Lecture 3 Concepts

Features of the Chymotrypsin Active Site and Mechanism

  • The chymotrypsin active site consists of several key features that enable its catalytic activity:

    • Catalytic Triad: Comprises three essential amino acids: serine (Ser), histidine (His), and aspartate (Asp).

    • Serine (Ser195): Acts as the nucleophile that attacks the peptide bond.

    • Histidine (His57): Acts as a general base, accepting a proton from serine to facilitate nucleophilic attack.

    • Aspartate (Asp102): Stabilizes the positively charged histidine, enhancing its ability to act as a base.

Types of General Enzymatic Mechanisms in Chymotrypsin Catalytic Cycle

  • The chymotrypsin catalytic cycle involves several enzymatic mechanisms:

    • Acid-Base Catalysis: Histidine acts as a base.

    • Covalent Catalysis: Formation of a transient covalent bond between the enzyme and substrate.

    • Electrostatic Catalysis: Charge stabilization through the reactive triad.

Equation and Binding of Ligand to a Binding Site

  • The binding of a ligand (L) to a receptor (R) can be described by the equation: R+L↔RLR + L \leftrightarrow RL

    • The dissociation constant (Kd) is defined as:
      Kd=[R][L][RL]K_d = \frac{[R][L]}{[RL]}

    • Bmax: Maximum binding capacity of the site.

Binding Isotherms and Kd Relationship

  • Binding isotherms illustrate the relationship between the concentration of ligand and the amount of binding:

    • The isotherm typically shows a hyperbolic curve.

    • The relationship of Kd and L indicates:

    • High affinity when Kd < [L] and low affinity when Kd > [L].

    • Kd is related to half-saturation by the condition that at half-saturation, [L] = Kd.

Cooperative Binding vs. Constant Kd

  • Cooperative Binding: occurs when ligand binding affects subsequent binding affinities, often described by a sigmoidal curve.

  • Unlike single-site binding with a constant Kd, cooperative binding illustrates varying affinity depending on occupancy.

Michaelis-Menten Equation

  • The Michaelis-Menten equation is expressed as:
    V<em>0=V</em>max[S]Km+[S]V<em>0 = \frac{V</em>{max}[S]}{K_m + [S]}

  • Where:

    • V0: Initial velocity of the reaction.

    • Vmax: Maximum rate achieved by the system at saturating substrate concentration.

    • [S]: Substrate concentration.

    • Km: Michaelis constant, reflecting the concentration of substrate at which the reaction velocity is half of Vmax.

kcat Definition and Rate Plot Relationship

  • kcat (turnover number): Defined as the number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated.

  • It provides information about catalytic efficiency but does not indicate saturation behavior; it reflects the intrinsic activity of the enzyme under maximum substrate conditions.