Biology - Biological Molecules

What are enzymes?

biological catalysts — globular proteins that speed up chemical reactions by lowering the activation energy by providing an alternate reaction pathway required for them to occur.

Outline the structure of an enzyme.
1. What structure makes up the active site
2. What bonds hold this structure together?
3. What is special about this structure?

1. Enzymes have a specific tertiary structure

2. Hydrogen bonds, Ionic bonds, Disulfide bridges and Hydrophobic interactions

3. The active site is a specific 3D shape complementary to the substrate. The shape of the active site depends on the enzyme’s tertiary structure,

Outline the lock and key mechanism of enzymes

• The substrate fits into the enzyme’s active site perfectly

• The enzyme–substrate complex forms → bonds in the substrate are destabilised → products form.

• Explains specificity, but not flexibility.

Outline the induced fit model of enzyme action

• When the substrate binds, the active site changes shape slightly to better fit the substrate.

• This puts strain on substrate bonds, making them easier to break→ lowers activation energy

• Explains how enzymes can catalyse multiple but similar substrates, and why enzymes are flexible.

Define activation energy and explain how enzymes decrease the activation energy

Activation energy is The minimum energy required for a chemical reaction to occur.

• Enzymes lower activation energy by:

  • Bringing substrates close together in the correct orientation.

  • Weakening bonds within the substrate by stressing them in the enzyme–substrate complex.

What are the five factors affect enzyme rate?

Temperature, pH, substrae concentration, enzyme concentration, presence of inhibitors

How does temperature affect the rate of enzyme action? What happens if the temperature goes above optimum?

• As temperature increases → enzyme and substrate molecules gain kinetic energy → more frequent successful collisions → rate increases.

• Above optimum:

  • Hydrogen and ionic bonds break, altering tertiary structure, so Active site denatured — no longer complementary to substrate.

How does pH affect the rate of enzyme action? What happens if the pH changes?

• Enzymes have an optimum pH where activity is highest.

• Changes in pH affect the H⁺ and OH⁻ concentration, which disrupt ionic and hydrogen bonds maintaining tertiary structure. The active site changes shape → substrate no longer fits → enzyme denatures.

How does substrate concentration affect rate of reaction?

• As substrate concentration increases: More enzyme–substrate complexes form. So the Rate increases until all active sites are occupied → enzyme saturation.

• Once saturated, increasing substrate further doesn’t increase rate — enzyme concentration becomes the limiting factor.

How does enzyme concentration affect rate of reaction?

• As enzyme concentration increases: More active sites available → rate increases linearly. Eventually substrate becomes limiting → rate plateaus.

What are the two types of inhibitors?

Competitive and Non-Competitive

How do competitive inhibitors work?

• Complementary shape to substrate → compete for active site.

• Effect: fewer ES complexes form.

• Can be overcome by increasing substrate concentration.

How do Non-Competitive inhibitors work?

• Bind to allosteric site

• Causes conformational change in enzyme → active site no longer complementary to substrate

• Increasing substrate concentration won’t overcome inhibition.

• Effect: reduces number of active enzymes, decreasing rate of reaction

Define Vmax

Vmax — the maximum rate

• Definition: The maximum rate an enzyme-catalysed reaction can reach when all active sites are occupied by substrate.

• Meaning: Beyond this point, adding more substrate won’t increase rate because every enzyme molecule is already working at full capacity.

Define Km

• Definition: The substrate concentration at which the rate = ½ Vmax.

• Meaning: It reflects how strongly an enzyme binds its substrate (its affinity).

High affinity = Low Km

Low affinity = High Km