measured in blood tests for heart attack diagnosis. Liver enzymes: ALT, AST indicate liver damage when elevated.

Industrial Applications

Industry Enzyme Application
Food Pectinase Clarifying fruit juices by breaking down pectin
Food Lactase Producing lactose-free milk
Food Amylase Breaking down starch in brewing and baking
Detergent Lipase, protease, amylase Removing fat, protein, and starch stains
Textile Cellulase Stone-washing denim, softening fabrics
Biotechnology DNA polymerase PCR for DNA amplification and analysis
HSC Connection

Q: Explain why enzymes used in biological washing powders work best at moderate temperatures (30-40°C) and why manufacturers warn against using very hot water. (3 marks)

8

Investigation: Effect of pH on Catalase Activity

This practical investigation explores how pH affects the activity of catalase, an enzyme that breaks down hydrogen peroxide into water and oxygen.

Background

Catalase is found in many living tissues, including liver and potato. It catalyses the decomposition of hydrogen peroxide (H₂O₂), a toxic byproduct of metabolism:

2H₂O₂ → 2H₂O + O₂

Materials

  • Fresh liver homogenate or potato extract (catalase source)
  • Hydrogen peroxide solution (3%)
  • Buffer solutions at pH 3, 5, 7, 9, 11
  • Test tubes, measuring cylinders, stopwatch
  • Gas syringe or inverted measuring cylinder for O₂ collection

Method

  1. Add 5 mL of buffer solution to each of 5 test tubes (one for each pH)
  2. Add 2 mL of liver homogenate to each tube and mix
  3. Add 5 mL of H₂O₂ to each tube and immediately start timing
  4. Measure oxygen production (volume of foam or gas collected) at 30-second intervals for 3 minutes
  5. Record results in a table

Results Table

pH Rate of O₂ production (mL/min) Relative enzyme activity
3
5
7
9
11

Discussion Questions

  1. At which pH was enzyme activity highest? Why?
  2. What happens to the enzyme at very low or very high pH?
  3. Why is it important to use buffer solutions rather than simply adding acid or base?
  4. How could you improve the reliability of your results?
  5. What is the biological significance of catalase having an optimum pH around 7?

✅ Quick Check Questions

1. Which of the following best describes the active site of an enzyme?

2. The induced fit model of enzyme action differs from the lock-and-key model because it proposes that:

3. Enzyme denaturation results from:

4. Which factor does NOT affect enzyme activity?

5. An enzyme accelerates a reaction by:

📝 Short Answer Questions

1. (3 marks) Describe the structure of an enzyme's active site and explain why the tertiary structure of the protein is essential for its function.

2. (4 marks) Explain how the induced fit model accounts for enzyme specificity, using the interaction between an enzyme and its substrate to illustrate your answer.

3. (5 marks) Describe the mechanism of enzyme denaturation and explain why high temperatures and extreme pH both result in loss of enzyme activity. Discuss the biological significance of this property.

4. (3 marks) Using your understanding of enzyme specificity, explain why people with lactose intolerance can often consume yoghurt without experiencing symptoms.

🔄 Revisit & Reflect

Think back to the clinical scenario at the beginning of this lesson. Can you now explain:

Understanding enzyme structure and function is essential for explaining countless biological processes and developing treatments for enzyme deficiency disorders.

🎉 Lesson Complete!

You've covered the fundamental principles of enzyme structure and action—the molecular basis of metabolism in living cells.

Key takeaways:

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