Competitive Enzyme Inhibition in Medicines

1. Enzymes as Biological Catalysts

• Enzymes are protein-based catalysts that speed up biochemical reactions in living systems.
• They are highly specific, meaning each enzyme typically catalyzes only one type of reaction.
• Enzymes have a specific region called the active site where the substrate binds and the reaction occurs.

1.1 Enzyme Structure

• Enzymes have four levels of structural organization:
  • Primary Structure: The sequence of amino acids in the polypeptide chain.
  • Secondary Structure: Localized folding patterns like alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
  • Tertiary Structure: The overall 3D structure of a single polypeptide chain, determined by various interactions (hydrophobic, ionic, hydrogen bonds, disulfide bridges) between amino acid side chains.
  • Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) to form a functional enzyme complex (not all enzymes have this).

1.2 Factors Affecting Enzyme Activity

• Temperature:
  • Increasing temperature generally increases reaction rate up to a certain point (optimum temperature).
  • Above the optimum temperature, the enzyme’s structure can be disrupted (denaturation), leading to loss of activity.
  • Decreasing temperature lowers enzyme activity because molecules move slower and collide less often.
• pH:
  • Enzymes have an optimum pH range for activity.
  • Changes in pH can disrupt the enzyme’s structure by:
    • Altering the ionization state of amino acid side chains, affecting bonding.
    • Leading to denaturation at extreme pH values.
    • Formation of zwitterions - a molecule with both positive and negative electrical charges.

KEY TAKEAWAY: Enzymes are biological catalysts with specific structures and active sites. Their activity is affected by temperature and pH, which can lead to denaturation.

2. Enzyme Inhibition

• Enzyme inhibitors are substances that reduce or prevent the activity of enzymes.
• Inhibition can be reversible or irreversible.
• Competitive inhibition is a type of reversible inhibition.

3. Competitive Inhibition

3.1 Mechanism of Competitive Inhibition

• A competitive inhibitor is an organic molecule that is structurally similar to the enzyme’s normal substrate.
• The inhibitor binds reversibly to the active site of the enzyme, preventing the substrate from binding.
• This “competes” with the substrate for the active site.
• The enzyme-inhibitor complex does not lead to a reaction.

3.2 Impact on Reaction Rate

• Competitive inhibition decreases the rate of enzyme activity.
• The degree of inhibition depends on:
  • The concentration of the inhibitor.
  • The concentration of the substrate.
  • The relative affinity of the enzyme for the inhibitor and the substrate.

3.3 Overcoming Competitive Inhibition

• Increasing the substrate concentration can overcome competitive inhibition.
• If there is much more substrate than inhibitor, the substrate is more likely to bind to the active site.

3.4 Lock-and-Key Mechanism

• The binding of the substrate or inhibitor to the active site follows a “lock-and-key” mechanism.
• The shape of the substrate or inhibitor must be complementary to the shape of the active site.

3.5 Diagram Description

(Description of a diagram showing an enzyme, its active site, a substrate, and a competitive inhibitor. The diagram should illustrate the inhibitor binding to the active site, blocking the substrate. A second part of the diagram shows that with increased substrate concentration, the substrate can outcompete the inhibitor.)

4. Competitive Inhibition as a Treatment Approach

• Competitive inhibitors can be used as therapeutic agents (medicines).
• By inhibiting specific enzymes, they can disrupt metabolic pathways and treat diseases.

4.1 Examples of Medicines as Competitive Inhibitors

*   Cortisol
*   Corticosterone
*   Aldosterone

4.2 How it Works

1. Identify an enzyme that plays a crucial role in a disease process.
2. Design or identify a molecule that can act as a competitive inhibitor for that enzyme.
3. Administer the inhibitor to the patient.
4. The inhibitor binds to the enzyme’s active site, reducing its activity and slowing down the disease process.

4.3 Advantages

• Specificity: Competitive inhibitors can be designed to target specific enzymes, minimizing side effects.

4.4 Disadvantages

• Reversibility: The inhibition is reversible, so the drug must be administered regularly to maintain its effect.
• Substrate Concentration: High levels of the natural substrate can reduce the effectiveness of the inhibitor.

EXAM TIP: When explaining competitive inhibition, always mention the active site, the structural similarity between inhibitor and substrate, and the effect of substrate concentration.

5. Key Terms

VCAA FOCUS: Understand the relationship between enzyme structure, active site, substrate, and inhibitor. Be able to explain how competitive inhibitors work at a molecular level.