Enzymatic Mechanisms

Lecture Overview

  • Title: Molecular Biology & Biochemistry Lecture 12: Enzyme Active Sites - Mechanism

  • Presenter: Dr. Emily Flack

Learning Outcomes

  • Understand key features of enzyme active sites.

  • Familiarity with experimental methods to study active sites and mechanisms.

  • Explain the mechanism of action for serine proteases and carbonic anhydrase, focusing on binding specificity and catalysis.

  • Analyze substrate binding and transition state formation in enzyme catalysis.

Enzyme Active Sites

  • Definition: The active site is crucial for substrate binding and catalysis.

  • Characteristics:

    • Small relative to the entire protein.

    • Three-dimensional cleft formed by amino acid residues from different parts of the primary structure.

    • Substrate binding is mediated by weak, non-covalent interactions (e.g., hydrogen bonds).

Mechanisms of Enzyme Catalysis

  • Types of Catalysis:

    • Covalent Catalysis (serine proteases)

    • Acid-base Catalysis (serine proteases and carbonic anhydrase)

    • Metal Ion Catalysis (carbonic anhydrase)

    • Proximity and Orientation Effects (various enzymes)

    • Electrostatic Catalysis (serine proteases)

    • Preferential Binding to Transition State (serine proteases)

Understanding Enzyme Mechanisms

  • Structural Biology Methods:

    • X-ray crystallography provides models of protein structure, useful for understanding enzyme mechanisms.

    • Substrate analogues bind to active site without being processed by the enzyme.

    • Mutations in key amino acids can be used to probe functions.

Proteases

  • Definition: Hydrolase enzymes that cleave peptide bonds.

  • Selectivity: Depends on the structure of the enzyme and substrate specificity.

Serine Proteases

  • Family of enzymes hydrolyzing proteins in the digestive system.

  • Roles in digestion, blood clotting, fertilization, and immune response.

  • Inactive forms are released from the pancreas as zymogens (proenzymes).

  • Example: Trypsin is activated from Trypsinogen.

Chymotrypsin Specificity

  • Substrate Preference: Favors bulky aromatic amino acids (e.g., tryptophan, phenylalanine).

  • Mechanism:

    • Active site includes a hydrophobic pocket accommodating aromatic residues.

    • Cleaves on the carbonyl side of aromatic residues.

Comparison of Serine Proteases

  • Trypsin: Cleaves after basic residues (Arg, Lys); contains Asp189 at the bottom of its pocket.

  • Elastase: Cleaves after small neutral residues; shallow pocket with bulky residues.

Catalytic Triad of Serine Proteases

  • Components: Serine, Histidine, Aspartate are key for enzymatic function.

  • Chymotrypsin Mechanism:

    1. Serine oxygen is activated through interaction with Histidine.

    2. Formation of a covalent bond between serine and substrate.

    3. Stabilization of intermediates in the oxyanion hole.

    4. Catalytic breakdown leading to product release.

Oxyanion Hole

  • Function: Stabilizes the transition state intermediate during catalysis.

  • Interactions help lower the energy barrier for the reaction.

Summary of Chymotrypsin Mechanism

  • Details on substrate specificity and catalytic steps including the role of serine as a nucleophile, involvement of histidine, and the stabilization of transition states.

Evolution of Serine Proteases

  • Divergent Evolution: Similar structures with different functions (e.g. Chymotrypsin, Factor X).

  • Convergent Evolution: Different folds with the same active site residues (e.g. Subtilisin).

Role of Metal Ions in Catalysis

  • Example: Carbonic Anhydrase (CA) utilizes zinc ions in catalysis.

  • Function of Zinc: Activates water, stabilizes hydroxide ion, and plays a role in rapid reactions.

Carbonic Anhydrase Functions

  • Catalyzes the hydration of CO2 for effective transport, pH regulation, and water balance.

  • Achieves high reaction rates (Kcat ≈ 10^6 s^-1) at neutral pH using metal ion-mediated catalysis.

Summary Points

  • Enzyme active sites are a small part of the protein's structure yet crucial for function.

  • Active sites involve residues and metal ions that contribute to catalysis and specificity.

  • Structural insights allow for understanding of enzyme mechanisms and their evolution.