Biochemistry Study Notes - Enzymes: Basic Concepts and Kinetics

Chapter 8: Enzymes: Basic Concepts and Kinetics

Introduction to Enzymes and Inhibition

  • Enzymes are biological catalysts that speed up the rates of chemical reactions without altering the equilibrium of the reactions. They allow the same amount of product to be produced but at a significantly higher rate.

  • To maintain proper cell function, the activity of these enzymes must be controllable. One method of control is through the use of specific molecules called enzyme inhibitors.

Types of Enzyme Inhibition

1. Enzyme Inhibition Overview

  • Enzymes may be inhibited by certain molecules to regulate metabolic pathways.

  • Diagram Explanation:

    • (a) Shows the normal reaction where the enzyme binds to the substrate, facilitating a chemical reaction, and releasing products.

    • (b) Shows inhibition where the enzyme binds to an inhibitor instead of the substrate, preventing the reaction.

2. Irreversible vs Reversible Inhibitors

  • Irreversible Inhibitors:

    • These bind covalently or noncovalently to the enzyme with negligible dissociation constant, meaning they do not easily unbind.

  • Reversible Inhibitors:

    • Characterized by rapid dissociation from the enzyme-inhibitor complex, allowing normal enzyme activity to resume.

3. Examples of Irreversible Inhibitors

  • Nerve Agents (Sarin and Soman):

    • Organophosphorus compounds that irreversibly inhibit acetylcholinesterase (AChE), causing accumulation of the neurotransmitter acetylcholine (ACh) and leading to overstimulation of cholinergic receptors.

    • Clinical Symptoms: Miosis, excessive secretions, paralysis, and respiratory arrest.

  • Penicillin:

    • A bactericidal antibiotic that irreversibly binds to penicillin-binding proteins (PBPs), essential for bacterial cell wall synthesis, leading to bacterial cell death.

  • Aspirin:

    • Covalently modifies cyclooxygenase (COX) enzymes, inhibiting platelet aggregation and reducing inflammation. Aspirin’s effects are longer-lived in platelets but can eventually be remedied as nucleated cells can regenerate COX enzymes over time.

Types of Reversible Inhibition

1. Competitive Inhibition

  • Definition: The inhibitor competes with substrate for the active site of the enzyme.

  • Characteristics:

    • Binding of the inhibitor and substrate is mutually exclusive.

    • An example includes the antibiotic sulfanilamide, which mimics PABA, crucial for bacterial folic acid synthesis.

2. Uncompetitive Inhibition

  • Definition: The inhibitor binds only to the enzyme-substrate complex, creating an inactive enzyme-substrate-inhibitor complex.

    • This type of inhibition is substrate-dependent as the binding of substrate facilitates the binding of the inhibitor.

3. Noncompetitive Inhibition

  • Definition: The inhibitor binds either the free enzyme or the enzyme-substrate complex, inhibiting the conversion of substrate to product but not preventing substrate binding.

  • Characteristics:

    • Cannot be overcome by increasing substrate concentration; maximum reaction rate (Vmax) is decreased while Km remains unchanged.

Kinetics of Enzyme Inhibition

1. Michaelis-Menten Kinetics

  • Key Parameters:

    • Vm is the maximum reaction rate (when enzyme is saturated with substrate).

    • Km is the Michaelis constant, the substrate concentration at which the reaction rate is half of Vm. Low Km indicates high substrate affinity, while high Km indicates low affinity.

2. Kinetics of Competitive Inhibition

  • Impact on Km and Vmax:

    • Vmax remains unchanged as the inhibition can be overcome by a high substrate concentration.

    • Km increases in the presence of an inhibitor (denoted as Kmapp).

3. Kinetics of Uncompetitive Inhibition

  • Impact on Km and Vmax:

    • Both Vmax and Km decrease equivalently since the inhibitor stabilizes the enzyme-substrate complex, reducing active enzyme concentration.

4. Kinetics of Noncompetitive Inhibition

  • Impact on Km and Vmax:

    • Vmax decreases due to reduced effective enzyme concentration, but Km does not change because the inhibitor does not compete for the active site.

Mapping the Active Site Using Irreversible Inhibitors

1. Types of Irreversible Inhibitors

  • Group-specific reagents, affinity labels, and suicide inhibitors can be used to identify active sites in enzymes.

2. Applications of Specific Inhibitors

  • DIPF: A group-specific reagent that covalently modifies serine residues in enzymes, aiding in the study of serine proteases.

  • Affinity Labels: Structurally resemble substrates and bind to the active site, covalently modifying enzyme residues to cause irreversible inhibition.

  • Suicide Inhibitors: Substrates that are modified during catalysis, converting to irreversible inhibitors (e.g., L-deprenyl for treating Parkinson’s disease).

3. Specific Examples of Inhibitors

  • TPCK: An affinity label for chymotrypsin that modifies histidine in the active site.

  • Bromoacetol Phosphate: Affects triose phosphate isomerase by targeting glutamic acid in its active site.

4. Penicillin Mechanism of Action

  • Inhibits the bacterial enzyme transpeptidase, crucial for peptidoglycan synthesis in bacterial cell walls.

  • Forms a penicilloyl-serine derivative, which is a stable and inactive enzyme form that prevents bacterial cell wall synthesis, leading to cell lysis.

5. Transition-State Analog Inhibition

  • Transition-state analogs are more effective inhibitors than substrate analogs as they resemble the transition state of the reaction, effectively binding to the enzyme and blocking activity (e.g., pyrrole 2-carboxylic acid inhibits proline racemase).

Conclusion

  • Understanding enzyme inhibition mechanisms, including reversible and irreversible types, is crucial in biochemistry and drug design, influencing approaches to treat various diseases.