Enzymes as Biological Catalysts

What is a Catalyst?

A catalyst is a substance that increases the rate of a chemical reaction without being used up or permanently changed in the process.

Characteristics of Catalysts:

• Speed up reactions (increase reaction rate)
• Are NOT consumed in the reaction
• Can be used repeatedly
• Lower activation energy required
• Do NOT change the equilibrium position

Why Enzymes are Important

Enzymes are essential for sustaining life because:

• Metabolic reactions in cells occur too slowly without them
• Body temperature is too low for rapid non-enzymatic reactions
• Enzymes allow reactions to proceed at viable rates for cellular function

Example: Without enzymes, digesting a protein would take weeks; with enzymes, it takes minutes.

Enzymes as Proteins

Key Facts

Enzyme Names

Most enzymes have names ending in “-ase”:

• Amylase (starch digestion)
• Pepsin (protein digestion in stomach)
• Lipase (fat digestion)
• Catalase (breaks down hydrogen peroxide)
• DNA polymerase (DNA synthesis)

Enzyme Action: Active Site and Substrate

Lock and Key Model

Enzymes work through a specific interaction between enzyme and substrate:

Active Site

• Small region on enzyme where substrate binds
• Complementary shape to substrate (like key fits lock)
• Contains specific amino acid arrangement

Substrate

• Molecule(s) on which enzyme acts
• Binds to active site

Reaction Process

1. Substrate binds to active site (forming enzyme-substrate complex)
2. Reaction occurs (substrate converted to product)
3. Product released
4. Enzyme free to bind another substrate

Enzyme-Substrate Complex

• Temporary structure formed when substrate binds
• Allows optimal orientation for reaction
• Stabilizes transition state
• Lowers activation energy

Factors Affecting Enzyme Activity

1. Temperature

Effect on Reaction Rate

Typical Optimum Temperature

• Human enzymes: ~37°C (body temperature)
• Plant enzymes: 20-30°C
• Thermophilic bacteria: 60-80°C+

Graph Characteristics

• Rate increases with temperature up to optimum (Q10 effect: ~2x increase per 10°C)
• Sharp decline after optimum as enzyme denatures
• Irreversible damage at high temperatures

Why Temperature Affects Enzymes

1. Low Temperature: Molecules have less kinetic energy

   • Fewer successful collisions per second
   • Less likely to overcome activation energy barrier

2. Optimum Temperature: Ideal conditions

   • Maximum collision frequency
   • Correct enzyme shape maintained
   • Maximum successful collisions

3. High Temperature: Enzyme denaturation

   • Heat breaks hydrogen bonds and other weak bonds
   • Active site shape changes
   • Substrate can no longer bind
   • Enzyme permanently damaged

2. pH

Effect on Reaction Rate

Typical Optimum pH Values

Graph Characteristics

• Bell-shaped curve similar to temperature effect
• Different enzymes have different optimum pH values
• Rate drops sharply away from optimum

Why pH Affects Enzymes

1. Extreme pH: Ionization changes

   • H⁺ or OH⁻ ions disrupt ionic bonds
   • Charge on amino acids changes
   • Active site shape altered
   • Substrate cannot bind effectively

2. Optimum pH: Ideal ionization

   • Correct ionic bonds maintain shape
   • Active site properly formed
   • Maximum catalytic efficiency

3. Enzyme Concentration

Effect (when substrate is in excess)

• More enzyme molecules = more active sites available
• Rate increases proportionally with enzyme concentration
• Linear increase until substrate becomes limiting

4. Substrate Concentration

Effect (when enzyme is constant)

Saturation Point

• Maximum rate achieved when all enzyme active sites are continuously occupied
• Adding more substrate has no effect
• Rate limited by number of enzymes, not substrate

5. Other Factors

Inhibitors

• Competitive: Bind to active site, blocking substrate (reversible)
• Non-competitive: Bind to other site, changing enzyme shape (reversible)

Co-factors

• Some enzymes require non-protein helpers:
  • Metal ions (e.g., Mg²⁺ for DNA polymerase)
  • Coenzymes (e.g., vitamins)

Detailed Mechanism Explanation

Step-by-Step Enzyme Action

1. Collision: Substrate molecule collides with enzyme

   • Must have correct orientation and sufficient kinetic energy

2. Binding: Substrate binds to active site

   • Complementary shape allows tight fit
   • Hydrogen bonds, ionic bonds, van der Waals forces form
   • Enzyme-substrate complex formed

3. Catalysis: Chemical reaction occurs

   • Bonds in substrate weakened or broken
   • Transition state stabilized
   • Activation energy lowered
   • Products formed

4. Release: Products released

   • Product shape no longer fits active site
   • Products diffuse away
   • Active site free for new substrate

5. Reuse: Enzyme ready for another cycle

   • Enzyme unchanged, can catalyse again

Common Exam Questions and Answers

Q1: Explain why enzymes are specific

Answer: Each enzyme has an active site with a unique shape determined by its amino acid sequence. Only substrates with complementary shape can bind to the active site. This is like a lock and key - only the correct key (substrate) fits the lock (active site). Different enzymes have different shapes, so they catalyse different reactions.

Q2: Describe what happens at high temperature

Answer: High temperature increases kinetic energy of molecules, increasing collision frequency. However, beyond optimum temperature, heat energy breaks hydrogen bonds and other weak bonds in the enzyme. This changes the shape of the active site (denaturation), preventing substrate binding. The enzyme is permanently damaged and cannot function.

Q3: Why do different enzymes have different optimum pH values?

Answer: Each enzyme is adapted to function in a specific environment. The optimum pH reflects the pH of that environment. For example, pepsin works in stomach acid (pH 2), while trypsin works in alkaline small intestine (pH 8). The pH optimum maintains the enzyme’s ionic bonds and active site shape for that particular environment.

Practical Applications

Industrial Uses of Enzymes

Temperature Control in Labs

• Incubators set to optimum temperature for specific enzyme
• Ice baths for temperature-sensitive reactions
• pH buffers maintain constant pH during experiments

Comparison: Enzymes vs Non-Enzymatic Catalysts

Glossary